Partitioning between recoding and termination at a stop codon–selenocysteine insertion sequence

Selenocysteine (Sec) is inserted into proteins by recoding a UGA stop codon followed by a selenocysteine insertion sequence (SECIS). UGA recoding by the Sec machinery is believed to be very inefficient owing to RF2-mediated termination at UGA. Here we show that recoding efficiency in vivo is 30–40% independently of the cell growth rate. Efficient recoding requires sufficient selenium concentrations in the medium. RF2 is an unexpectedly poor competitor of Sec. We recapitulate the major characteristics of SECIS-dependent UGA recoding in vitro using a fragment of fdhF-mRNA encoding a natural bacterial selenoprotein. Only 40% of actively translating ribosomes that reach the UGA codon insert Sec, even in the absence of RF2, suggesting that the capacity to insert Sec into proteins is inherently limited. RF2 does not compete with the Sec incorporation machinery; rather, it terminates translation on those ribosomes that failed to incorporate Sec. The data suggest a model in which early recruitment of Sec-tRNA^Sec–SelB–GTP to the SECIS blocks the access of RF2 to the stop codon, thereby prioritizing recoding over termination at Sec-dedicated stop codons.     

Genome-wide prediction of stop codon readthrough during translation in the yeast Saccharomyces cerevisiae

In-frame stop codons normally signal termination during mRNA translation, but they can be read as ‘sense’ (readthrough) depending on their context, comprising the 6 nt preceding and following the stop codon. To identify novel contexts directing readthrough, under-represented 5′ and 3′ stop codon contexts from Saccharomyces cerevisiae were identified by genome-wide survey in silico. In contrast with the nucleotide bias 3′ of the stop codon, codon bias in the two codon positions 5′ of the termination codon showed no correlation with known effects on stop codon readthrough. However, individually, poor 5′ and 3′ context elements were equally as effective in promoting stop codon readthrough in vivo, readthrough which in both cases responded identically to changes in release factor concentration. A novel method analysing specific nucleotide combinations in the 3′ context region revealed positions +1,2,3,5 and +1,2,3,6 after the stop codon were most predictive of termination efficiency. Downstream of yeast open reading frames (ORFs), further in-frame stop codons were significantly over-represented at the +1, +2 and +3 codon positions after the ORF, acting to limit readthrough. Thus selection against stop codon readthrough is a dominant force acting on 3′, but not on 5′, nucleotides, with detectable selection on nucleotides as far downstream as +6 nucleotides. The approaches described can be employed to define potential readthrough contexts for any genome.     

The Strength of Selection Against the Yeast Prion [PSI+]

The [PSI⁺] prion causes widespread readthrough translation and is rare in natural populations of Saccharomyces, despite the fact that sex is expected to cause it to spread. Using the recently estimated rate of Saccharomyces outcrossing, we calculate the strength of selection necessary to maintain [PSI⁺] at levels low enough to be compatible with data. Using the best available parameter estimates, we find selection against [PSI⁺] to be significant. Inference regarding selection on modifiers of [PSI⁺] appearance depends on obtaining more precise and accurate estimates of the product of yeast effective population size N_e and the spontaneous rate of [PSI⁺] appearance m. The ability to form [PSI⁺] has persisted in yeast over a long period of evolutionary time, despite a diversity of modifiers that could abolish it. If mN_e < 1, this may be explained by insufficiently strong selection. If mN_e > 1, then selection should favor the spread of [PSI⁺] resistance modifiers. In this case, rare conditions where [PSI⁺] is adaptive may permit its persistence in the face of negative selection.     

Response of human REV1 to different DNA damage: preferential dCMP insertion opposite the lesion

REV1 functions in the DNA polymerase ζ mutagenesis pathway. To help understand the role of REV1 in lesion bypass, we have examined activities of purified human REV1 opposite various template bases and several different DNA lesions. Lacking a 3′→5′ proofreading exonuclease activity, purified human REV1 exhibited a DNA polymerase activity on a repeating template G sequence, but catalyzed nucleotide insertion with 6-fold lower efficiency opposite a template A and 19–27-fold lower efficiency opposite a template T or C. Furthermore, dCMP insertion was greatly preferred regardless of the specific template base. Human REV1 inserted a dCMP efficiently opposite a template 8-oxoguanine, (+)-trans-anti-benzo[a]pyrene-N ²-dG, (–)-trans-anti-benzo[a]pyrene-N ²-dG and 1,N ⁶-ethenoadenine adducts, very inefficiently opposite an acetylaminofluorene-adducted guanine, but was unresponsive to a template TT dimer or TT (6–4) photoproduct. Surprisingly, the REV1 specificity of nucleotide insertion was very similar in response to different DNA lesions with greatly preferred C insertion and least frequent A insertion. By combining the dCMP insertion activity of human REV1 with the extension synthesis activity of human polymerase κ, bypass of the trans-anti-benzo[a]pyrene-N ² -dG adducts and the 1,N ⁶-ethenoadenine lesion was achieved by the two-polymerase two-step mechanism. These results suggest that human REV1 is a specialized DNA polymerase that may contribute to dCMP insertion opposite many types of DNA damage during lesion bypass.     

Impact of the six nucleotides downstream of the stop codon on translation termination

The efficiency of translation termination is influenced by local contexts surrounding stop codons. In Saccharomyces cerevisiae, upstream and downstream sequences act synergistically to influence the translation termination efficiency. By analysing derivatives of a leaky stop codon context, we initially demonstrated that at least six nucleotides after the stop codon are a key determinant of readthrough efficiency in S. cerevisiae. We then developed a combinatorial-based strategy to identify poor 3′ termination contexts. By screening a degenerate oligonucleotide library, we identified a consensus sequence –CA(A/G)N(U/C/G)A–, which promotes >5% readthrough efficiency when located downstream of a UAG stop codon. Potential base pairing between this stimulatory motif and regions close to helix 18 and 44 of the 18S rRNA provides a model for the effect of the 3′ stop codon context on translation termination.     

The [URE3] phenotype: evidence for a soluble prion in yeast

The aggregation of the two yeast proteins Sup35p and Ure2p is widely accepted as a model for explaining the prion propagation of the phenotypes [PSI⁺] and [URE3], respectively. Here, we demonstrate that the propagation of [URE3] cannot simply be the consequence of generating large aggregates of Ure2p, because such aggregation can be found in some conditions that are not related to the prion state of Ure2p. A comparison of [PSI⁺] and [URE3] aggregation demonstrates differences between these two prion mechanisms. Our findings lead us to propose a new unifying model for yeast prion propagation.     

Error-prone lesion bypass by human DNA polymerase eta

DNA lesion bypass is an important cellular response to genomic damage during replication. Human DNA polymerase η (Polη), encoded by the Xeroderma pigmentosum variant (XPV) gene, is known for its activity of error-free translesion synthesis opposite a TT cis-syn cyclobutane dimer. Using purified human Polη, we have examined bypass activities of this polymerase opposite several other DNA lesions. Human Polη efficiently bypassed a template 8-oxoguanine, incorporating an A or a C opposite the lesion with similar efficiencies. Human Polη effectively bypassed a template abasic site, incorporating an A and less frequently a G opposite the lesion. Significant –1 deletion was also observed when the template base 5′ to the abasic site is a T. Human Polη partially bypassed a template (+)-trans-anti-benzo[a]pyrene-N²-dG and predominantly incorporated an A, less frequently a T, and least frequently a G or a C opposite the lesion. This specificity of nucleotide incorporation correlates well with the known mutation spectrum of (+)-trans-anti-benzo[a]pyrene-N²-dG lesion in mammalian cells. These results show that human Polη is capable of error-prone translesion DNA syntheses in vitro and suggest that Polη may bypass certain lesions with a mutagenic consequence in humans.     

The GA2 Locus of Arabidopsis thaliana Encodes ent-Kaurene Synthase of Gibberellin Biosynthesis

The ga2 mutant of Arabidopsis thaliana is a gibberellin-deficient dwarf. Previous biochemical studies have suggested that the ga2 mutant is impaired in the conversion of copalyl diphosphate to ent-kaurene, which is catalyzed by ent-kaurene synthase (KS). Overexpression of the previously isolated KS cDNA from pumpkin (Cucurbita maxima) (CmKS) in the ga2 mutant was able to complement the mutant phenotype. A genomic clone coding for KS, AtKS, was isolated from A. thaliana using CmKS cDNA as a heterologous probe. The corresponding A. thaliana cDNA was isolated and expressed in Escherichia coli as a fusion protein. The fusion protein showed enzymatic activity that converted [³H]copalyl diphosphate to [³H]ent-kaurene. The recombinant AtKS protein derived from the ga2–1 mutant is truncated by 14 kD at the C-terminal end and does not contain significant KS activity in vitro. Sequence analysis revealed that a C-2099 to T base substitution, which converts Gln-678 codon to a stop codon, is present in the AtKS cDNA from the ga2–1 mutant. Taken together, our results show that the GA2 locus encodes KS.     

The Spontaneous Appearance Rate of the Yeast Prion [PSI+] and Its Implications for the Evolution of the Evolvability Properties of the [PSI+] System

Epigenetically inherited aggregates of the yeast prion [PSI+] cause genomewide readthrough translation that sometimes increases evolvability in certain harsh environments. The effects of natural selection on modifiers of [PSI+] appearance have been the subject of much debate. It seems likely that [PSI+] would be at least mildly deleterious in most environments, but this may be counteracted by its evolvability properties on rare occasions. Indirect selection on modifiers of [PSI+] is predicted to depend primarily on the spontaneous [PSI+] appearance rate, but this critical parameter has not previously been adequately measured. Here we measure this epimutation rate accurately and precisely as 5.8 × 10⁻⁷ per generation, using a fluctuation test. We also determine that genetic “mimics” of [PSI+] account for up to 80% of all phenotypes involving general nonsense suppression. Using previously developed mathematical models, we can now infer that even in the absence of opportunities for adaptation, modifiers of [PSI+] are only weakly deleterious relative to genetic drift. If we assume that the spontaneous [PSI+] appearance rate is at its evolutionary optimum, then opportunities for adaptation are inferred to be rare, such that the [PSI+] system is favored only very weakly overall. But when we account for the observed increase in the [PSI+] appearance rate in response to stress, we infer much higher overall selection in favor of [PSI+] modifiers, suggesting that [PSI+]-forming ability may be a consequence of selection for evolvability.THE yeast phenotype [PSI+] is characterized by prion aggregates of the protein Sup35. Cells are in either a [psi−] (normal) or [PSI+] state, depending on the absence or presence of the prion aggregates (Figure 1, a and b). Sup35 prion aggregates replicate in a similar fashion to mammalian prions but are cytoplasmic and, as such, the prion state is cytoplasmically inherited (Wickner et al. 1995).Open in a separate windowFigure 1.—Comparison between the three possible modes ([PSI+], genetic mimic, point mutation revertant) of the expression of 3′-UTR sequences in yeast. (a) The normal [psi−] phenotypic state; (b) the [PSI+] prion causes readthrough and low-level expression of 3′-UTRs across multiple genes, appearing at rate m_PSI; (c) a genetic mimic of [PSI+] such as the sal3-4 mutant of Sup35 (Eaglestone et al. 1999) appearing at rate m_mimic not reversible by the application of guanidine hydrochloride; (d) a point mutation in a single stop codon at rate μ_point, leading to incorporation of formerly 3′-UTR into a single coding sequence. (e) [PSI+] can act as a “stop-gap” mechanism, buying a lineage more time to acquire one or more adaptive stop codon readthrough point mutations. When this genetic assimilation is complete, [PSI+] can revert to [psi−] (Masel and Bergman 2003; Griswold and Masel 2009).When not part of an aggregate, Sup35 helps mediate translation termination in yeast (Stansfield et al. 1995b; Zhouravleva et al. 1995). Sup35 molecules that are incorporated into nonfunctional prion aggregates are presumably not available for translation termination, which can lead to the translation of stop codons by near-cognate tRNAs (Figure 1b) (Tuite and Mclaughlin 1982; Pure et al. 1985; Lin et al. 1986). This partial loss of Sup35 function leads to an increased frequency of readthrough translation of 3′-untranslated regions (3′-UTR) across all genes (Figure 1b). This increase is modest in wild-type yeast, from an average readthrough rate of 0.3% in [psi−] cells up to 1% in [PSI+] cells (Firoozan et al. 1991). Some [PSI+] yeast strains grow faster than [psi−] controls in certain harsh environments, suggesting that readthrough translation of some 3′-UTRs may be adaptive in certain conditions (True and Lindquist 2000; Joseph and Kirkpatrick 2008). This directly shows that [PSI+]-mediated capacitance may increase evolvability in the laboratory. [PSI+]-mediated phenotypes have a complex genetic basis, involving multiple loci (True et al. 2004).As an epigenetically inherited protein aggregate, [PSI+] can easily be lost after some generations (Cox et al. 1980). This returns the lineage to its normal [psi−] state and restores translation fidelity. If a subset of revealed phenotypic variation is adaptive, it may have lost its dependence on [PSI+] by this time (True et al. 2004). This process of genetic assimilation may, for example, involve one or more point mutations in stop codons, increasing readthrough up to 100% (Figure 1e) (Griswold and Masel 2009). This leaves the yeast with a new adaptive trait and with no permanent load of other, deleterious variation.In general, stop codons can be lost either directly through point mutations or indirectly through upstream indels. This leads to novel coding sequence coming from in-frame and out-of-frame 3′-UTRs, respectively. [PSI+] is expected to facilitate only the former, while mutation bias favors the latter. Yeasts show a much higher ratio of in-frame to out-of-frame 3′-UTR incorporation events than mammals do (Giacomelli et al. 2007), confirming a role for [PSI+] in capacitance-mediated evolvability in natural populations.The adaptive evolution both of evolvability in general (Sniegowski and Murphy 2006; Lynch 2007; Pigliucci 2008) and of capacitance in particular (Dickinson and Seger 1999; Wagner et al. 1999; Partridge and Barton 2000; Brookfield 2001; Pal 2001; Meiklejohn and Hartl 2002; Ruden et al. 2003) is highly controversial. In general, any costs of evolvability are borne in the present, while the benefits lie in the future, making it difficult for natural selection to favor an evolvability allele. For example, mutation rates seem to be set according to a trade-off between metabolic cost (favoring higher mutation rates) and the avoidance of deleterious effects (favoring lower mutation rates) (Sniegowski et al. 2000). The fact that mutation creates variation, the ultimate source of evolvability, is merely a fortuitous consequence of the metabolic cost of fidelity.Previous theoretical population genetic studies have, however, suggested that modifier alleles promoting the formation of [PSI+] might, unlike mutator alleles, be favored for their evolvability properties (King and Masel 2007; Masel et al. 2007; Griswold and Masel 2009; Masel and Griswold 2009). These models depend, however, on a number of parameter estimates. In particular, a number of predictions depend on the spontaneous rate of [PSI+] formation (Masel and Griswold 2009).

[PSI+] appearance rates and the fluctuation test:

The most widely cited spontaneous appearance rate for [PSI+] is m_PSI ∼ 10⁻⁷–10⁻⁵, on the basis of experiments by Lund and Cox (1981). This estimate was calculated as the proportion of colonies scored as [PSI+] after growth over multiple generations from a single founding [psi−] clone. If [PSI+] happens to appear in the first generation of growth, this leads to a “jackpot” event with only one switching event, but many [PSI+] colonies. The proportion of colonies scored as [PSI+] therefore yields a systematic overestimation of the [PSI+] appearance rate.Various implementations of the fluctuation test (Luria and Delbrück 1943) can address such effects. The mutation rate experiment is replicated many times using independent populations, and a Luria–Delbrück distribution is fitted to the results across all replicates. In a simulation study, Stewart (1994) examined a number of estimators of the underlying Luria–Delbrück distribution and found that the maximum-likelihood estimator performed the best.Originally developed to study mutation rates, the fluctuation test can also be used for estimating epimutation rates. Fluctuation tests have been used to estimate the rate of gene silencing in Chinese hamster ovary cells (Holliday and Ho 1998) and in the yeast Schizosaccharomyces pombe (Singh and Klar 2002). However, fluctuation tests do not appear to be used routinely for epimutation rate estimates. For example, although the rates of spontaneous appearance and disappearance of [ISP+], a prion-like element in yeast, have been measured using the fluctuation test (Volkov et al. 2002), to the best of our knowledge there are no published estimates of the spontaneous rate of [PSI+] appearance as measured using a fluctuation test. Although results from the fluctuation test can be confounded by reverse epimutation, or back-switching, this is an issue only if the rate of back-switching is very high, e.g., 10⁻¹–10⁻² per generation (Saunders et al. 2003). This is not the case for [PSI+], for which the reverse epimutation rate (loss of [PSI+]) is <2 × 10⁻⁴ (Tank et al. 2007).

Other [PSI+]-like phenotypes, including genetic mimics:

[PSI+] causes partial loss of Sup35 function, leading to elevated rates of translational readthrough at all stop codons (Figure 1b). There are many other spontaneous changes, presumably mutations, that also lead to elevated translational readthrough (Lund and Cox 1981). Mutations that affect readthrough at all stop codons (Figure 1c) (sometimes called “[PSI+]-like”) can be considered as genetic “mimics” because they produce the same phenotype as the Sup35 aggregate, but are generally not epigenetically inherited. A specific example of such a genetic mimic was characterized by Eaglestone et al. (1999), who identified the sal3-4 point mutation in the SUP35 gene. This leads to a defect in the Sup35 protein structure rendering the termination process less efficient (Eaglestone et al. 1999). The sal3-4 mutant can therefore be considered a partial loss-of-function genetic mimic of [PSI+], since it generates the same readthrough phenotype. Translation termination could also potentially be impaired through other point mutations or deletions, for example, in either the SUP35 or the SUP45 gene (Stansfield et al. 1995a) or in a tRNA that mutates to recognize stop codons at a higher rate. The presence of genetic mimics, whose effects are less reversible than those of [PSI+], can affect the evolution of the evolvability properties of the [PSI+] system such as its epimutation rate (Lancaster and Masel 2009). Note that genetic mimics are quite different from much rarer point mutations that convert stop codons into coding sequence (Figure 1d), resulting in readthrough at a single gene rather than multiple genes.Here we performed experiments to obtain accurate and precise estimates of the baseline appearance rates of both [PSI+] and [PSI+]-like phenotypes in permissive laboratory conditions, excluding stop codon point mutations that affect only a single gene. Our estimates are superior to previous estimates, since we use the fluctuation test. We consider the consequences of these estimates for the evolution of the [PSI+] system.     

Genomic analysis of codon,sequence and structural conservation with selective biochemical-structure mapping reveals highly conserved and dynamic structures in rotavirus RNAs with potential cis-acting functions

Rotaviruses are a major cause of acute, often fatal, gastroenteritis in infants and young children world-wide. Virions contain an 11 segment double-stranded RNA genome. Little is known about the cis-acting sequences and structural elements of the viral RNAs. Using a database of 1621 full-length sequences of mammalian group A rotavirus RNA segments, we evaluated the codon, sequence and RNA structural conservation of the complete genome. Codon conservation regions were found in eight ORFs, suggesting the presence of functional RNA elements. Using ConStruct and RNAz programmes, we identified conserved secondary structures in the positive-sense RNAs including long-range interactions (LRIs) at the 5′ and 3′ terminal regions of all segments. In RNA9, two mutually exclusive structures were observed suggesting a switch mechanism between a conserved terminal LRI and an independent 3′ stem–loop structure. In RNA6, a conserved stem–loop was found in a region previously reported to have translation enhancement activity. Biochemical structural analysis of RNA11 confirmed the presence of terminal LRIs and two internal helices with high codon and sequence conservation. These extensive in silico and in vitro analyses provide evidence of the conservation, complexity, multi-functionality and dynamics of rotavirus RNA structures which likely influence RNA replication, translation and genome packaging.     

Complex Adaptations Can Drive the Evolution of the Capacitor [PSI+], Even with Realistic Rates of Yeast Sex

The [PSI⁺] prion may enhance evolvability by revealing previously cryptic genetic variation, but it is unclear whether such evolvability properties could be favored by natural selection. Sex inhibits the evolution of other putative evolvability mechanisms, such as mutator alleles. This paper explores whether sex also prevents natural selection from favoring modifier alleles that facilitate [PSI⁺] formation. Sex may permit the spread of “cheater” alleles that acquire the benefits of [PSI⁺] through mating without incurring the cost of producing [PSI⁺] at times when it is not adaptive. Using recent quantitative estimates of the frequency of sex in Saccharomyces paradoxus, we calculate that natural selection for evolvability can drive the evolution of the [PSI⁺] system, so long as yeast populations occasionally require complex adaptations involving synergistic epistasis between two loci. If adaptations are always simple and require substitution at only a single locus, then the [PSI⁺] system is not favored by natural selection. Obligate sex might inhibit the evolution of [PSI⁺]-like systems in other species.     

Prion Switching in Response to Environmental Stress

Evolution depends on the manner in which genetic variation is translated into new phenotypes. There has been much debate about whether organisms might have specific mechanisms for “evolvability,” which would generate heritable phenotypic variation with adaptive value and could act to enhance the rate of evolution. Capacitor systems, which allow the accumulation of cryptic genetic variation and release it under stressful conditions, might provide such a mechanism. In yeast, the prion [PSI⁺] exposes a large array of previously hidden genetic variation, and the phenotypes it thereby produces are advantageous roughly 25% of the time. The notion that [PSI⁺] is a mechanism for evolvability would be strengthened if the frequency of its appearance increased with stress. That is, a system that mediates even the haphazard appearance of new phenotypes, which have a reasonable chance of adaptive value would be beneficial if it were deployed at times when the organism is not well adapted to its environment. In an unbiased, high-throughput, genome-wide screen for factors that modify the frequency of [PSI⁺] induction, signal transducers and stress response genes were particularly prominent. Furthermore, prion induction increased by as much as 60-fold when cells were exposed to various stressful conditions, such as oxidative stress (H₂O₂) or high salt concentrations. The severity of stress and the frequency of [PSI⁺] induction were highly correlated. These findings support the hypothesis that [PSI⁺] is a mechanism to increase survival in fluctuating environments and might function as a capacitor to promote evolvability.     

The Evolutionary Potential of Phenotypic Mutations

Errors in protein synthesis, so-called phenotypic mutations, are orders-of-magnitude more frequent than genetic mutations. Here, we provide direct evidence that alternative protein forms and phenotypic variability derived from translational errors paved the path to genetic, evolutionary adaptations via gene duplication. We explored the evolutionary origins of Saccharomyces cerevisiae IDP3 - an NADP-dependent isocitrate dehydrogenase mediating fatty acids ß-oxidation in the peroxisome. Following the yeast whole genome duplication, IDP3 diverged from a cytosolic ancestral gene by acquisition of a C-terminal peroxisomal targeting signal. We discovered that the pre-duplicated cytosolic IDPs are partially localized to the peroxisome owing to +1 translational frameshifts that bypass the stop codon and unveil cryptic peroxisomal targeting signals within the 3’-UTR. Exploring putative cryptic signals in all 3’-UTRs of yeast genomes, we found that other enzymes related to NADPH production such as pyruvate carboxylase 1 (PYC1) might be prone to peroxisomal localization via cryptic signals. Using laboratory evolution we found that these translational frameshifts are rapidly imprinted via genetic single base deletions occurring within the very same gene location. Further, as exemplified here, the sequences that promote translational frameshifts are also more prone to genetic deletions. Thus, genotypes conferring higher phenotypic variability not only meet immediate challenges by unveiling cryptic 3’-UTR sequences, but also boost the potential for future genetic adaptations.     

Gaining Insights into the Codon Usage Patterns of TP53 Gene across Eight Mammalian Species

TP53 gene is known as the “guardian of the genome” as it plays a vital role in regulating cell cycle, cell proliferation, DNA damage repair, initiation of programmed cell death and suppressing tumor growth. Non uniform usage of synonymous codons for a specific amino acid during translation of protein known as codon usage bias (CUB) is a unique property of the genome and shows species specific deviation. Analysis of codon usage bias with compositional dynamics of coding sequences has contributed to the better understanding of the molecular mechanism and the evolution of a particular gene. In this study, the complete nucleotide coding sequences of TP53 gene from eight different mammalian species were used for CUB analysis. Our results showed that the codon usage patterns in TP53 gene across different mammalian species has been influenced by GC bias particularly GC₃ and a moderate bias exists in the codon usage of TP53 gene. Moreover, we observed that nature has highly favored the most over represented codon CTG for leucine amino acid but selected against the ATA codon for isoleucine in TP53 gene across all mammalian species during the course of evolution.     

Mitochondrial Contribution to the Anoxic Ca2+ Signal in Maize Suspension-Cultured Cells

Anoxia induces a rapid elevation of the cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) in maize (Zea mays L.) cells, which is caused by the release of the ion from intracellular stores. This anoxic Ca²⁺ release is important for gene activation and survival in O₂-deprived maize seedlings and cells. In this study we examined the contribution of mitochondrial Ca²⁺ to the anoxic [Ca²⁺]_cyt elevation in maize cells. Imaging of intramitochondrial Ca²⁺ levels showed that a majority of mitochondria released their Ca²⁺ in response to anoxia and took up Ca²⁺ upon reoxygenation. We also investigated whether the mitochondrial Ca²⁺ release contributed to the increase in [Ca²⁺]_cyt under anoxia. Analysis of the spatial association between anoxic [Ca²⁺]_cyt changes and the distribution of mitochondrial and other intracellular Ca²⁺ stores revealed that the largest [Ca²⁺]_cyt increases occurred close to mitochondria and away from the tonoplast. In addition, carbonylcyanide p-trifluoromethoxyphenyl hydrazone treatment depolarized mitochondria and caused a mild elevation of [Ca²⁺]_cyt under aerobic conditions but prevented a [Ca²⁺]_cyt increase in response to a subsequent anoxic pulse. These results suggest that mitochondria play an important role in the anoxic elevation of [Ca²⁺]_cyt and participate in the signaling of O₂ deprivation.     

Crystal structures of a DNA octaplex with I-motif of G-quartets and its splitting into two quadruplexes suggest a folding mechanism of eight tandem repeats

Recent genomic analyses revealed many kinds of tandem repeats of specific sequences. Some of them are related to genetic diseases, but their biological functions and structures are still unknown. Two X-ray structures of a short DNA fragment d(gcGA[G]₁Agc) show that four base-intercalated duplexes are assembled to form an octaplex at a low K⁺ concentration, in which the eight G₅ residues form a stacked double G-quartet in the central part. At a higher K⁺ concentration, however, the octaplex is split into just two halves. These structural features suggest a folding process of eight tandem repeats of d(ccGA[G]₄Agg), according to a double Greek-key motif. Such a packaging of the repeats could facilitate slippage of a certain sequence during DNA replication, to induce increase or decrease of the repeats.     

In vitro and in vivo modulations of benzo[c]phenanthrene-DNA adducts by DNA mismatch repair system

Benzo[c]phenanthrene dihydrodiol epoxide (B[c] PhDE) is well known as an important environmental chemical carcinogen that preferentially modifies DNA in adenine residues. However, the molecular mechanism by which B[c]PhDE induces tumorigenesis is not fully understood. In this report, we demonstrate that DNA mismatch repair (MMR), a genome maintenance system, plays an important role in B[c]PhDE-induced carcinogensis by promoting apoptosis in cells treated with B[c]PhDE. We show that purified human MMR recognition proteins, MutSα and MutSβ, specifically recognized B[c]PhDE-DNA adducts. Cell lines proficient in MMR exhibited several-fold more sensitivity to killing than cell lines defective in either MutSα or MutLα by B[c]PhDE; the nature of this sensitivity was shown to be due to increased apoptosis. Additionally, wild-type mice exposed to B[c]PhDE had intestinal crypt cells that underwent apoptosis significantly more often than intestinal crypt cells found in B[c]PhDE-treated Msh2^–/– or Mlh1^–/– mice. These findings, combined with previous studies, suggest that the MMR system may serve as a general sensor for chemical-caused DNA damage to prevent damaged cells from mutagenesis and carcinogenesis by promoting apoptosis.     

Relating repair susceptibility of carcinogen-damaged DNA with structural distortion and thermodynamic stability

A key issue in the nucleotide excision repair (NER) of bulky carcinogen–DNA adducts is the ability of the NER machinery to recognize and repair certain adducts while failing to repair others. Unrepaired adducts can survive to cause mutations that initiate the carcinogenic process. Benzo[c]phenanthrene (B[c]Ph), a representative fjord region polycyclic aromatic hydrocarbon, can be metabolically activated to the enantiomeric benzo[c]phenanthrene diol epoxides (B[c]PhDEs), (+)-(1S,2R,3R,4S)-3,4- dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenzo[c]phe nanthrene and the corresponding (–)-(1R,2S,3S,4R) isomer. These react predominantly with adenine residues in DNA to produce the stereoisomeric 1R (+)- and 1S (–)-trans-anti-B[c]Ph-N⁶-dA adducts. Duplexes containing the 1R (+) or 1S (–) B[c]Ph-dA adduct in codon 61 of the human N-ras mutational hotspot sequence CA*A, with B[c]Ph modification at A*, are not repaired by the human NER system. However, the analogous stereoisomeric DNA adducts of the bay region benzo[a]pyrene diol epoxide (B[a]PDE), 10S (+)- and 10R (–)-trans-anti-B[a]P-N⁶-dA, are repaired in the same base sequence. In order to elucidate structural and thermodynamic origins of this phenomenon, we have carried out a 2 ns molecular dynamics simulation for the 1R (+)- and 1S (–)-trans-anti-B[c]Ph-N⁶-dA adducts in an 11mer duplex containing the human N-ras codon 61 sequence, and compared these results with our previous study of the B[a]P-dA adducts in the same sequence. The molecular mechanics Poisson– Boltzmann surface area (MM-PBSA) method was applied to calculate the free energies of the pair of stereoisomeric B[c]Ph-dA adducts, and a detailed structural analysis was carried out. The different repair susceptibilities of the B[a]P-dA adducts and the B[c]Ph-dA adducts can be attributed to different degrees of distortion, stemming from combined effects of differences in the quality of Watson–Crick hydrogen bonding, unwinding, stretching and helix backbone perturbations. These differences are due to the different intrinsic topologies of the rigid, planar bay region adducts versus the twisted, sterically hindered fjord region adducts.