Comparative analysis of base biases around the stop codons in six eukaryotes

Using full-length cDNA sequences, a comparative analysis of sequence patterns around the stop codons in six eukaryotes was performed. Here, it was showed that the codon immediately before and after the stop codons (defined as -1 codon and +1 codon, respectively) were much more biased than other examined positions, especially at the second position of -1 codons and the first position of +1 codons which were rich in As/Us and purines, respectively, for most species. The author speculated that strongly biased sequence pattern from position -2 to +4 might act as an extended translation termination signal. Translation termination was catalyzed by release factors that recognized the stop codons. The multiple amino acid sequence alignment of eukaryotic release factor 1 (eRF1) of 20 species showed that there were 16 residue sites that were strictly conserved, especially the invariant amino acids Ile70 and Lys71. Accordingly, it could be inferred that those candidate amino acids might involve in the recognition process. Moreover, the possible stop signal recognition hypothesis was also discussed herein.     

Two genetic codes, one genome: frameshifted primate mitochondrial genes code for additional proteins in presence of antisense antitermination tRNAs

Genomic amino acid usages coevolve with cloverleaf formation capacities of corresponding primate mitochondrial tRNAs, also for antisense tRNAs, suggesting translational function for sense and antisense tRNAs. Some antisense tRNAs are antitermination tRNAs (anticodons match stops (UAR: UAA, UAG; AGR: AGA, AGG)). Genomes possessing antitermination tRNAs avoid corresponding stops in frames 0 and +1, preventing translational antitermination. In frame +2, AGR stop frequencies and corresponding antisense antitermination tRNAs coevolve positively. This suggests expression of frameshifted overlapping genes, potentially shortening genomes, increasing metabolic efficiency. Blast analyses of hypothetical proteins translated from one and seven +1, respectively, +2 frameshifted human mitochondrial protein coding genes align with eleven GenBank sequences (31% of the mitochondrial coding regions). These putative overlap genes contain few UARs, AGRs align with arginine. Overlap gene numbers increase in presence of, and with time since evolution of antitermination tRNA AGR in 57 primate mitochondrial genomes. Numbers of putative proteins translated from antisense protein coding sequences and detected by blast also coevolve positively with antitermination tRNAs; expression of two of these ‘antisense’ mRNAs increases under low resource availability. Although more direct evidence is still lacking for the existence of proteins translated from overlapping mitochondrial genes and for antisense tRNAs activity, coevolutions between predicted overlap genes and the antitermination tRNAs required to translate them suggest expression of overlapping genes by an overlapping genetic code. Functions of overlapping genes remain unknown, perhaps originating from dual lifestyles of ancestral free living-parasitic mitochondria. Their amino acid composition suggests expression under anaerobic conditions.     

Avoidance of antisense, antiterminator tRNA anticodons in vertebrate mitochondria

Protein synthesis (translation) stops at stop codons, codons not complemented by tRNA anticodons. tRNAs matching stops, antitermination (Ter) tRNAs, prevent translational termination, producing dysfunctional proteins. Genomes avoid tRNAs with anticodons whose complement (the anticodon of the ‘antisense’ tRNA) matches stops. This suggests that antisense tRNAs, which also form cloverleaves, are occasionally expressed. Mitochondrial antisense tRNA expression is plausible, because both DNA strands are transcribed as single RNAs, and tRNA structures signal RNA maturation. Results describe potential antisense Ter tRNAs in mammalian mitochondrial genomes detected by tRNAscan-SE, and evidence for adaptations preventing translational antitermination: genomes possessing Ter tRNAs use less corresponding stop codons; antisense Ter tRNAs form weaker cloverleaves than homologuous non-Ter antisense tRNAs; and genomic stop codon usages decrease with stabilities of codon-anticodon interactions and of Ter tRNA cloverleaves. This suggests that antisense tRNAs frequently function in translation. Results suggest that opposite strand coding is exceptional in modern genes, yet might be frequent for mitochondrial tRNAs. This adds antisense tRNA templating to other mitochondrial tRNA functions: sense tRNA templating, formation and regulation of secondary (light strand DNA) replication origins. Antitermination probably affects mitochondrial degenerative diseases and ageing: pathogenic mutations are twice as frequent in tRNAs with antisense Ter anticodons than in other tRNAs, and species lacking mitochondrial antisense Ter tRNAs have longer mean maximal lifespans than those possessing antisense Ter tRNAs.     

Complete DNA Sequence of the Mitochondrial Genome of the Black Chiton, Katharina Tunicata

The DNA sequence of the 15,532-base pair (bp) mitochondrial DNA (mtDNA) of the chiton Katharina tunicata has been determined. The 37 genes typical of metazoan mtDNA are present: 13 for protein subunits involved in oxidative phosphorylation, 2 for rRNAs and 22 for tRNAs. The gene arrangement resembles those of arthropods much more than that of another mollusc, the bivalve Mytilus edulis. Most genes abut directly or overlap, and abbreviated stop codons are inferred for four genes. Four junctions between adjacent pairs of protein genes lack intervening tRNA genes; however, at each of these junctions there is a sequence immediately adjacent to the start codon of the downstream gene that is capable of forming a stem-and-loop structure. Analysis of the tRNA gene sequences suggests that the D arm is unpaired in tRNA(ser(AGN)), which is typical of metazoan mtDNAs, and also in tRNA(ser(UCN)), a condition found previously only in nematode mtDNAs. There are two additional sequences in Katharina mtDNA that can be folded into structures resembling tRNAs; whether these are functional genes is unknown. All possible codons except the stop codons TAA and TAG are used in the protein-encoding genes, and Katharina mtDNA appears to use the same variation of the mitochondrial genetic code that is used in Drosophila and Mytilus. Translation initiates at the codons ATG, ATA and GTG. A + T richness appears to have affected codon usage patterns and, perhaps, the amino acid composition of the encoded proteins. A 142-bp non-coding region between tRNA(glu) and CO3 contains a 72-bp tract of alternating A and T.     

Codons AGA and AGG are read as glycine in ascidian mitochondria

Summary A 1.2-kb DNA fragment of the cytochrome oxidase subunit I (CO I) gene of mitochondria isolated from an ascidian,Halocynthia roretzi, was amplified by polymerase chain reaction (PCR) and sequenced. Codons AGA and AGG appeared in its reading frame, indicating that these are sense codons in this organelle. Sequence comparisons with the corresponding regions of other animal mitochondrial CO I genes suggest that codons AGA and AGG correspond to glycine in the ascidian mitochondrial genome, but not to serine as in most invertebrate genomes, nor to stops as in vertebrate genomes. The other codons are identical to those of vertebrate mitochondria.     

In search of the nature of specific nucleic acid-protein interactions

The theory of "codon-amino acid coevolution" was first proposed by Woese in 1967. It suggests that there is a stereochemical matching - that is, affinity - between amino acids and certain of the base triplet sequences that code for those amino acids. We have constructed a Common Periodic Table of Codons and Amino Acids, where the Nucleic Acid Table showed perfect axial symmetry for codons and the corresponding Amino Acid Table also displayed periodicity regarding the biochemical properties (charge and hydrophobicity) of the 20 amino acids and the position of the stop signals. The Table indicates that the middle (2nd) amino acid in the codon has a prominent role in determining some of the structural features of the amino acids. The possibility that physical contact between codons and amino acids might exist was tested on restriction enzymes. Many recognition site-like sequences were found in the coding sequences of these enzymes and as many as 73 examples of codon-amino acid co-location were observed in the 7 known 3D structures (December 2003) of endonuclease-nucleic acid complexes. These results indicate that the smallest possible units of specific nucleic acid-protein interaction are indeed the stereochemically compatible codons and amino acids.     

RNA editing makes mistakes in plant mitochondria: editing loses sense in transcripts of a rps19 pseudogene and in creating stop codons in coxI and rps3 mRNAs of Oenothera.

An intact gene for the ribosomal protein S19 (rps19) is absent from Oenothera mitochondria. The conserved rps19 reading frame found in the mitochondrial genome is interrupted by a termination codon. This rps19 pseudogene is cotranscribed with the downstream rps3 gene and is edited on both sides of the translational stop. Editing, however, changes the amino acid sequence at positions that were well conserved before editing. Other strange editings create translational stops in open reading frames coding for functional proteins. In coxI and rps3 mRNAs CGA codons are edited to UGA stop codons only five and three codons, respectively, downstream to the initiation codon. These aberrant editings in essential open reading frames and in the rps19 pseudogene appear to have been shifted to these positions from other editing sites. These observations suggest a requirement for a continuous evolutionary constraint on the editing specificities in plant mitochondria.     

简并性还是多态性?——遗传密码子设定的进化特点的剖析(英文)

遗传密码子的设定表现出令人困惑的多态性特点 :不同氨基酸拥有的密码子的数目 ,除 5个外 ,从 1个到 6个都有 .这种特点显示出密码子无论在翻译行为还是进化轨迹上 ,都存在诸多的异质性 .因此 ,简并性一词的收敛含义 ,并不能表征这种多态性的进化内涵 .没有同义密码子的AUG(Met)和UGG (Trp)并无简并现象 .其余的密码子则可分为两大类 :一类是 ,4个同义密码子为 1组 ,具有相同的第 1、2位碱基 ,并遵循“3中读 2”的读出规则 .同组的 4个同义密码子 ,不过是来自同一个双字母原始密码子 (XYN)的孑遗物 ,从这个意义上讲 ,也不宜视为简并现象 ;另一类则主要是 ,2个同义密码子为一组 ,并遵循“3中读 3”读出规则 .它们是由编码 2个氨基酸的双义原始密码子 ,第 3位的未定碱基N进一步设定形成 .至于有 6个同义密码子的 ,特别令人困感不解的组别 ,实际上是 4 + 2个 ,这启示它们可能源于上述两大类 .遗传密码子多态性的起源 ,可能始于最初阶段 ,氨基酸同某类寡核苷酸的起始二联体的相互作用 ,而完成于所有的双义原始密码子的第 3位碱基的分化 .这种进化轨迹被传统的简并性一词所模糊 ,并导致鉴定各有关理论可信性的坚实依据和令不同观点取得共识的基础被掩盖起来 .这可能就是在遗传密码子起源领域里 ,长期存在着众     

Codon usage in Entamoeba histolytica.

The codon usage of 10 E. histolytica genes comprising 4455 codons was analysed. The codon usage revealed an extremely biased use of synonymous codons with a preference for NNU (44%) and NNA (41.4%) codons. Codons CGG (arg), AGG (arg) and CCG (pro) were absent in the E. histolytica genes examined. The codon usage of E. histolytica resembled that of Plasmodium falciparum.     

Phylogenetic position of turtles among amniotes: evidence from mitochondrial and nuclear genes

Maximum likelihood analysis, accounting for site-heterogeneity in evolutionary rate with the Gamma-distribution model, was carried out with amino acid sequences of 12 mitochondrial proteins and nucleotide sequences of mitochondrial 12S and 16S rRNAs from three turtles, one squamate, one crocodile, and eight birds. The analysis strongly suggests that turtles are closely related to archosaurs (birds+crocodilians), and it supports both Tree-2: (((birds, crocodilians), turtles), squamates) and Tree-3: ((birds, (crocodilians, turtles)), squamates). A more traditional Tree-1: (((birds, crocodilians), squamates), turtles) and a tree in which turtles are basal to other amniotes were rejected with high statistical significance. Tree-3 has recently been proposed by Hedges and Poling [Science 283 (1999) 998-1001] based mainly on nuclear genes. Therefore, we re-analyzed their data using the maximum likelihood method, and evaluated the total evidence of the analyses of mitochondrial and nuclear data sets. Tree-1 was again rejected strongly. The most likely hypothesis was Tree-3, though Tree-2 remained a plausible candidate.     

Molecular characterization of of alpha-gliadin genes from wild emmer wheat (Triticum dicoccoides).

According to the two distal and conserved regions of known alpha-gliadin genes, gene-specific primers for alpha-gliadin were designed, and the coding regions of four gliadin genes (i.e. GliTd-1, GliTd-2, GliTd-3 and GliTd-4) with the length of about 800 bp were isolated from the genomic DNA of wild emmer wheat (Triticum dicoccoides). No introns were observed. Sequence comparison indicated that these genes should be classified as alpha-gliadins. GliTd-3 (GenBank accession No.DQ140351) and GliTd-4 (DQ140352) were potentially functional, whereas GliTd-1 (DQ140349) and GliTd-2 (DQ140350) were both pseudogenes by the definition of in-frame stop codons and frameshifts. Six conserved cysteine residues were observed. Sequence analysis suggested that the motif units of repetitive domain for the four newly detected genes were different from the known genes, and the QQQP sequence before the position 60 was more toxic to coeliac patients. Codons for proline were strongly biased. Codons (CAG and CAA) for glutamine were clustered into the specific regions, and the high percentage of pseudogenes resulted from the mutation of CAG --> TAG.     

The Usage of Codons Which are Similar to Stop Codons in the Genomes of Xylella fastidiosa and Xanthomonas citri

During the evolution of living organisms, a natural selection event occurs toward the optimization of their genomes regarding the usage of codons. During this process which is known as codon bias, a set of preferred codons is naturally defined in the genome of a given organism, since there are 61 possible codons (plus 3 stop codons) to 20 amino acids. Such event leads to optimization of metabolic cellular processes such as translational efficiency, RNA stability and energy saving. Although we know why, we do not know how exactly a set of preferred codons for each amino acid is defined for a given genome considering that the usage frequency of each synonymous codons is peculiar to each organism. In order to help answering this question, we analyzed the usage frequency of codons which are similar to stop codons, since a minor mutation on these codons may lead to a stop codon into the open reading frame compromising the protein expression as a result. We found a reduced use of those codons in Xanthomomas axonopodis pv. citri which presents an optimized genome regarding codon usage. On the other hand, such codons are more often used in Xylella fastidiosa, which does not seem to have established codon preferences as previously shown. Our results support that a set of preferred codons is not randomly selected and propose new ideas to the field warranting further experiments in this regard.     

Convergence and constraint in eukaryotic release factor 1 (eRF1) domain 1: the evolution of stop codon specificity

Class 1 release factor in eukaryotes (eRF1) recognizes stop codons and promotes peptide release from the ribosome. The ‘molecular mimicry’ hypothesis suggests that domain 1 of eRF1 is analogous to the tRNA anticodon stem–loop. Recent studies strongly support this hypothesis and several models for specific interactions between stop codons and residues in domain 1 have been proposed. In this study we have sequenced and identified novel eRF1 sequences across a wide diversity of eukaryotes and re-evaluated the codon-binding site by bioinformatic analyses of a large eRF1 dataset. Analyses of the eRF1 structure combined with estimates of evolutionary rates at amino acid sites allow us to define the residues that are under structural (i.e. those involved in intramolecular interactions) versus non-structural selective constraints. Furthermore, we have re-assessed convergent substitutions in the ciliate variant code eRF1s using maximum likelihood-based phylogenetic approaches. Our results favor the model proposed by Bertram et al. that stop codons bind to three ‘cavities’ on the protein surface, although we suggest that the stop codon may bind in the opposite orientation to the original model. We assess the feasibility of this alternative binding orientation with a triplet stop codon and the eRF1 domain 1 structures using molecular modeling techniques.     

Nonstandard genetic codes and translation termination

Genetic code is not universal. Various nonstandard versions of the code are known for some mitochondrial, prokaryotic, and eukaryotic genomes. The most common deviation is stop codon reassignment; i.e., a stop codon is decoded as a sense codon rather than as a signal for translation termination. Class 1 release factors (RFs: prokaryotic RF1 and RF2 and eukaryotic eRF1) recognize the stop codons and induce hydrolysis of peptidyl-tRNA in the ribosome. The specificity of class 1 RFs changes in organisms with a nonstandard code. The rare amino acids selenocysteine and pyrrolysine utilize essentially different decoding strategies. The review considers several hypotheses of the origin of nonstandard genetic codes. A new hypothesis is advanced, assuming a change in the specificity of class 1 RFs as a starting point for stop codon reassignment.     

Hydropathic anti-complementarity of amino acids based on the genetic code

An interesting pattern in the genetic code has been discovered. Codons for hydrophilic and hydrophobic amino acids on one strand of DNA are complemented by codons for hydrophobic and hydrophilic amino acids on the other DNA strand, respectively. The average tendency of codons for "uncharged" (slightly hydrophilic) amino acids is to be complemented by codons for "uncharged" amino acids.     

Interaction of silent and replacement changes in eukaryotic coding sequences

We examined the codon usages in well-conserved and less-well-conserved regions of vertebrate protein genes and found them to be similar. Despite this similarity, there is a statistically significant decrease in codon bias in the less-well-conserved regions. Our analysis suggests that although those codon changes initially fixed under amino acid replacements tend to follow the overall codon usage pattern, they also reduce the bias in codon usage. This decrease in codon bias leads one to predict that the rate of change of synonymous codons should be greater in those regions that are less well conserved at the amino acid level than in the better-conserved regions. Our analysis supports this prediction. Furthermore, we demonstrate a significantly elevated rate of change of synonymous codons among the adjacent codons 5' to amino acid replacement positions. This provides further support for the idea that there are contextual constraints on the choice of synonymous codons in eukaryotes.