Untranslated T7 Phage mRNA is stabilized in <Emphasis Type="Italic">suA</Emphasis> Host

IT has been shown¹ that relief from the polar effect of nonsense mutations in the tryptophan Operon by the suA allele² is associated with increased survival of mRNA operator distal to the nonsense mutation. Experiments described here confirm the effect of suA on mRNA distal to amber mutations and directly demonstrate the complete mRNA molecule only in the presence of the suA mutation.     

Increased tRNA level in yeast cells with mutant translation termination factors eRF1 and eRF3

We have earlier characterized Saccharomyces cerevisiae strains with mutations of essential SUP45 and SUP35, which code for translation termination factors eRF1 and eRF3, respectively. In this work, the sup45 and sup35 nonsense mutants were compared with respect to the levels of eight tRNAs: tRNA^Tyr, tRNA^Gln, tRNA^Trp, tRNA^Leu, tRNA^Arg (described as potential suppressor tRNAs), tRNA^Pro, tRNA^His, and tRNA^Gly. The mutants did not display a selective increase in tRNAs, capable of a noncanonical read-through at stop codons. Most of the mutations increased the level of all tRNAs under study. The mechanisms providing for the viability of the sup45 and sup35 nonsense mutants are discussed.     

De Novo Mutations in CHAMP1 Cause Intellectual Disability with Severe Speech Impairment

CHAMP1 encodes a protein with a function in kinetochore-microtubule attachment and in the regulation of chromosome segregation, both of which are known to be important for neurodevelopment. By trio whole-exome sequencing, we have identified de novo deleterious mutations in CHAMP1 in five unrelated individuals affected by intellectual disability with severe speech impairment, motor developmental delay, muscular hypotonia, and similar dysmorphic features including short philtrum and a tented upper and everted lover lip. In addition to two frameshift and one nonsense mutations, we found an identical nonsense mutation, c.1192C>T (p.Arg398^∗), in two affected individuals. All mutations, if resulting in a stable protein, are predicted to lead to the loss of the functionally important zinc-finger domains in the C terminus of the protein, which regulate CHAMP1 localization to chromosomes and the mitotic spindle, thereby providing a mechanistic understanding for their pathogenicity. We thus establish deleterious de novo mutations in CHAMP1 as a cause of intellectual disability.     

Evidence that the supE44 Mutation of Escherichia coli Is an Amber Suppressor Allele of glnX and that It Also Suppresses Ochre and Opal Nonsense Mutations

Translational readthrough of nonsense codons is seen not only in organisms possessing one or more tRNA suppressors but also in strains lacking suppressors. Amber suppressor tRNAs have been reported to suppress only amber nonsense mutations, unlike ochre suppressors, which can suppress both amber and ochre mutations, essentially due to wobble base pairing. In an Escherichia coli strain carrying the lacZU118 episome (an ochre mutation in the lacZ gene) and harboring the supE44 allele, suppression of the ochre mutation was observed after 7 days of incubation. The presence of the supE44 lesion in the relevant strains was confirmed by sequencing, and it was found to be in the duplicate copy of the glnV tRNA gene, glnX. To investigate this further, an in vivo luciferase assay developed by D. W. Schultz and M. Yarus (J. Bacteriol. 172:595-602, 1990) was employed to evaluate the efficiency of suppression of amber (UAG), ochre (UAA), and opal (UGA) mutations by supE44. We have shown here that supE44 suppresses ochre as well as opal nonsense mutations, with comparable efficiencies. The readthrough of nonsense mutations in a wild-type E. coli strain was much lower than that in a supE44 strain when measured by the luciferase assay. Increased suppression of nonsense mutations, especially ochre and opal, by supE44 was found to be growth phase dependent, as this phenomenon was only observed in stationary phase and not in logarithmic phase. These results have implications for the decoding accuracy of the translational machinery, particularly in stationary growth phase.Translation termination is mediated by one of the three stop codons (UAA, UAG, or UGA). When such stop codons arise in coding sequences due to mutations, referred to as nonsense mutations, they lead to abrupt arrest of the translation process. However, the termination efficiency of such nonsense codons is not 100%, as certain tRNAs have the ability to read these nonsense codons. Genetic code ambiguity is seen in several organisms. Stop codons have been shown to have alternate roles apart from translation termination. In organisms from all three domains of life, UGA encodes selenocysteine through a specialized mechanism. In Methanosarcinaceae, UAG encodes pyrrolysine (3). UAA and UAG are read as glutamine codons in some green algae and ciliates such as Tetrahymena and Diplomonads (24), and UAG alone encodes glutamine in Moloney murine leukemia virus (32). UGA encodes cysteine in Euplotes; tryptophan in some ciliates, Mycoplasma species, Spiroplasma citri, Bacillus, and tobacco rattle virus; and an unidentified amino acid in Pseudomicrothorax dubius and Nyctotherus ovalis (30). In certain cases the context of the stop codon in translational readthrough has been shown to play a role; for example, it has been reported that in vitro in tobacco mosaic virus, UAG and UAA are misread by tRNA^Tyr in a highly context-dependent manner (34, 9).Termination suppressors are of three types, i.e., amber, ochre, and opal suppressors, which are named based on their ability to suppress the three stop codons. Amber suppressors can suppress only amber codons, whereas ochre suppressors can suppress ochre codons (by normal base pairing) as well as amber codons (by wobbling) and opal suppressors can read opal and UGG tryptophan codon in certain cases. As described by Sambrook et al. (27), a few amber suppressors can also suppress ochre mutations by wobbling. The suppression efficiency varies among these suppressors, with amber suppressors generally showing increased efficiency over ochre and opal suppressors. supE44, an amber suppressor tRNA, is an allele of and is found in many commonly used strains of Escherichia coli K-12. Earlier studies have shown that supE44 is a weak amber suppressor and that its efficiency varies up to 35-fold depending on the reading context of the stop codon (8).Translational accuracy depends on several factors, which include charging of tRNAs with specific amino acids, mRNA decoding, and the presence of antibiotics such as streptomycin and mutations in ribosomal proteins which modulate the fidelity of the translational machinery. Among these, mRNA decoding errors have been reported to occur at a frequency ranging from about 10⁻³ to 10⁻⁴ per codon. Translational misreading errors also largely depend on the competition between cognate and near-cognate tRNA species. Poor availability of cognate tRNAs increases misreading (18).Several studies with E. coli and Saccharomyces cerevisiae have shown the readthrough of nonsense codons in suppressor-free cells. In a suppressor-free E. coli strain, it has been shown in vitro that glutamine is incorporated at the nonsense codons UAG and UAA (26). It has been reported that overexpression of wild-type tRNA^Gln in yeast suppresses amber as well as ochre mutations (25). In this study, we have confirmed the presence of an amber suppressor mutation in the glnX gene in a supE44 strain by sequence analysis. This was done essentially because we observed that supE44 could also suppress lacZ ochre mutations, albeit inefficiently. On further investigation using an in vivo luciferase reporter assay system for tRNA-mediated nonsense suppression (28), we found that the efficiency of suppression of amber lesion by supE44 is significantly higher than that reported previously in the literature. An increased ability to suppress ochre and opal nonsense mutations was observed in cells bearing supE44 compared to in the wild type. Such an effect was observed only in the stationary phase and was abolished in logarithmic phase.     

Reduced Life- and Healthspan in Mice Carrying a Mono-Allelic BubR1 MVA Mutation

Mosaic Variegated Aneuploidy (MVA) syndrome is a rare autosomal recessive disorder characterized by inaccurate chromosome segregation and high rates of near-diploid aneuploidy. Children with MVA syndrome die at an early age, are cancer prone, and have progeroid features like facial dysmorphisms, short stature, and cataracts. The majority of MVA cases are linked to mutations in BUBR1, a mitotic checkpoint gene required for proper chromosome segregation. Affected patients either have bi-allelic BUBR1 mutations, with one allele harboring a missense mutation and the other a nonsense mutation, or mono-allelic BUBR1 mutations combined with allelic variants that yield low amounts of wild-type BubR1 protein. Parents of MVA patients that carry single allele mutations have mild mitotic defects, but whether they are at risk for any of the pathologies associated with MVA syndrome is unknown. To address this, we engineered a mouse model for the nonsense mutation 2211insGTTA (referred to as GTTA) found in MVA patients with bi-allelic BUBR1 mutations. Here we report that both the median and maximum lifespans of the resulting BubR1 ^+/GTTA mice are significantly reduced. Furthermore, BubR1 ^+/GTTA mice develop several aging-related phenotypes at an accelerated rate, including cataract formation, lordokyphosis, skeletal muscle wasting, impaired exercise ability, and fat loss. BubR1 ^+/GTTA mice develop mild aneuploidies and show enhanced growth of carcinogen-induced tumors. Collectively, these data demonstrate that the BUBR1 GTTA mutation compromises longevity and healthspan, raising the interesting possibility that mono-allelic changes in BUBR1 might contribute to differences in aging rates in the general population.     

Suppression of TGA Mutations in the Bacillus subtilis spoIIR Gene by prfB Mutations

An unexpectedly high proportion of TGA nonsense mutations was obtained in a collection of chemically induced mutations in the spoIIR locus of Bacillus subtilis. Of 11 different mutations obtained, TGA mutations were found in four codons, whereas only three codons yielded missense mutations. Six suppressors of the TGA mutations were isolated, and five of the suppressing mutations were mapped to the prfB gene encoding protein release factor 2. These are the first mutations shown to map to the B. subtilis prfB locus. The sequence of the prfB gene was completed, and two revisions of the published sequence were made. The five prfB mutations also resulted in suppression of the catA86-TGA mutation to between 19 and 54% of the expression of catA86⁺, compared to the readthrough level of 6% in the prfB⁺ strain. N-terminal sequencing of suppressed catA86-TGA-specified protein demonstrated that the amino acid inserted at UGA because of the prfB1 mutations was tryptophan.     

De Novo Nonsense Mutations in KAT6A,a Lysine Acetyl-Transferase Gene,Cause a Syndrome Including Microcephaly and Global Developmental Delay

Chromatin remodeling through histone acetyltransferase (HAT) and histone deactylase (HDAC) enzymes affects fundamental cellular processes including the cell-cycle, cell differentiation, metabolism, and apoptosis. Nonsense mutations in genes that are involved in histone acetylation and deacetylation result in multiple congenital anomalies with most individuals displaying significant developmental delay, microcephaly and dysmorphism. Here, we report a syndrome caused by de novo heterozygous nonsense mutations in KAT6A (a.k.a., MOZ, MYST3) identified by clinical exome sequencing (CES) in four independent families. The same de novo nonsense mutation (c.3385C>T [p.Arg1129^∗]) was observed in three individuals, and the fourth individual had a nearby de novo nonsense mutation (c.3070C>T [p.Arg1024^∗]). Neither of these variants was present in 1,815 in-house exomes or in public databases. Common features among all four probands include primary microcephaly, global developmental delay including profound speech delay, and craniofacial dysmorphism, as well as more varied features such as feeding difficulties, cardiac defects, and ocular anomalies. We further demonstrate that KAT6A mutations result in dysregulation of H3K9 and H3K18 acetylation and altered P53 signaling. Through histone and non-histone acetylation, KAT6A affects multiple cellular processes and illustrates the complex role of acetylation in regulating development and disease.     

Mutation at the am locus of Neurospora crassa

Forty-eight new mutations at the am locus of Neurospora crassa have been characterized. Nineteen mutations were induced by UV; of these, eight were missense, two were frameshifts, two were nonsense, three were deletions and four were unidentified. Twenty-nine mutations were induced with nitrous acid; of these, twenty-one were missense, three were frameshifts, one was nonsense, two were deletions and one was genetically unstable.     

Overexpression of genes encoding tRNA<Superscript>Tyr</Superscript> and tRNA<Superscript>Gln</Superscript> increases the viability of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> strains with nonsense mutations in the <Emphasis Type="Italic">SUP45</Emphasis> gene

At present, the machinery supporting the viability of organisms possessing nonsense mutations in essential genes is not entirely understood. Nonsense mutants of Saccharomyces cerevisiae yeast containing a premature translation termination codon in the essential SUP45 gene are known. These strains are viable in the absence of mutant suppressor tRNAs; hence, the existence of alternative mechanisms providing nonsense suppression and mutant viability is conjectured. Analysis of clones obtained by transformation of a strain bearing a nonsense-mutant allele of SUP45 with a multicopy yeast genomic library revealed three genes encoding wild-type tRNA^Tyr and four genes encoding wild-type tRNA^Gln, which increased nonsense mutant viability. Moreover, overexpression of these genes leads to an increase in the amount of the full-length eRF1 protein in cells and compensates for heat sensitivity in the nonsense mutants. Probable ways of tRNA^Tyr and tRNA^Gln influence on the increase in the viability of strains with nonsense mutations in SUP45 are discussed.     

Loss of Dispensable Endonuclease Activity in Relief of Polarity by <Emphasis Type="Italic">suA</Emphasis>

NONSENSE mutations in several microorganisms exert a pleio-tropic “polar” effect, reducing the level of expression of those genes in the same Operon which lie on the operator-distal side of the mutant gene'. Although ribosomes terminate translation and are quickly discharged from the messenger when they encounter a nonsense codon², they can efficiently re-attach to the messenger at the “start” codons of downstream genes, both in RNA phage^2–5 and in E. coli⁶. Thus, polarity must be caused by unavailability of the messenger template beyond the nonsense codon. In RNA phage, this unavailability has been attributed to a conformational masking of the messenger^2–5     

Genetic analysis of the histidine operon control region of Salmonella typhimurium

Mutants of the histidine operon control region (hisO) include two classes: (1) those completely unable to express the operon (His auxotrophs), and (2) prototrophs that are unable to achieve fully induced levels of operon expression (still His⁺ but sensitive to the drug amino-triazole). Using new, as well as previously existing hisO mutants, we constructed a fine-structure deletion map of hisO. Mutations that presumably alter the his promoter map at one end of hisO; mutations that alter the his attenuator map at the other end of hisO. Between the promoter and the attenuator lie a number of mutations that affect either the translation of the his leader peptide gene, or the formation and stability of his leader messenger RNA structures. All of the point mutations mapping in this central region revert to His⁺ at a very high frequency (10^?5 to 10^?6); this frequency is increased by both base substitution and frameshift-inducing mutagens. Many of the His^? mutants are suppressed by informational suppressors; all three types of nonsense mutations have been identified, demonstrating that translation of a region of hisO between the promoter and attenuator is essential for his operon expression. All of the hisO mutations tested are cis-dominant.     

mRNA surveillance mitigates genetic dominance in Caenorhabditis elegans

Nonsense mutant mRNAs are unstable in all eucaryotes tested, a phenomenon termed nonsense-mediated mRNA decay (NMD) or mRNA surveillance. Functions of the seven smg genes are required for mRNA surveillance in Caenorhabditis elegans. In Smg(+) genetic backgrounds, nonsense-mutant mRNAs are unstable, while in Smg(?) backgrounds such mRNAs are stable. Previous work has demonstrated that the elevated level of nonsense-mutant mRNAs in Smg(?) animals can influence the phenotypic effects of heterozygous nonsense mutations. Certain nonsense alleles of a muscle myosin heavy chain gene are recessive in Smg(+) backgrounds but strongly dominant in Smg(?) backgrounds. Such alleles probably express disruptive myosin polypeptide fragments whose abundance is elevated in smg mutants due to elevation of mRNA levels. We report here that mutations in a variety of C. elegans genes are strongly dominant in Smg(?), but recessive or only weakly dominant in Smg(+) backgrounds. We isolated 32 dominant visible mutations in a Smg(?) genetic background and tested whether their dominance requires a functional NMD system. The dominance of 21 of these mutations is influenced by NMD. We demonstrate, furthermore, that in the case of myosin, the dominant-negative effects of nonsense alleles are likely to be due to expression of N-terminal nonsense-fragment polypeptides, not to mistranslation of the nonsense codons. mRNA surveillance, therefore, may mitigate potentially deleterious effects of many heterozygous germline and somatic nonsense or frameshift mutations. We also provide evidence that smg-6, a gene previously identified as being required for NMD, performs essential function(s) in addition to its role in NMD.     

Viable nonsense mutants for the <Emphasis Type="Italic">SUP45</Emphasis> gene in the yeast <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> are lethal at increased temperature

Nonlethal nonsense mutations obtained earlier in the essential gene SUP45 encoding the translation termination factor eRF1 in the yeast Saccharomyces cerevisiae were further characterized. Strains carrying these mutations retain the viability, since the full-length eRF1 protein is present in these strains, although in decreased amounts as compared to wild-type cells, together with a trucated eRF1. All nonsense mutations are likely to be located in a weak termination context, because a change in the stop codon UGAA (in the case of mutation sup45-107) to UAGA (sup45-107.2) led to the alteration of the local context from a weak to strong and to the lethality of the strain carrying sup45-107.2. All nonsense mutations studied are characterized by thermosensitivity expressed as cell mortality after cultivation at 37°C. When grown under nonpermissive conditions (37°C), cells of nonsense mutants sup45-104, sup45-105, and sup45-107 display a decrease in the amount of the truncated eRF1 protein without reduction in the amount of the full-length eRF1 protein. The results of this study suggest that the N-terminal eRF1 fragment is indispensable for cell viability of nonsense mutants due to the involvement in termination of translation.     

Mutations in RAB28, Encoding a Farnesylated Small GTPase,Are Associated with Autosomal-Recessive Cone-Rod Dystrophy

The majority of the genetic causes of autosomal-recessive (ar) cone-rod dystrophy (CRD) are currently unknown. A combined approach of homozygosity mapping and exome sequencing revealed a homozygous nonsense mutation (c.565C>T [p.Glu189^∗]) in RAB28 in a German family with three siblings with arCRD. Another homozygous nonsense mutation (c.409C>T [p.Arg137^∗]) was identified in a family of Moroccan Jewish descent with two siblings affected by arCRD. All five affected individuals presented with hyperpigmentation in the macula, progressive loss of the visual acuity, atrophy of the retinal pigment epithelium, and severely reduced cone and rod responses on the electroretinogram. RAB28 encodes a member of the Rab subfamily of the RAS-related small GTPases. Alternative RNA splicing yields three predicted protein isoforms with alternative C-termini, which are all truncated by the nonsense mutations identified in the arCRD families in this report. Opposed to other Rab GTPases that are generally geranylgeranylated, RAB28 is predicted to be farnesylated. Staining of rat retina showed localization of RAB28 to the basal body and the ciliary rootlet of the photoreceptors. Analogous to the function of other RAB family members, RAB28 might be involved in ciliary transport in photoreceptor cells. This study reveals a crucial role for RAB28 in photoreceptor function and suggests that mutations in other Rab proteins may also be associated with retinal dystrophies.     

Identification of novel alleles induced by EMS-mutagenesis in key genes of kernel hardness and starch biosynthesis in wheat by TILLING

To identify novel allelic variations in key genes of wheat quality, the present study used the targeting induced local lesions in genomes platform to detect point mutations in target genes. The wheat variety Longfumai 17 was treated by the mutagen ethyl methanesulfonate to produce a bulk M₂ generation, and the population included 1122 plants. A total length of 3906.80 kb nucleotides was analyzed, and the average mutation density was 1/244.17 kb. The identified mutations included G>A substitutions (43.75%), C>T substitutions (31.25%), A insertions (12.50%), T insertions (6.25%), and deletions (6.25%). These point mutations led to changes in amino acids and thus the encoded protein sequences, ultimately producing 18.75% of missense mutations, 12.50% of frame shift mutations, 6.25% of nonsense mutations, 25.00% of silent mutations and 37.50% of non-coding region mutations. In the kernel hardness gene Pinb and 3 starch synthesis genes waxy, Agp2 and SSIIa-A, we detected 16 different point mutations in 25 mutant lines. The Pinb gene harbored two missense mutations and a nonsense mutation; the C>T missense mutation resulted in a novel allele, this novel allele and the nonsense mutation alerted protein 3D structure; the waxy gene presented missense and frame shift mutations; the Agp2 gene carried a missense mutation; the SSIIa-A incurred a missense mutation and a frame shift mutation that resulted in premature protein termination. All the frame shift mutations, nonsense mutations and the Pinb novel allele resulted in allelic variation of their corresponding genes, which in turn affected their gene functions. The identified mutant lines can be used as intermediate materials in wheat quality improvement schemes.     

Fine Structure Analysis of the ade3 Locus in SACCHAROMYCES CEREVISIAE

Twenty-six spontaneous mutants at the ade3 locus of Saccharomyces cerevisiae have been mapped and characterized with respect to revertibility, osmotic remediability and temperature sensitivity. Twelve of the twenty-six are temperature sensitive, 25 of 26 are osmotic remedial and 21 of 26 revert. Two of the mutants map as deletions. At least five of the 26 are nonsense mutations but are also, unexpectedly, osmotic remedial. Three nonsense mutations are also temperature sensitive, again an unexpected result. The two multisite mutations are both temperature sensitive and osmotic remedial. For mutants at this locus osmotic remediability and temperature sensitivity cannot be considered diagnostic criteria for missense mutations.     

Mapping of the polA Locus of ESCHERICHIA COLI K12: Genetic Fine Structure of the Cistron

The close linkage of the glnA gene with polA was exploited to construct a fine structure map of polA by means of generalized transduction with phage P1. Nine different polA^- alleles were mapped by recombinational crosses. The results indicate a gene order consistent with previous observations (Kelley and Grindley 1976a; Murray and Kelley 1979). Three mutations, polA5, polA6 and polA12 map within the "carboxy-terminal" or "large-fragment" portion of the gene in unambiguous order. Four alleles, known to affect the "aminoterminal" portion of the gene, polA107, polA214, polA480ex and polA4113, appear to be closely linked with certain ambiguities in their exact order. All four of these mutations are known to alter the 5''→3'' exonuclease activity of DNA polymerase I and three of them result in the conditional lethal polA^- phenotype. The polA1 nonsense mutation maps between these two groups in a position consistent with its known effect, production of an amber fragment that includes the 5''→3'' exonuclease. The final allele, resA1, is another nonsense mutation that maps at the extreme "amino-terminus" of the cistron.——A number of control experiments were conducted to determine the effects of polA^- mutations on the P1-mediated recombinational event. These experiments indicated that abortive transduction occurs quite frequently, but the formation of abortive transductants and segregation of unselected transduced markers among daughter progeny is like that observed by other investigators. There was no evidence that any individual polA^- allele behaved in an exceptional fashion during recombination.     

Prion-Dependent Lethality of sup45 Mutants in Saccharomyces cerevisiae

In yeast Saccharomyces cerevisiae translation termination factors eRF1 (Sup45) and eRF3 (Sup35) are encoded by the essential genes SUP45 and SUP35 respectively. Heritable aggregation of Sup35 results in formation of the yeast prion [PSI⁺]. It is known that combination of [PSI⁺] with some mutant alleles of the SUP35 or SUP45 genes in one and the same haploid yeast cell causes synthetic lethality. In this study, we perform detailed analysis of synthetic lethality between various sup45 nonsense and missense mutations on one hand, and different variants of [PSI⁺] on the other hand. Synthetic lethality with sup45 mutations was detected for [PSI⁺] variants of different stringencies. Moreover, we demonstrate for the first time that in some combinations, synthetic lethality is dominant and occurs at the postzygotic stage after only a few cell divisions. The tRNA suppressor SUQ5 counteracts the prion-dependent lethality of the nonsense alleles but not of the missense alleles of SUP45, indicating that the lethal effect is due to the depletion of Sup45. Synthetic lethality is also suppressed in the presence of the C-proximal fragment of Sup35 (Sup35C) that lacks the prion domain and cannot be included into the prion aggregates. Remarkably, the production of Sup35C in a sup45 mutant strain is also accompanied by an increase in the Sup45 levels, suggesting that translationally active Sup35 up-regulates Sup45 or protects it from degradation.Key Words: Sup45, Sup35, eRF1, eRF3, amyloid, [PSI⁺], translation termination, Saccharomyces cerevisiae     

Site Specificity and Variability in the Mutator and Antimutator Effects of Phage T4 Gene 43 Mutants

Spontaneous, 2-aminopurine- and 5-bromouracil-induced mutations at six rII nonsense codons were studied in phage T4 strains possessing wild-type and mutant gene 43 alleles. The mutation pathways studied included interconversions and reversions of nonsense codons. The tsCB87 allele, which specifies an antimutator DNA polymerase, reduced base-analogue-induced mutation frequencies along all pathways. However, GC base pairs were less affected than AT base pairs. The frequency of spontaneous UAA→UAG conversions was also reduced by tsCB87, but that of spontaneous UAA→UGA conversions was often increased. Mutation in the presence of the mutator allele tsL56 was increased along all pathways, with no preference for either AT or GC base pairs. Mutation frequencies in the presence of the two mutant DNA polymerases were highly variable. A strong correlation was found between 2-aminopurine-induced mutation frequencies in ts⁺ and tsCB87 phage along the reversion and UAA→UAG (but not UAA→UGA) pathways.