Spontaneous mutators of salmonella typhimurium LT2 generated by insertion of transposable elements

DNA Mismatch Repair

DNA mismatch repair (MMR) corrects replication errors in newly synthesized DNA. It also has an antirecombination action on heteroduplexes that contain similar but not identical sequences. This review focuses on the genetics and development of MMR and not on the latest biochemical mechanisms. The main focus is on MMR in Escherichia coli, but examples from Streptococcuspneumoniae and Bacillussubtilis have also been included. In most organisms, only MutS (detects mismatches) and MutL (an endonuclease) and a single exonucleaseare present. How this system discriminates between newlysynthesized and parental DNA strands is not clear. In E. coli and its relatives, however, Dam methylation is an integral part of MMR and is the basis for strand discrimination. A dedicated site-specific endonuclease, MutH, is present, andMutL has no endonuclease activity; four exonucleases can participate in MMR. Although it might seem that the accumulated wealth of genetic and biochemical data has given us a detailed picture of the mechanism of MMR in E. coli, the existence of three competing models to explain the initiation phase indicates the complexity of the system. The mechanism of the antirecombination action of MMR is largely unknown, but only MutS and MutL appear to be necessary. A primary site of action appears to be on RecA, although subsequent steps of the recombination process can also be inhibited. In this review, the genetics of Very Short Patch (VSP) repair of T/G mismatches arising from deamination of 5-methylcytosineresidues is also discussed.

DNA base repair--recognition and initiation of catalysis

Endogenous DNA damage induced by hydrolysis, reactive oxygen species and alkylation modifies DNA bases and the structure of the DNA duplex. Numerous mechanisms have evolved to protect cells from these deleterious effects. Base excision repair is the major pathway for removing base lesions. However, several mechanisms of direct base damage reversal, involving enzymes such as transferases, photolyases and oxidative demethylases, are specialized to remove certain types of photoproducts and alkylated bases. Mismatch excision repair corrects for misincorporation of bases by replicative DNA polymerases. The determination of the 3D structure and visualization of DNA repair proteins and their interactions with damaged DNA have considerably aided our understanding of the molecular basis for DNA base lesion repair and genome stability. Here, we review the structural biochemistry of base lesion recognition and initiation of one-step direct reversal (DR) of damage as well as the multistep pathways of base excision repair (BER), nucleotide incision repair (NIR) and mismatch repair (MMR).

Virulence and mutation rates of Salmonella typhimurium strains with increased mutagenic strength in a mouse model

Two strains of Salmonella typhimurium presenting increased mutation rates, either spontaneous or mediated by DNA damage, have been constructed. One of the strains carries a null mutS mutation, while the other harbors plasmid pRW30, which contains the Escherichia coli umuDC operon. The virulence of these strains has been determined by inoculating BALB/c or Swiss mice. The 50% lethal dose of both strains is identical to that obtained for the wild-type. Likewise, the two strains and the wild-type contribute equally to animal death in mixed infections. The frequency of Nal(R) mutants recovered from animals inoculated with either wild-type or MutS(-) cells was not affected by the presence of pRW30. These results indicate that the DNA damage which S. typhimurium cells can suffer during the infectious process by host cell metabolites does not cause induction of the SOS response at levels able to trigger the error-prone DNA repair pathway.

Detection of mutator subpopulations in Salmonella typhimurium LT2 by reversion of his alleles

Defects in the methyl-directed mismatch repair lead to both the hypermutability phenotype and removal of a barrier to genetic exchange between species. Mutator bacteria carrying such defects occur frequently among bacterial pathogens, suggesting that subpopulations of mutators are contained within pathogen clones and give rise to the genetic variants that are acted upon by selective forces to allow survival or successful infection. We report here on the detection of the mutator subpopulation in Salmonella typhimurium and determination of its frequency in laboratory cultures. The analysis involved screening for mutators among revertants of S. typhimurium histidine auxotrophs selected for the His+ phenotype, since the frequency of mutators is expected to be increased in the selected mutant population they helped to spawn. The increases in spontaneous reversion of histidine mutations were first measured in isogenic strains carrying mismatch repair-defective mutH, mutL, mutS, or uvrD alleles, relative to their mismatch repair-proficient counterparts. Screening for the mutator phenotype in nearly 12,000 revertants of repair-proficient strains carrying his mutations highly stimulated for reversion in mutator backgrounds, the base substitution in hisG428 and frameshift in hisC3076, yielded five mutator strains (0.04%). The his+ reversion mutations contained within the newly-arisen mutator strains were characteristic of the predominant nucleotide changes expected in such mutators, as assessed by comparison with the spectra for reversion events in wild-type and mismatch correction-defective backgrounds. The results show that subpopulations of mutators, residing in normal populations at a finite frequency, can be culled from the culture by strong selection for a required phenotype. We calculate that the frequency of mutators in the unselected population of S. typhimurium is 1-4x10-6, an incidence 10-fold lower than that expected based on studies of laboratory cultures of Escherichia coli.

RNase III deficient Salmonella typhimurium LT2 contains intervening sequences (IVSs) in its 23S rRNA

Salmonella typhimurium LT2 contains intervening sequences (IVSs) of 90-110 nt within all its 23S rRNA that are cleaved out by RNase III, resulting in rRNA fragmentation. In order to determine the functionality of 23S rRNA that contains unexcised IVSs, we constructed an S. typhimurium RNase III (rnc) deficient strain by transducing a mini-Tn10 (rnc-14::Tn10) from Escherichia coli K-12. The resulting strain of S. typhimurium was viable, contained IVSs within all of its 23S rRNA, and showed a growth reduction similar to that observed for the RNase III deficient strain of E. coli. These results indicate that ribosomes containing 23S rRNA in which IVSs are not excised are functional in translation, and make it unlikely that RNase III excision of IVSs from strain LT2 23S rRNA is dictated by a selective pressure to uphold the functional integrity of ribosomes.

DNA adenine methylase mutants of Salmonella typhimurium and a novel dam-regulated locus

Mutants of Salmonella typhimurium lacking DNA adenine methylase were isolated; they include insertion and deletion alleles. The dam locus maps at 75 min between cysG and aroB, similar to the Escherichia coli dam gene. Dam- mutants of S. typhimurium resemble those of E coli in the following phenotypes: (1) increased spontaneous mutations, (2) moderate SOS induction, (3) enhancement of duplication segregation, (4) inviability of dam recA and dam recB mutants, and (5) suppression of the inviability of the dam recA and dam recB combinations by mutations that eliminate mismatch repair. However, differences between S. typhimurium and E. coli dam mutants are also found: (1) S. typhimurium dam mutants do not show increased UV sensitivity, suggesting that methyl-directed mismatch repair does not participate in the repair of UV-induced DNA damage in Salmonella. (2) S. typhimurium dam recJ mutants are viable, suggesting that the Salmonella RecJ function does not participate in the repair of DNA strand breaks formed in the absence of Dam methylation. We also describe a genetic screen for detecting novel genes regulated by Dam methylation and a locus repressed by Dam methylation in the S. typhimurium virulence (or "cryptic") plasmid.

Isolation and characterization of a mutator strain of Streptomyces ambofaciens ATCC23877 exhibiting an increased level of genetic instability

In Streptomyces ambofaciens ATCC23877, 0.7% of pigment-defective mutants (Pig−) can be observed in the progeny of wild-type colonies. A mutator (Mut−) strain was isolated from the offspring of the wild-type strain. The Mut− strain produced colonies that sported nonpigmented papillae. Furthermore, the frequency of Pig− colonies obtained in the progeny of this strain was fivefold higher than in the wild-type strain. This strain showed the same level of sensitivity to ultraviolet light and mitomycin C as the wild-type strain. This Mut− phenotype was found to be reversible at high frequency (3 × 10−3). Genomic analysis using pulsed-field gel electrophoresis (PFGE) showed that the Pig− mutants arisen from the Mut− strain were less frequently rearranged (32% were deleted) compared with the mutants arising from the wild type (59% were deleted). Moreover, the Pig− papillae mutants possessed no visible rearrangement as revealed by PFGE analyses.Key words: Streptomyces, genetic instability, mutator strain, papillae.

Characterization of the mouse Rep-3 gene: sequence similarities to bacterial and yeast mismatch-repair proteins

The mouse Rep-3 gene is transcribed divergently from the same promoter region as the dihydrofolate reductase-encoding gene and has a deduced amino-acid sequence that shares identity with the bacterial protein, MutS, which is involved in DNA mismatch repair. We have cloned Rep-3, mapped it and sequenced all of the known exons and their intron junction sequences. We find that the open reading frame is considerably larger than initially reported and that the most abundant form of Rep-3 mRNA encodes a protein of 123 kDa. The gene spans at least 134 kb and consists of 26 exons, including several alternatively spliced exons. All of the exon/intron junctions match the expected consensus sequences with the exception of the splice junctions for intron 6, which has AT and AC dinucleotides instead of the usual GT and AG bordering the exon sequences. The junction sequences for this intron share consensus sequences with three intron sequences from other genes, thereby helping to establish an alternative consensus sequence.

The pleiotropic effects of his overexpression in Salmonella typhimurium do not involve AICAR-induced mutagenesis

Inhibition of cell division associated with overexpression of hisH and hisF in Salmonella typhimurium is strongly reminiscent of a cellular response to DNA damage. On these grounds, we investigated the involvement of a metabolite which appeared to represent a possible candidate for an endogenous mutagen: the base analog 5-amino-4-carboxamide imidazole riboside 5'-phosphate (AICAR), a by-product of HisH and HisF activity. However, we showed that AICAR is not an endogenous mutagen in S. typhimurium. Other types of DNA damage induced by his overexpression seem also unlikely, since similar mutation rates are found in hisO+ and hisOc strains. We also show that AICAR production is not involved in the pleiotropic effects of his overexpression, since these are still observed in strains devoid of AICAR. Thus inhibition of cell division resulting from HisH and HisF overexpression must operate through a mechanism unrelated to the role of these proteins in histidine biosynthesis.

The yeast gene MSH3 defines a new class of eukaryotic MutS homologues

We have identified a gene in Saccharomyces cerevisiae, MSH3, whose predicted protein product shares extensive sequence similarity with bacterial proteins involved in DNA mismatch repair as well as with the predicted protein product of the Rep-3 gene of mouse. MSH3 was obtained by performing a polymerase chain reaction on yeast genomic DNA using degenerate oligonucleotide primers designed to anneal with the most conserved regions of a gene that would be homologous to Rep-3 and Salmonella typhimurium mutS. MSH3 seems to play some role in DNA mismatch repair, inasmuch as its inactivation results in an increase in reversion rates of two different mutations and also causes an increase in postmeiotic segregation. However, the effect of MSH3 disruption on reversion rates and postmeiotic segregation appears to be much less than that of previously characterized yeast DNA mismatch repair genes. Alignment of the MSH3 sequence with all of the known MutS homologues suggests that its primary function may be different from the role of MutS in repair of replication errors. MSH3 appears to be more closely related to the mouse Rep-3 gene and other similar eukaryotic mutS homologues than to the yeast gene MSH2 and other mutS homologues that are involved in replication repair. We suggest that the primary function of MSH3 may be more closely related to one of the other known functions of mutS, such as its role in preventing recombination between non-identical sequences.

Nucleotide sequence of the Salmonella typhimurium mutB gene, the homolog of Escherichia coli mutY

The mutB gene of Salmonella typhimurium is involved in a methylation-independent repair pathway specific for A/G or A/C mismatches and is the homolog of the Escherichia coli mutY gene. The mutB gene of S. typhimurium was cloned and sequenced. The isolated mutB clone reduced the mutation rate of the mutB mutant to wild-type levels and also restored A/G mismatch-specific nicking activity, which is defective in mutB extracts. The amino acid sequence encoded by the mutB gene is 91% homologous to that encoded by the E. coli mutY gene.

A genetic analysis of Xis and FIS interactions with their binding sites in bacteriophage lambda

The bacteriophage P22-based challenge-phage system was used to study the binding of Xis and FIS to their sites in attP of bacteriophage lambda. Challenge phages were constructed that contained the X1, X2, and F sites within the P22 Pant promoter, which is required for expression of antirepressor. If Xis and FIS bind to these sites in vivo, they repress transcription from Pant, allowing lysogenization to occur. Challenge phages carrying the XIX2F region in either orientation exhibited lysogenization dependent on both Xis and FIS. Neither Xis nor FIS was capable of functioning by itself as an efficient repressor in this system. This was the first time challenge phages have been constructed that require two different proteins bound simultaneously to act as a repressor. Mutations in the X1, X2, and F sites that inhibit Xis and FIS from binding were isolated by selecting mutant phages that still expressed antirepressor synthesis in the presence of Xis and FIS. DNA sequence analysis of the mutants revealed 38 unique mutations, including single-base-pair substitutions, multiple-base-pair changes, deletions, and insertions throughout the entire X1, X2, and F regions. Some of the mutations verified the importance of certain bases within the proposed consensus sequences for Xis and FIS, while others provided evidence that the DNA sequence outside of the proposed binding sites may affect the binding of the individual proteins or the cooperativity between them.

[Genetic engineering]

This paper deals with the progress made in genetic engineering techniques, capable of altering the genetic potential of an organism, either by the introduction or the suppression of new structural genes. Some of the general applications are described as are also, more particularly, their uses in the field of medicine. A critical analysis of the benefits and risks involved is also undertaken.

Specific A/G-to-C.G mismatch repair in Salmonella typhimurium LT2 requires the mutB gene product

An assay has been developed that permits analysis of repair of A/G mismatches to C.G base pairs in cell extracts of Salmonella typhimurium LT2. This A/G mismatch repair is independent of ATP, dam methylation, and mutS gene function. The gene product of mutB has been shown to be involved in the dam-independent pathway through the in vitro assay. Moreover, specific DNA-protein complexes and an endonuclease can be detected in S. typhimurium extracts by using DNA fragments containing an A/G mismatch. These activities are not observed with substrates which have a T/G mismatch or no mismatch. The S. typhimurium endonuclease, like the A/G endonuclease found in Escherichia coli (A-L. Lu and D.-Y. Chang, Cell 54:805-812, 1988), makes incisions at the first phosphodiester bond 3' to and the the second phosphodiester bond 5' to the dA of the A/G mismatch. No incision site was detected on the other DNA strand. Extracts prepared from mutB mutants cannot form A/G mismatch-specific DNA-protein complexes and do not contain the A/G endonuclease activity. Thus the A/G mismatch specific binding and nicking activities are probably involved in the A/G mismatch repair pathway. Preliminary analysis of the mutational spectrum of the mutB strain has indicated that this mutator allele causes an increase in C.G-to-A.T transversions without affecting the frequencies of other transversion or transition events. In addition, the mutB gene has been mapped to the 64-min region of the S. typhimurium chromosome. Together, this biochemical and genetic evidence suggests that the mutB gene product of S. typhimurium is the homolog of the E. coli micA (and/or mutY) gene product.

The barrier to recombination between Escherichia coli and Salmonella typhimurium is disrupted in mismatch-repair mutants

The requirement for DNA sequence homology in generalized genetic recombination is greatly relaxed in bacterial mutL, mutS and mutH mutants deficient in mismatch repair. In such mutants, intergeneric recombination occurs efficiently between Escherichia coli and Salmonella typhimurium, which are approximately 20% divergent in DNA sequence. This finding has implications for speciation, for regulating recombination between diverged repeated sequences, and for hitherto difficult interspecies hybridizations.

Nucleotide sequence of the Salmonella typhimurium mutL gene required for mismatch repair: homology of MutL to HexB of Streptococcus pneumoniae and to PMS1 of the yeast Saccharomyces cerevisiae

The mutL gene of Salmonella typhimurium LT2 is required for dam-dependent methyl-directed DNA mismatch repair. We have cloned and sequenced the mutL gene of S. typhimurium LT2 and compared its sequence with those of the hexB gene product of the gram-positive bacterium Streptococcus pneumoniae and the PMS1 gene product of the yeast Saccharomyces cerevisiae. MutL was found to be quite similar to the HexB mismatch repair protein of S. pneumoniae and to the mismatch repair protein PMS1 of the yeast S. cerevisiae. The significant similarities among these proteins were confined to their amino-terminal regions and suggest common evolution of the mismatch repair machinery in those organisms. The DNA sequence for mutL predicted a gene encoding a protein of 618 amino acid residues with a molecular weight of 67,761. The assignment of reading frame was confirmed by the construction of a chimeric protein consisting of the first 30 amino acids of LacZ fused to residues 53 through 618 of MutL. Interestingly, the presence of excess amounts of this fusion protein in wild-type mutL+ cells resulted in a trans-dominant effect causing the cell to exhibit a high spontaneous mutation frequency.

A mutation in the DNA adenine methylase gene (dam) of Salmonella typhimurium decreases susceptibility to 9-aminoacridine-induced frameshift mutagenesis

A mutant of Salmonella typhimurium with a reduced response to mutation induction by 9-aminoacridine (9AA) has been isolated. The mutation (dam-2) is located in the DNA adenine methylase gene. The dam-2 mutant strain exhibits a level of sensitivity to 2-aminopurine (2AP) intermediate between that of the dam+ and the DNA adenine methylation-deficit dam-1 strain, and 2AP sensitivity was reversed by introduction of a mutH mutation or of the plasmid pMQ148 (which carries a functional Escherichia coli dam+ gene). However, the dam-2 strain is not grossly defective in DNA adenine methylase activity. Whole cell DNA appears full methylated at -GATC- sites. The levels of 9AA required to induce equivalent levels of frameshift mutagenesis in the dam-2 strain were approximately 2-fold higher than for the dam+ strain. Introduction of pMQ148 dam+ reduced the level of 9AA required for induction of frameshift mutations 4-fold in the dam-2 strain and 2-fold in the dam+ strain. The dam-2 mutation had no effect on the levels of ICR191 required for induction of frameshift mutations, but introduction of pMQ148 reduced the ICR191-induced mutagenesis 2-fold. The dam+/pMQ148, dam-2/pMQ148 and dam-1/pMQ148 strains showed identical dose-response curves for both 9AA and ICR191. These results are consistent with a slightly reduced (dam-2) or increased (pMQ148) rate of methylation at the replication fork. The 2AP sensitivity of the dam-2 strain cannot be simply explained. Furthermore, addition of methionine to the assay medium reverses the 2AP sensitivity of the dam-2 strain, but has no effect on 9AA mutagenesis.

Isolation and characterization of a UV-sensitive mutator (mutB1) mutant of Haemophilus influenzae

The mutB1 mutant of Haemophilus influenzae is very sensitive to UV radiation but only slightly sensitive to methylmethane sulfonate or N-methyl-N'-nitro-N-nitrosoguanidine. Cultures of mutB1 cells contain high numbers of spontaneous mutants and show hypermutability after exposure to the latter mutagen. Normally high-efficiency transforming markers, as well as low-efficiency ones, transform mutB1 recipients at similarly low efficiencies. Significant host cell reactivation was observed when mutB1 cells were exposed to UV-damaged phage; however, these mutants showed a decrease in phage recombination. This mutant did not degrade its DNA following exposure to UV. It is speculated that the mutB1 mutation is similar to the Escherichia coli uvrD mutation.

The mutY gene: a mutator locus in Escherichia coli that generates G.C----T.A transversions

We have used a strain with an altered lacZ gene, which reverts to wild type via only certain transversions, to detect transversion-specific mutators in Escherichia coli. Detection relied on a papillation technique that uses a combination of beta-galactosides to reveal blue Lac+ papillae. One class of mutators is specific for the G.C----T.A transversion as determined by the reversion pattern of a set of lacZ mutations and by the distribution of forward nonsense mutations in the lacI gene. The locus responsible for the mutator phenotype is designated mutY and maps near 64 min on the genetic map of E. coli. The mutY locus may act in a similar but reciprocal fashion to the previously characterized mutT locus, which results in A.T----C.G transversions.

Nucleotide sequence of the Salmonella typhimurium mutS gene required for mismatch repair: homology of MutS and HexA of Streptococcus pneumoniae

The mutS gene product of Escherichia coli and Salmonella typhimurium is one of at least four proteins required for methyl-directed mismatch repair in these organisms. A functionally similar repair system in Streptococcus pneumoniae requires the hex genes. We have sequenced the S. typhimurium mutS gene, showing that it encodes a 96-kilodalton protein. Amino-terminal amino acid sequencing of purified S. typhimurium MutS protein confirmed the initial portion of the deduced amino acid sequence. The S. typhimurium MutS protein is homologous to the S. pneumoniae HexA protein, suggesting that they arose from a common ancestor before the gram-negative and gram-positive bacteria diverged. Overall, approximately 36% of the amino acids of the two proteins are identical when the sequences are optimally aligned, including regions of stronger homology which are of particular interest. One such region is close to the amino terminus. Another, located closer to the carboxy terminus, includes homology to a consensus sequence thought to be diagnostic of nucleotide-binding sites. A third one, adjacent to the second, is homologous to the consensus sequence for the helix-turn-helix motif found in many DNA-binding proteins. We found that the S. typhimurium MutS protein can substitute for the E. coli MutS protein in vitro as it can in vivo, but we have not yet been able to demonstrate a similar in vitro complementation by the S. pneumoniae HexA protein.

Identification and characterization of the mutL and mutS gene products of Salmonella typhimurium LT2

The gene products of the mutL and mutS loci play essential roles in the dam-directed mismatch repair in both Salmonella typhimurium LT2 and Escherichia coli K-12. Mutations in these genes result in a spontaneous mutator phenotype. We have cloned the mutL and mutS genes from S. typhimurium by generating mutL- and mutS-specific probes from an S. typhimurium mutL::Tn10 and an mutS::Tn10 strain and using these to screen an S. typhimurium library. Both the mutL and mutS genes from S. typhimurium were able to complement E. coli mutL and mutS strains, respectively. By a combination of Tn1000 insertion mutagenesis and the maxicell technique, the products of the mutL and mutS genes were shown to have molecular weights of 70,000 and 98,000 respectively. A phi (mutL'-lacZ+) gene fusion was constructed; no change in the expression of the fusion could be detected by treatment with DNA-damaging agents. In crude extracts, the MutS protein binds single-stranded DNA, but not double-stranded DNA, with high affinity.

Spontaneous mutagenesis: the roles of DNA repair, replication, and recombination

There appears to be no dearth of mechanisms to explain spontaneous mutagenesis. In the case of base substitutions, data for bacteriophage T4 and especially for E. coli and S. cerevisiae suggest important roles in spontaneous mutagenesis for the error-prone repair of DNA damage (to produce mutations) and for error-free repair of DNA damage (to avoid mutagenesis). Data from the very limited number of studies on the subject suggest that about 50% of the spontaneous base substitutions in E. coli, and perhaps 90% in S. cerevisiae are due to error-prone DNA repair. On the other hand, spontaneous frameshifts and deletions seem to result from mechanisms involving recombination and replication. Spontaneous insertions have been shown to be important in the strongly polar inactivation of certain loci, but it is less important at other loci. Perhaps with continued study, the term "spontaneous mutagenesis" will be replaced by more specific terms such as 5-methylcytosine deamination mutagenesis, fatty acid oxidation mutagenesis, phenylalanine mutagenesis, and imprecise-recombination mutagenesis. While most studies have concentrated on mutator mutations, the most conclusive data for the actual source of spontaneous mutations have come from the study of antimutator mutations. Further study in this area, perhaps along with an understanding of chemical antimutagens, should be invaluable in clarifying the bases of spontaneous mutagenesis.

The use of transposon Tn5 mutagenesis in the rapid generation of correlated physical and genetic maps of DNA segments cloned into multicopy plasmids--a review

The properties of transposon Tn5 that render it useful for in vivo mutagenesis of cloned DNA sequences are reviewed. Transposition frequency, insertional specificity, polarity and stability of Tn5 insertion mutations are among the topics discussed. Examples are cited from the published literature which illustrate the applications of Tn5 mutagenesis to the analysis of cloned prokaryotic and eukaryotic genes. A methods section is included which outlines precisely how to carry out transposon Tn5 mutagenesis analysis of cloned DNA segments.

The Salmonella typhimurium LT2 uvrD gene is regulated by the lexA gene product

The uvrD gene product apparently plays a role in the repair of UV damage, in mismatch repair, and in genetic recombination. A lower level of expression of the Salmonella typhimurium LT2 uvrD gene was observed in maxicells prepared from an Escherichia coli strain that contained a lexA+ plasmid than in maxicells prepared from an E. coli strain that lacked functional LexA protein. These results suggest that the uvrD+ gene is repressed by the LexA protein and is thus a member of the set of genes whose expression is increased by "SOS"-inducing treatments.

Virus-induced gene mutations of eukaryotic cells

Most animal viruses studied so far induce chromosomal aberrations. In addition, adenoviruses, papovaviruses, and retroviruses are known to induce gene mutations like mutagenic bacteriophages. At least in one case studied retrovirus induced mutagenesis involves gene and/or scripton splitting analogous to the mutagenic mechanism of action of mutatorphage Mu and other movable DNA elements. On the contrary, several results obtained by independent means indicate that Simian virus 40, a papovavirus, does not act by splitting the affected gene but presumably by generation of base pair substitutions or of other minor DNA damages leading to amino acid substitutions. The mechanisms involved are still unknown. There a some hints, however, that these mechanisms might have some step(s) in common with processes leading to malignancy. In fact those viruses proved unequivocally so far to be capable of inducing gene mutations are oncogenic viruses.

Identification of the uvrD gene product of Salmonella typhimurium LT2

The product of the uvrD gene of Salmonella typhimurium LT2 and Escherichia coli K-12 is thought to play a role in both the correction of mismatched bases and the repair of DNA damage, since insertion mutations in the uvrD gene increase the spontaneous mutation frequency and make the cells more sensitive to killing by UV irradiation. To clone the uvrD gene of S. typhimurium, we first generated a uvrD-specific probe by using DNA from an S. typhimurium uvrD421::Tn5 mutant. This probe was used to screen a lambda library of S. typhimurium DNA. Bacteriophage carrying intact uvrD+ genes were subsequently identified, and the uvrD+ gene was subcloned onto a low-copy-number vector. By using a combination of Tn1000 insertion mutagenesis and the maxicell technique, the product of the uvrD gene was shown to be a 75,000-dalton protein, and the relative direction of transcription of this protein was determined. Introduction of a low-copy-number plasmid carrying the S. typhimurium uvrD+ gene into uvrD insertion mutants of either S. typhimurium or E. coli restored the spontaneous mutation frequency and degree of UV sensitivity to the levels in the corresponding uvrD+ strains.

The Escherichia coli uvrD gene is inducible by DNA damage

The product of the uvrD gene of Escherichia coli is involved in the repair of DNA damage, mismatch repair, and recombination. Phage Mud(Amp, Lac) was used to form a uvrD-lacZ fusion allowing uvrD expression to be followed by measuring the activity of beta-galactosidase, the product of the lacZ gene. uvrD expression was inducible by DNA damage and was under the control of lexA-recA regulatory system. Mutations in the uvrD gene that result in different phenotypes in respect to DNA repair and spontaneous mutation have been previously found. The phenotype of the uvrD::Mud(Amp, Lac) mutant was mutator and UV-sensitive but not as deficient in host cell reactivation or repair of methyl methanesulfonate damage as the previously described uvrD3 mutant.

Mutagenesis, by methylating and ethylating agents, in mutH, mutL, mutS, and uvrD mutants of Salmonella typhimurium LT2

Salmonella typhimurium LT2 mutH, mutL, mutS, and uvrD mutants were especially sensitive to mutagenesis by both the recA+-dependent mutagen methyl methane sulfonate and the recA+-independent mutagen ethyl methane sulfonate, but not to mutagenesis by agents such as 4-nitroquinoline-1-oxide and UV irradiation. Similarly, these mutator strains were very sensitive to mutagenesis by the methylating agents N-methyl-N'-nitro-N-nitrosoguanidine and N-methyl-N-nitrosourea. The increased susceptibility to mutagenesis by small alkylating agents due to mutH, mutL, mutS, and uvrD mutations was not accompanied by an increased sensitivity to killing by these agents. Various models are discussed in an effort to explain why strains thought to be deficient in methyl-instructed mismatch repair are sensitive to mutagenesis by methylating and ethylating agents.