Restoration of mutability in non-mutable Escherichia coli carrying different plasmids

Translesion DNA Synthesis

All living organisms are continually exposed to agents that damage their DNA, which threatens the integrity of their genome. As a consequence, cells are equipped with a plethora of DNA repair enzymes to remove the damaged DNA. Unfortunately, situations nevertheless arise where lesions persist, and these lesions block the progression of the cell's replicase. In these situations, cells are forced to choose between recombination-mediated "damage avoidance" pathways or a specialized DNA polymerase (pol) to traverse the blocking lesion. The latter process is referred to as Translesion DNA Synthesis (TLS). As inferred by its name, TLS not only results in bases being (mis)incorporated opposite DNA lesions but also bases being (mis)incorporated downstream of the replicase-blocking lesion, so as to ensure continued genome duplication and cell survival. Escherichia coli and Salmonella typhimurium possess five DNA polymerases, and while all have been shown to facilitate TLS under certain experimental conditions, it is clear that the LexA-regulated and damage-inducible pols II, IV, and V perform the vast majority of TLS under physiological conditions. Pol V can traverse a wide range of DNA lesions and performs the bulk of mutagenic TLS, whereas pol II and pol IV appear to be more specialized TLS polymerases.

Unusual insertion element polymorphisms in the promoter and terminator regions of the mucAB-like genes of R471a and R446b

We have previously identified umu-complementing genes on two incL/M plasmids, R471a and R446b (C. Ho et al., J. Bacteriol., 175 (1993) 5411-5419). Molecular analysis of these genes revealed that they are more structurally and functionally related to mucAB from the incN plasmid pKM101 than to other members of the previously identified Umu-like family. As a consequence, we have termed these new homologs mucAB(R471a) and mucAB(R446b) respectively. Interestingly, while the location of the mucAB-like genes is essentially the same in both R471a and R446b, the regions immediately flanking the mucAB-like genes are highly polymorphic. For example, 5' to mucAB(R471a) we found an insert that appears to be a novel retroelement encoding a putative reverse transcriptase (RT). This RT is related to the reverse transcriptases encoded by group II introns but is embedded in a retron-like context. Immediately 3' to the mucAB(R471a) locus is a putative insertion element of a sparsely-dispersed class not previously reported from enteric bacteria. Both the RT and insertion element are absent in R446b. These observations suggest that the mucAB-like genes from R471a and R446b are located within regions of the R-plasmids that perhaps were once (or still are) mobile genetic elements. Such observations might help explain the distribution of umu-like genes on R-plasmids and bacterial chromosomes.

Mutagenic activity of the dacarbazine analog p-(3,3-dimethyl-1-triazeno)benzoic acid potassium salt in bacterial cells

The mutagenic activity of the antimetastatic agent p-(3,3-dimethyl-1-triazeno)benzoic acid potassium salt (DM-COOK) was studied in procaryotic cells and compared with that of dacarbazine (DTIC) which is clinically used in the management of human neoplasms. The results indicated that DM-COOK has a very low mutagenic activity on the Salmonella typhimurium strains tested, while it is more effective in inducing trp+ revertants in E. coli B strains. The magnitude of these effects was always less pronounced than that displayed by DTIC. The mutagenic activity of DM-COOK appeared to be independent from the addition of a metabolic activating system and had a different pattern from that displayed by MM-COOK. It is therefore unlikely that DM-COOK acts through conversion into the monomethyl derivative.

Complementation of a pKM101 derivative that decreases resistance to UV killing but increases susceptibility to mutagenesis

The drug resistance plasmid pKM101 makes Escherichia coli resistant to the lethal effects of ultraviolet (UV) irradiation and more susceptible to mutagenesis by a variety of agents. The plasmid operon responsible for increasing mutagenesis has been termed mucAB (Mutagenesis, UV and chemical). We have isolated a derivative of pKM101 called pGW1975 which makes cells more sensitive to killing by UV but which retains the ability of pKM101 to increase susceptibility to methyl methanesulfonate (MMS) mutagenesis. pGW1975 increases UV mutagenesis less than pKM101 in a uvrA+ strain but more than pKM101 in a uvrA- strain. muc- point and insertion mutants of pKM101 and pGW1975 complement to restore the plasmid-mediated: (i) ability to reactivate UV-irradiated phage, (ii) resistance to killing by UV, and (iii) level of susceptibility to UV mutagenesis. We have identified a 2.0 kb region of pKM101 which is responsible for the complementation and which maps counterclockwise of mucAB.

The effects of cis-platinum (II)diamminodichloride, UV light and N-methyl-N'-nitro-N-nitrosoguanidine on Escherichia coli:plasmid mediated resistance to mutagens

Protease deficient recA431 mutants of Escherichia coli are defective in their capacity for induction of SOS responses and were intermediate in their sensitivities to ultraviolet light (UV) and cis-platinum (II) diamminodichloride (cis-PDD). Survival after treatment determined as colony forming ability was greater in rec+ strains and decreased in recA13 mutants which are defective in both recA proteolytic and recombination capabilities. In contrast, recA431 mutants were as sensitive to N-methyl-N'-nitro-N-nitrosoguanidine (NTG) as the recA13 cells. When cells carried either the pKM101 or N3 plasmid, survival after treatment with the three mutagens was increased. Presence of these plasmids in cells also resulted in hypermutagenicity as indicated by reversion of the argE3 mutation using a modified Ames test. Mutagenesis by NTG and cis-PDD was increased, as was survival of cells treated with UV light, cis-PDD and NTG in both recA+ and recA431 (protease deficient) strains. No plasmid mediated enhancement of mutagenesis or cell survival was observed in recA13 mutants. Thus, the ability of the plasmids to enhance cell survival and mutagenesis was dependent on recombination proficiency of the recA gene product and not its regulatory proteolytic activity. Unlike UV or NTG, presence of one of these plasmids was needed to detect reversion of the argE3 mutation by cis-PDD.

Expression of eight unrelated Muc+ plasmids in eleven DNA repair-deficient E. coli strains

23 plasmids from different incompatibility groups were tested for their ability to increase post-UV survival and UV-induced reversion to Arg+ in Escherichia coli strain AB1157 argE3 8 plasmids increased mutagenesis, of which 7 increased UV resistance. The exception, plasmid R391, sensitized AB1157 to UV. All 8 plasmids were absolutely dependent upon host recA+ and lexA+ genotypes for expression of these functions, but were independent of uvrA+, uvrB+, umuC+, recF+, polA+, uvrD+ or recL+. E. coli KMBL91 uvrE was sensitized to UV by R391, but protected by only 3 plasmids. All 8 plasmids restored mutation frequency in the non-mutable TK501 uvrB umuC strain to levels found in the JC3890 uvrB umuC+ parent strain. R391 sensitized TK501 to UV, but all other plasmids increased survival in the strain by over 1000-fold to levels found in the JC3890 uvrB umuC+ R+ strains. Plasmid R391 reduced the UV-protecting effect of R46 when both were present in strain TK501. Mutation frequencies were higher in TK501 (R46) than in TK501 (R391); in TK501 (R46/R391) they were slightly lower than in TK501 (R46).

Sensitivity of different E. coli and Salmonella strains in mutagenicity testing calculated on the basis of selected literature

Two microbial screening test systems for gene (point) mutations, the Salmonella typhimurium assay (TA1535, TA1537, TA1538, TA98 and TA100) and the Escherichia coli WP2 reverse-mutation system (WP2, WP2uvrA, WP2pKM101 and WP2uvrApKM101), were compared with regard to sensitivity toward a broad spectrum of compounds that cause base-pair or frameshift mutations and that have known carcinogenic qualities. Based on available published literature we found that all 44 carcinogens and 9 non-carcinogens examined in both test systems also met with criteria for data acceptance drawn up by us. The results obtained are: firstly, that the Salmonella assay is decidedly better validated than the E. coli WP2 test; and secondly, that the E. coli test system sensitivity (91%) is fully on a par with the sensitivity of the Salmonella assay (72%). This last is in divergence from earlier reports, e.g. Brusick et al. (1980), and this difference must be ascribed to the new plasmid-containing strains. The many compounds not tested in the E. coli department result in fewer false negatives in the E. coli test system and their omission constitutes a bias in favour of the E. coli assay. By eliminating compounds that are negative in Salmonella and dropped from the WP2 analysis owing to insufficient data, the sensitivity of the Salmonella system is raised to 84% as compared with 91% for the WP2 assay. The results further indicate that some of the tester strains are superfluous, and show an exceedingly sensitive test can be performed by combining the best tester strains from the two test systems.

Functional organization of plasmid pKM101

Tn5 insertion mutants and in vitro-generated deletion mutants of the mutagenesis-enhancing plasmid pKM101 have been used to identify several genetic regions on the pKM101 map. In clockwise order on the pKM101 map are: (i) the bla gene, coding for a beta-lactamase; (ii) the Slo region, responsible for retarding cell growth on minimal medium; (iii) the tra genes, enabling pKM101 to transfer conjugally; (iv) sensitivity to IKe phage (this function[s] maps within the tra region); (v) the muc gene(s), responsible for enhancing ultraviolet light and chemically induced mutagenesis in the cell; and (vi) the Rep region, essential for plasmid replication. The muc gene(s) and the Rep region are contained in a deoxyribonucleic acid region bounded by inverted repeated sequences.

The influence of colicinogenic plasmids ColIb-P9, ColIa-CA53 and ColV-K30 on the repair, mutagenesis and induction of colicin E1 synthesis

The presence of colicinogenic plasmids ColIb-P9 and ColIa-CA53 in E. coli K-12 cells, wild-type with respect to repair, enhanced the survival of cells after UV irradiation and increased the frequency of UV-induced argE3 and his-4 reversions, while the presence of ColV-K30 negatively affected repair and mutagenesis. The plasmid ColIb-P9 showed a UV-protective effect in E. coli cells carrying mutations in genes uvrA, uvrB, uvrC, polA, recB, recF, though in none of the mutants did cell survival reach the wild-type level. The effect of ColIb-P9 on mutagenesis did not depend on the uvrA or recB genes. The plasmid's protective effect and the enhancement of mutagenesis depended on the recA+ lexA+ genotype. The frequency of 2-aminopurine-induced mutations was not affected by ColIb-P9 or ColV-K30. The presence of ColIb-P9 decreased the ability of ColE1-carrying cells to induce colicin E1 synthesis caused by DNA-damaging agents: UV, MNNG, mitomycin C, whereas ColV-K30 increased the percentage of colicin E1-producing cells. These plasmid effects on the level of induction of colicin E1 synthesis were not observed in the case of induction caused by chloramphenicol which did not depend on the products of recA and lexA genes.

Thymineless elimination of N group plasmids is Res-(RII)-dependent and determined by a different gene than the Uvp plasmid phenotype

The wild-type N group plasmid RN3, which is phenotypically Res+Mod+(RII) is not eliminated by thymine starvation of its bacterial host. Derivatives of RN3 selected for the Res−phenotype are eliminated. The presence or absence of the RII modification specificity does not affect thymineless elimination of RN3 Res−plasmids. A Res−Mod(am) RN3 mutant is not eliminated by thymine starvation from either amber suppressing or non-suppressing hosts, suggesting that it carries a cryptic mutation in a novel genetic locus required for elimination. Thymineless elimination is shown to berecA+-dependent and the presence of the X group plasmid R6K significantly inhibits elimination of RN3 Res−Mod+. However, since R6K has no effect on two other plasmid-mediated functions of UV protection and UV-induced mutagenesis, which are alsorecA+-dependent, it would appear that elimination is determined by a separate plasmid gene than that encoding the UV functions. This is confirmed using derivatives of another N group plasmid R390, which eliminate but which have lost the ability to increase UV-induced mutagenesis in their host.

Comparative mutagenicity of linear and angular furocoumarins in Escherichia coli strains deficient in known repair functions

Four furocoumarins, two having a linear molecule, psoralen and 8-methyl-psoralen and two having an angular molecule, angelicin and 4,5'-dimethyl-angelicin were tested for mutagenesis in Escherichia coli B wild type and in various strains deficient in known repair systems. The results indicate that both monoadducts and crosslinks are mutagenic. The mutagenic efficiency of the furocourmarins ranks in the following order 8-methylpsoralen psoralen greater than angelicin greater than 4,5'-dimethylangelicin.

Spontaneous mutational specificity of drug resistance plasmid pKM101 in Escherichia coli

Plasmid pKM101 enhances the frequency of spontaneous and ultraviolet light-induced mutations in Escherichia coli and protects the cells against the lethal effects of ultraviolet irradiation. By analyzing reversion patterns of defined trpA alleles, we showed that pKM101 caused all types of spontaneous base-pair substitution mutations with the possible exception of guanine . cytosine leads to adenine. thymine transitions. Neither insertion nor deletion frameshift mutations were enhanced. Transversions were more strongly enhanced than transitions, and adenine . thymine base pairs appeared more susceptible to pKM101 mutator activity than guanine . cytosine base pairs. In addition, there were effects from neighboring base pairs and genetic background that influenced the mutator activity of pKM101.

MMS mutagenesis in strains of Escherichia coli carrying the R46 mutagenic enhancing plasmid: phenotypic analysis of Arg+ revertants

Arg+ revertants of E. coli AB1157 and derivative strains were selected after MMS mutagenesis and subjected to a phenotypic analysis which permitted the partitioning of revertants into 4 classes. The distribution of these revertant classes was influenced by mutations affecting DNA-repair systems, mutagen treatment and revertant-selection methods. Introduction of the R46 plasmid into strains also affected this mutational specificity, and it was concluded that the plasmid's mutagenic enhancing effect does not merely augment the cellular error-prone capacity to repair MMS damage to DNA.

Effect of host lex, recA, recF, and uvrD genotypes on the ultraviolet light-protecting and related properties of plasmid R46 in Escherichia coli

The ability of plasmid R46 to reduce the lethal but enhance the mutagenic effect of ultraviolet (UV) irradiation was tested in sets of Escherichia coli K-12 derivatives, wild type or with different mutations affecting DNA repair capacity, but otherwise isogenic. UV protection and enhancement of UV mutagenic effect were obtained in uvrA6, uvrB5, uvrD3, and recF143 hosts, but not in a recA56 strain. The plasmid gave some UV protection in two lexA1 and two lexA101 strains and in one lexA102 host, but produced no such effect in another lexA102 host. The plasmid restored UV mutagenic effect in a lexB30 strain, the yield of induced mutants per survivor of irradiation (10 J/m2) being about the same for the lexB30(R46) and lex+(R46) strains; by contrast the plasmid, though it reduced the UV sensitivity of the lexB30 strain, did not make it as UV-resistant as the lex+ R-strain.

Segregation of the mutator property of plasmid R46 from its ultraviolet-protecting property

Plasmid R46 (an R factor conferring resistance to ampicillin, sulfonamides, streptomycin and tetracycline) reduces the bactericidal effect of UV irradiation but increases its mutagenic effect (reversion of hisG46), and raises the frequency of spontaneous reversion (mutator effect). Putative deletion mutants of R46 were obtained by transduction of the plasmid, then two successive conjugal transfers. Plasmids of five of six deletion classes, each with a different combination of drug resistance traits, retained conjugative ability and the UV-protecting, mutagenesis-enhancing and mutator effects of R46. (pKM101, used in the Ames system to enhance responsiveness to chemical mutagens, is one such mutant of R46.) Plasmids of a sixth class, represented by pKM115, conferred resistance only to streptomycin and were non-conjugative. All of several such plasmids (of independent origin) had a much stronger mutator effect than did R46, but lacked UV-protecting ability and did not enhance the mutagenic effect of UV irradiation. We infer that R46 possesses: (i) a gene, uvp, which increases capacity for error-prone repair of UV-damaged DNA, and thus causes both UV protection and enhancement of UV mutagenesis; (ii) gene(s) whose action in the absence of gene uvp greatly increases the frequency of spontaneous reversion of hisG46. A plasmid of another incompatibility group, pLS51, has UV-protecting and mutagenesis-enhancing effect but lacks the mutator property; introduction of pLS51 into a clone of hisG46 carrying a pKM115-type plasmid greatly reduced its spontaneous reversion rate, as expected if pLS51 also has a uvp gene able to modulate the mutator effect of R46-derived gene(s) in the pKM115-type plasmid.

R plasmid-mediated enhancement of mutagenesis in strains of Escherichia coli deficient is known repair functions

The response to four mutagens (UV radiation, methyl methanesulfonate (MMS), cis-platinum dichlorodiamine (cis-PDD), and a 2-aminopurine (AP)) known to cause different types of DNA damage was investigated in the WP2 strain wild-type for DNA repair and in uvrA-, lexA-, polA-, uvrD- and recL- strains. Each strain was also tested after introduction of either the pKM101 or R648 plasmid. The number of revertants produced by a given mutagen in a given bacterial strain depended in a complex way on: (1) the nature of the mutagen and the type of lesion it created in DNA; (2) the prensence and the nature of defects in the chromosomally determined DNA-repair system; and (3) the presence and the nature of plasmids with mutator effect. The results confirm that plasmids enhance mutagenesis through an error-prone DNA-repair system, which is expressed at different levels for different plasmids. Or, alternatively, different repair mechanisms for different plasmids may exist.