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An experimental system ensuring fusion of bacterial genes to the lac operon of the Mu dl(Aplac) phage was used. Fusion operons in which the lac operon was under the control of promoters of the elt gene, responsible for synthesis of the LT toxin, of the tetracyclin-resistance tet gene, and sfiA gene encoding filament production, was studied. Using this experimental system, plasmid pKM101 was shown to be capable of activating the expression of the above Escherichia coli and Salmonella typhimurium genes, which is manifested as the activation of beta-galactosidase synthesis. The activation of the elt gene expression by the pKM101 plasmid was also confirmed in experiments on detecting the LT toxin synthesized by bacteria carrying this plasmid. Effect of the plasmid on the activation of elt operon expression, unlike the effect of this plasmid on mutability, does not depend on the functioning of the lexA and recA genes, i.e., this is not a SOS-regulated process. The mutant plasmid pGW12, a derivative of pKM101, deficient in the mucAB genes responsible for mutagenesis, causes a more pronounced activation of the elt gene than plasmid pKM101.

The effect of plasmid pKM101 on UV irradiated excision proficient and excision deficient cells was investigated. The plasmid increased the survival of excision proficient cells while partially inhibiting thymine dimer excision. The frequency of mutations was almost unchanged. In excision deficient cells the effect of the plasmid on survival was less pronounced while cell mutability was increased. Our data indicate that the mucAB genes (carried by the plasmid) influence the two types of cells in a different way.

The ability of 23 chemicals (carcinogens and non-carcinogens) to induce precise excision of Tn10 and point mutations was studied in experiments with a single strain. The mutation assay was shown to detect a wider spectrum of genotoxic agents than the assay of Tn10 precise excision. The latter was induced only by potent SOS mutagens, which is in accordance with data on the SOS dependence of the induction of precise excision of Tn10. The precise excision assay as an additional test contributing to the knowledge of particular features of the action of a tested mutagen is discussed. The induction of precise excision of Tn10 by pyrene (and its failure to induce point mutations in this strain) demonstrates the value of using the transposon excision assay in cases of 'problem' mutagens.

Mutation in the thr gene with the highest UV-induced reversion frequency has been chosen from a number of mutations arisen from insertion of transposon Tn10 into the umuDC- Salmonella typhimurium. This mutation has been transferred by transduction into the Salmonella strain carrying E. coli umuDC+ genes. umuDC- and umuDC+ Salmonella strains, bearing the same insertion mutation, were used to study the effect of umuDC genes and of plasmid pKM101 on spontaneous and UV-induced precise excision of Tn10. This process, like point mutagenesis, is shown to be umuDC-stimulated. Plasmid pKM101 eliminates the effect of umuDC+ genes on UV-induced Tn10 precise excision, in contrast to its effect on point mutagenesis, and does not enhance spontaneous precise excision of Tn10.

The UV-sensitivity of wild type Salmonella strains has been compared to that of wild type E. cole and its UV-sensitive mutants. Many wild type Salmonella strains are 4-5 times more sensitive than wild type E. coli and their inactivation curves is similar to that for E. coli with a mutation in the polA gene. Alkaline sucrose gradient centrifugation has shown a deficiency of these strains in normal excision repair of UV-damaged DNA. This deficiency is not a Salmonella genus feature because one strain as resistant as wild type E. coli was found. This resistant strain showed normal excision repair in alkaline sucrose gradient centrifugation experiments. The possible influence of plasmids and mutations in repair genes on the ability of Salmonella to repair UV-damages DNA is discussed.

Salmonella typhimurium recombinants have been constructed which have acquired UV-induced mutability. Such recombinants were obtained after transfer of plasmid F'126 from E. coli. UV-mutability was acquired by UV-stable Salmonella as a nonselective marker after selection of trp+ marker in 2.5% of the cases. Possible deficiency in UV-stable Salmonella of the umuC gene is discussed.

Two missense mutations, trpA58 and trpA78, and one nonsense mutation-trp-ochre, were used to determine the types of base-pair substitution caused by ultra, violet irradiation and methyl methanesulfonate (MMS) in Escherichia coli. UV irradiation of the wild-type bacteria led to the formation of revertants mainly arising as a result of GC yields AT transitions (suppressor revertants of the trpA58 mutant). True revertants of the trp- mutant (arising via transitions of AT pairs) and 5-methyl tryptophan-sensitive (MT-s) Trp+ of the trpA78 mutant (arising via unidentified transversions) occurred at a lower frequency. The polAI mutation did not change the frequency of the UV-induced transitions GC yields AT or that of the substitutions of the AT pairs. The uvrE502 mutation significantly increased the frequency of the UV-induced revertants arising via the transition GC yields AT. Treatment of the wild-type bacteria with MMS resulted in the formation of revertants mainly due to the GC yields AT substitution, and with a lower frequency to the AT yields GC transitions. MMS also induced, with a low frequency, some transversions. The frequency of the MMS-induced GC yields AT transitions was enhanced in the uvrE502 mutant. On the other hand, the uvrE502 mutation eliminated or significantly lowered MMS-induced revertants arising as a result of AT yields GC transitions or transversions.

Introduction of the su58 missense suppressor into the chromosome of the uvr502 mutant, either by mutation or by transduction, results in a marked increase of ultraviolet resistance of the uvr502 mutant. In the uvr(+) genetic background, the su58 suppressor causes some decrease of ultraviolet resistance and marked increase in the spontaneous mutation frequency. The presence of the su58 suppressor did not decrease the high frequency of spontaneous mutants in the population of the uvr502 strain. However, the significant increase in spontaneous mutant frequency in the uvr(+)su58 strain makes the difference between the uvr502 su58 and the uvr(+)su58 strains 18 times lower than that between the uvr502 and the uvr(+) suppressor-free strains. Since the missense suppressors act at the level of translation, the results suggest that the product of the uvr(502) gene is a protein.