Transduction of Escherichia coli trp genes in Salmonella typhimurium and effect of N-methyl-N'-nitro-N-nitrosoguanidine on transduction with heterogenotic DNA.

P1Kc-mediated transduction of Escherichia coli trp genes occurred at a frequency of about 10(-8) in Salmonella typhimurium trp strains carrying mutations determining sensitivity to P1 and a low level of restriction enzymes. Heterospecific transductants were analysed by using them as donors in second-stage transductions mediated by bacteriophage KBint. One class of heterospecific transductants had the phenotype Trp+ Pro- but were extremely unstable and reverted at high frequency (up to 80%) to the parental phenotype. The Trp+ Pro- phenotype probably represents insertions of the E. coli trp genes in the S. typhimurium pro genes. It was stable in a RecA background. Haploid Pl-mediated heterogenotes exhibited various degrees of homology with the trp region of S. typhimurium in KB-mediated second-stage transductions: one heterogenote (MA427), which transduced the linked markers trp, cysB and pyrF very poorly, was used to look at the effects of the mutagen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in stimulating heterologous recombination and to derive a method for positive selection of linked mutations. Following treatment with MNNG, heterospecific Trp+ transductants were obtained with MA427 as donor. Their yield was maximal immediately after MNNG treatment but decreased and even disappeared when transductions were carried out a few hours later. This transient process is thought to represent MNNG-induced conversion of abortive transductants to complete transductants. On the other hand, phenotypic analysis of MNNG-induced Trp+ heterologous transductants revealed the presence of mutations of various phenotypes. In particular, the antimutator phenotype was recognized in 15 clones among 110 MNNG-induced recombinants tested. However, most of these mutations seemed not to influence heterologous recombination.