[Starvation-induced mutagenesis in Salmonella typhimurium]

Spontaneous reversion to prototrophy of two his alleles of Salmonella typhimurium--hisG428 (ochre) and hisC527 (amber)--was studied. Strains containing hisG428 allele in the chromosome and within the multicopy plasmid pAQ1 were used. When the hisG428 allele was chromosomally located, no sharp fluctuations in the number of His+ revertant were observed in fluctuation experiments and the distribution of revertants obeyed the Poisson law. The pattern of revertants distribution in strains carrying plasmid pAQ1 was intermediate between the Poisson distribution and jackpot distribution described by Luria and Delbruck. Conversely, sharp fluctuations were observed in the distribution of His+ revertants of the hisC527 allele obtained from independent cultures. The data presented here suggest that His+ revertants, including ochre suppressors, appear under the influence of histidine starvation, when the hisG428 allele is chromosomally located. At the same time, reversion to prototrophy at the hisC527 allele does not strictly depend on the presence or absence of this amino acid in the medium. Plasmid location and novobiocin-resistance mutation promoted the identifying of preexisting mutants among His+ revertants of the hisG428 allele. The data obtained also demonstrate that the presence of the mutator plasmid pKM101 preferentially increases the frequency of adaptive His+ reversions.