STARVATION-INDUCED MUTAGENESIS IN SALMONE LLA-TYPHIMURIUM

Spontaneous reversion to prototrophy of two-his alleles of Salmonella typhimurium - hisG428 (ochre) and hisC527 (amber) - was studied. Strai ns containing hisG428 allele in the chromosome and within the multicop y plasmid pAQ1 were used. When the hisC428 allele was chromosomally lo cated, no sharp fluctuations in the number of His(+) revertant were ob served in fluctuation experiments and the distribution of revertants o beyed the Poisson law. The pattern of revertants distribution in strai ns carrying plasmid pAQ1 was intermediate between the Poisson distribu tion and jackpot distribution described by Luria and Delbruck. Convers ely, sharp fluctuations were observed in the distribution of His(+) re vertants of the hisC527 allele obtained from independent cultures. The data presented here suggest that His(+) revertants, including ochre s uppressors, appear under the influence of histidine starvation, when t he hisG428 allele is chromosomally located. At the same time, reversio n to prototrophy at the hisC527 allele does not strictly depend on the presence or absence of this amino acid in the medium. Plasmid locatio n and novobiocin-resistance mutation promoted the identifying of preex isting mutants among His(+) revertants by the hisC428 allele. The data obtained also demonstrate that the presence of the mutator plasmid pK M101 preferentially increases the frequency of adaptive His(+) reversi ons.