[Analysis of the dependence of the frequency of emergence of His+-reversions in Salmonella typhimurium on the density of the cell population]. / Analiz zavisimosti chastoty vozniknoveniia His+-reversii u Salmonella typhimurium ot plotnosti kletochnoi populiatsii.

The dependence of the appearance of mutants on the number of viable cells plated on a selective medium was analyzed with the spontaneous His+ reversion system in the histidine auxotroph of Salmonella typhimurium strain BA13. The frequency of spontaneous His+ revertants was shown to be inversely proportional to the number of plated cells. Evaluation of residual culture growth on a histidine-deficient medium suggests that this factor cannot be the reason behind the inverse dependence. The results of experiments involving previously added His+ revertants showed that the inverse relationship between mutant frequency and population density is not connected with the inhibition of preadaptive revertant growth by nonmutant cells. Moreover, an inhibitory effect of the culture medium on the frequency of spontaneous His+ revertants upon histidine starvation was detected. On the basis of these results, it was assumed that the His+ reversion generated by histidine starvation is suppressed by metabolites of starved cells.

[Specificity and time of the appearance of His+ reversions induced by histidine starvation in Salmonella typhimurium]. / Spetsifichnost' i vremia vozniknoveniia His+ reversii, indutsirovannykh gistidinovym golodaniem, u Salmonella typhimurium.

It was previously established that reversion of the hisG46 allele of Salmonella typhimurium to prototrophy occurred upon histidine starvation. In this paper, it was shown that histidine starvation does not affect the appearance of mutants resistant to L-arabinose and rifampicin. Threonine starvation did not change the frequency of His+ revertants. Analysis of His+ revertant clones did not reveal additional L-arabinose resistance mutations. Thus, these experiments allowed the conclusion that amino acid starvation does not lead to a nonspecific increase in the mutation rate. In addition, it was shown that spontaneous His+ revertants start to arise after two to three hours of histidine starvation, this process lasting for four days. Nevertheless, original His+ cells did not grow in a culture generating His+ revertants. Traces of histidine and novobiocin added to a minimal medium retarded reversion realization. However, the occurrence of revertants was not markedly inhibited by chloramphenicol. Based on the results, it is assumed that adaptive His+ reversions occurred due to a special mode of replication induced upon histidine starvation and requiring no de novo protein synthesis.

[Starvation-induced mutagenesis in Salmonella typhimurium]. / Indutsirovannyi golodaniem mutagenez u 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.

[The nature of the emergence of His+-revertants in Salmonella typhimurium]. / K prirode vozniknoveniia His+-revertantov u Salmonella typhimurium.

The His(-)-->His+ spontaneous reversion of the Salmonella typhimurium alleles hisD3052 and hisG46 was studied. The expected jackpot distribution of His+ revertants was not observed in the fluctuation tests. The experimental distributions were close to Poisson. It was also shown that the mean number of His+ reversion events and the mean number of revertants per plate were similar. These data demonstrate that the His+ reversion occurred under the influence of histidine starvation. At the same time, kanamycin resistant mutants had the jackpot distribution. Selection for His+ revertants did not increase the Kans-->Kanr mutations.

[Partial suppression of rec B rec C mutations in E. coli by plasmid pBR 322 containing a Bacillus subtilis chromosome insertion]. / Chastichnaia supressiia mutatsii rec B rec C E. coli plazmidoi pBR 322, soderzhashchei vstavku khromosomy Bacillus subtilis.

ATP-dependent DNAase genes of Bac. subtilis were originally cloned in E. coli plasmid pBR322. These genes are expressed in rec BC mutant E. coli cells leading to a complete recovery of the enzyme activity. Bac. subtilis enzyme suppresses reparative properties of the rec BC mutant to a considerable extent but is unable to replace functionally the E. coli mutant enzyme in recombination process.