Mutants of Salmonella typhimurium Deficient in DNA Polymerase I: Further Characterization and Genetic Analysis

Abstract

Tests with a plasmid-borne ochre suppressor (sup-BI2) and a chromosomal amber suppressor (supD50I) revealed that one of three mutants of S. typhimurium deficient in DNA polymerase I was an amber mutant. Assays performed on crude extracts established that derivatives of this mutant (designatedpoIA3) carrying ochre and amber suppressors had about 13 and 20% respectively of the enzyme activity found in the wild-type parent. The unsuppressed mutant showed less than 1 % of the wild-type level of activity. Other properties of the polA3 mutant that were also partially or in some cases completely reversed by the sup-BI2 and supD50I suppressors included: u.v. sensitivity, methyl methanesulphonate (MMS) sensitivity, reduced ability to effect host-cell reactivation of u.v.-irradiated or MMS-treated bacteriophages, inability to maintain the (Col El) plasmid, and reduced ability to plate the phage mutant P22 c2 hpi-30B. Mapping by PI-mediated transduction showed that all three polA mutations lie between metE and rha on the S. typhimurium chromosome, and that the polAI mutation is cotransduced with metE at a frequency of 20 % and with rha at a frequency of 8 %.