Plasmid copy number and mutant frequencies in S. typhimurium TA102.

Tetracycline and chloramphenicol increase the number of mutant colonies of strain TA102, which carries the reverting gene on the plasmid pAQ1. Determination of the plasmid content by agarose gel analysis shows that the increase of the mutant colony number is paralleled closely by an increase of the number of pAQ1 plasmids per cell, indicating that the two compounds do not increase the frequency of mutants "per gene," but only enhance the number of the genes at which mutations can occur. Thus, not considering the molecular processes could result in mistakenly attributing the increase in the number of mutants per plate (respective to the number of mutants per cell) to a mutagenic activity of the antibiotics.