Sex pili mutants isolated by macarbomycin treatment.

Macarbomycin (MM) preferentially kills Escherichia coli carrying episomes derepressed for sex pili formation (drd episomes). Using MM, we have isolated various types of mutants from both F-lac.tet and R100-1 episomes and have classified them into two types by sex pili formation. Among 26 mutants, 20 could not produce pili and had lost both the ability for conjugal transmission and sensitivity to male phages. From 12 of these 20 mutants, revertants capable of pili formation could be obtained. Most of these had simultaneously regained susceptibility to MM, indicating that the MM susceptibility of E. coli carrying drd episomes is due to pili formation. Three of the revertants could not confer MM susceptibility on their host and two of them could not produce normal pili, indicating that there are some types of pili insensitive to MM. The six MM-resistant mutants could produce pili, but the frequency of pili formation was slightly lower than that of their parent episomes. Mutants of this type exhibited various degrees of decrease in conjugal transferability or male phage sensitivity, and one mutant produced pili that were altered in shape. It is suggested that this latter mutant became MM-resistant as a result of changes in the chemical composition of its pili.