Suppression of lex Mutations Affecting Deoxyribonucleic Acid Repair in Escherichia coli K-12 by Closely Linked Thermosensitive Mutations

A major class of ultraviolet (UV)-resistant derivatives oflex − strains ofEscherichia coli K-12 grows normally at 30 C but at 42.5 C fails to produce colonies on complete or minimal agar. At 42.5 C these thermosensitive strains form filaments without septa, due to an apparent defect in cell division. Deoxyribonucleic acid degradation in UV-irradiated cultures of the thermosensitive strains is slow, in contrast to the rapid degradation in UV-irradiated cultures of the parentallex − strains. The thermosensitive mutations (tsl ) are tightly linked (less than 0.04 min on theE. coli K-12 linkage map) to the site of thelex mutation in the parental strain and could lie within the same gene. Thetsl + /tsl − heterozygotes grow at 42.5 C and are UV resistant when grown at 30 or 42.5 C. Thetsl mutations are, therefore, recessive in contrast tolex mutations, which are dominant. It appears likely that thetsl mutations alter the diffusible product that gives rise to the Lex− mutant phenotype. This product appears to be necessary for deoxyribonucleic acid repair and cell division.