Thermoresistant revertants of an Escherichia coli strain carrying tif-1 and ruv mutations: non-suppressibility of ruv by sfi.

Spontaneous thermoresistant revertants were isolated from Tif1 Ruv- and Tif1 Ruv+ strains of Escherichia coli K-12. They were divided into five groups; backmutants to tif+ and recA structural gene mutants accounted for at least two of these groups. Mutations with an unconditional RecA- phenyotype were detected at a higher frequency in the Tif1 Ruv- strains (65%) than in the Tif1 Ruv+ strains (25%). A third group consisted of revertants exhibiting a RecA- phenotype at low temperature. Revertants with normal recombination ability and UV resistance, but with a thermosensitive defect in propagating lambda bio11 phage, were also isolated (group 4). The alleles responsible for this property were cotransducible with the srl gene, suggesting that they are located at the recA locus. Other revertants, which might carry lex, LEXB, or zab mutations, were UV sensitive and were able to propagate lambda bio11 phage (group 5). The sfi mutation, which suppresses filamentation in the Tif1 and UV-sensitive Lon- strains, does not restore UV resistance of the Ruv- mutant.