2ND-SITE MUTATIONS IN CAPR(LON) STRAINS OF ESCHERICHIA-COLI K-12 THAT PREVENT RADIATION SENSITIVITY AND ALLOW BACTERIOPHAGE-LAMBDA TO LYSOGENIZE

The capR (lon) mutants of E. coli K-12 are mucoid and sensitive to UV and X-ray radiation and to nitrofurantoin. The mutants form filaments after exposure to these agents. The capR mutants are also conditionally lethal since they die when plated on complex medium even without UV treatment; this phenomenon is designated complex medium-induced killing. Furthermore, capR mutants are poorly lysogenized by bacteriophage .lambda.. Second-site revertants were isolated by plating on media containing nitrofurantoin. All 17 of the independent revertants studied were still mucoid but resistant to UV radiation. Sixteen of the 17 revertants contained a mutation, sulA, that cotransduced with pyrD (21 min). A 2nd locus, sulB, was also found that cotransduced with leu (2 min). Studies with partial diploids (F''pyrD+ sulA+/pyrD36 sulA17 capR9 (lon) demonstrated that sulA+ is dominant to sulA; thus the indicated partial diploid is UV sensitive, but the haploid parent is UV resistant. Two other phenotypic traits of capR (lon) mutants were reversed by the sul mutation: complex medium-induced killing and the inability of phage .lambda. to efficiently lysogenize capR strains. On the basis of these and other results, a model is suggested to explain capR (lon) and sul gene interactions. The capR (lon) is a regulator gene for the structural genes sulA+ and sulB+. Derepression of both sul operons results in UV sensitivity and decreased ability of .lambda. to lysogenize, by inactivation of either sul+ protein by mutation to sul prevents these phenomena.