On the nature of sbcA mutations in E. coli K12

Summary We have recently shown (Kaiser and Murray 1979) that many E. coli K12 strains carry a defective prophage (Rac) located a few minutes clockwise of the trp operon on the genetic map. The Rac genome contains recE, the determinant for the ATP-independent exonuclease, ExoVIII. E. coli K12 strains which carry sbcA mutations express recE constitutively. This paper describes an investigation of several such strains. We show that the SbcA phenotype may arise from more than one type of mutational change. The most readily explained SbcA phenotype is that of sbcA8 strains in which a large section of the Rac genome (including one hybrid attachment site and probably the prophage repressor gene) is deleted. Three sbcA^- strains carry multiple (and probably tandemly repeated) copies of the Rac genome while two others carry a single Rac prophage that is indistinguishable in its hybridisation behaviour from that carried by sbcA⁺ strains.