Mutation-dependent suppression of recB21 recC22 by a region cloned from the Rac prophage of Escherichia coli K-12.

Using pBR322 as a vector, we cloned a 5.95-kilobase fragment of the Rac prophage together with 1.70 kilobases of a flanking Escherichia coli chromosome sequence. The resulting plasmid (pRAC1) was unable to suppress the mitomycin and UV sensitivity and recombination deficiency of a recB21 recC22 strain. Five spontaneous mitomycin-resistant derivatives contained deletion mutant plasmids. These plasmids also suppressed the UV sensitivity and recombination deficiency of their recB21 recC22 hosts. All five deletions were contained within a 2.45-kilobase EcoRI-to-HindIII segment of the plasmid. By substituting the corresponding 2.45-kilobase EcoRI-toHindIII fragments of Rac prophage isolated from sbcA+, sbcA6, and sbcA23 strains for the shortened segment of one of the deletion mutant plasmids, we were able to show that sbcA mutations map in this region. Also in this region is the site (or closely linked sites) at which previous studies had shown that insertion of Tn5 and IS50 leads to suppression of recB21 recC22. The sequence in this region that must be altered or circumvented to allow suppression is discussed. Also presented are data correlating the expression of nuclease activity with the degree of suppression.