GENETIC-ANALYSIS OF DOUBLE-STRAND BREAK REPAIR IN ESCHERICHIA-COLI

We had reported that a double-strand gap (ca. 300 bp long) in a duplex DNA is repaired through gene conversion copying a homologous duplex i n a recB21 recC22 sbcA23 strain of Escherichia coli, as predicted on t he basis of the double-strand break repair models. We have now examine d various mutants for this repair capacity. (i) The recE159 mutation a bolishes the reaction in the recB21C22 sbcA23 background. This result is consistent with the hypothesis that exonuclease VIII exposes a 3'-e nded single strand from a double-strand break. (ii) Two recA alleles, including a complete deletion, fail to block the repair in this recBC sbcA background. (iii) Mutations in two more SOS-inducible genes, recN and recQ, do not decrease the repair. In addition, a lexA (Ind-) muta tion, which blocks SOS induction, does not block the reaction. (iv) Th e recJ, recF, recO, and recR gene functions are nonessential in this b ackground. (v) The RecBCD enzyme does not abolish the gap repair. We t hen examined genetic backgrounds other than recBC sbcA, in which the R ecE pathway is not active. We failed to detect the double-strand gap r epair in a rec+, a recA1, or a recB21 C22 strain, nor did we find the gap repair activity in a recD mutant or in a recB21 C22 sbcB15 sbcC201 mutant. We also failed to detect conservative repair of a simple doub le-strand break, which was made by restriction cleavage of an inserted linker oligonucleotide, in these backgrounds. We conclude that the Re cBCD, RecBCD-, and RecF pathways cannot promote conservative double-st rand break repair as the RecE and lambda Red pathways can.