ISOLATION OF A DNAE MUTATION WHICH ENHANCES RECA-INDEPENDENT HOMOLOGOUS RECOMBINATION IN THE ESCHERICHIA-COLI CHROMOSOME

The mechanism of recombination of tandem repeats in the chromosome of Escherichia coli was investigated by genetic means, Tandem repeats 624 bp long were introduced into the lacZ gene of E. coli and the efficie ncy of deletion of one repeat was compared in different recombination mutants, No effects of the recA, recBC, recF, ruvA or ruvA recG mutati ons were detected, Hence, tandem repeat deletion appears to not procee d via the RecBCD or RecF homologous recombination pathways. A new muta nt in which RecA-independent recombination is increased 15-fold was is olated, The mutation lies in the dnaE gene coding for the alpha subuni t of polymerase III: it is a Gly to Asp change at codon 133. Another d naE mutation, dnaE486, was tested and also shown to stimulate RecA-ind ependent recombination, It is proposed that tandem-repeat recombinatio n occurs by a replication slippage mechanism, RecA-independent recombi nation is also enhanced in a rep mutant, in which chromosomal replicat ion is slowed down by the absence of the Rep helicase, suggesting that replication pausing may facilitate slippage.