Increased episomal replication accounts for the high rate of adaptive mutation in recD mutants of Escherichia coli

Adaptive mutation has been studied extensively in FC40, a strain of Escheri chia coli that cannot metabolize lactose (Lac(-)) because of a frameshift m utation affecting the lacZ gene on its episome. recD mutants of FC40, in wh ich the exonuclease activity of RecBCD (ExoV) is abolished but its helicase activity is retained, have an increased rate of adaptive mutation. The res ults presented here show that, in several respects, adaptive mutation to La c(+) involves different mechanisms in recD mutant cells than in wild-type c ells. about half of the apparent increase in the adaptive mutation rate of recD mutant cells is due to a RecA-dependent increase in episomal copy numb er and to growth of the Lac- cells on the lactose plates. The remaining inc rease appears to be due to continued replication of the episome, with the e xtra copies being degraded or passed to recD(+) recipients. In addition, th e increase in adaptive mutation rate in recD mutant cells is (i) dependent on activities of the single-stranded exonucleases, RecJ and ExoI, which are not required for (in fact, slightly inhibit) adaptive mutation in wild-typ e cells, and (ii) enhanced by RecG, which opposes adaptive mutation in wild -type cells.