AN SOS-INDUCIBLE DEFECTIVE RETRONPHAGE (PHI-R86) IN ESCHERICHIA-COLI STRAIN-B

In Escherichia coli, RecA protein regulates the DNA damage-inducible survival-enhancing SOS response. Mutant allele recA730, which causes constitutive SOS expression, is lethal at high temperatures in B/r, a derivative of wild-type B, but not in K-12 or in certain B/r-K-12 hybrids. We present evidence that killing is due to SOS induction of a defective retronphage, phiR86, which is integrated into the B/r chromosome at 19 min, but is absent in K-12. PhiR86 contains retron EC-86 which encodes reverse transcriptase and a small multicopy DNA-RNA complex, msDNA-RNA. Induction of phiR86 in recA730 B/r strains results in inhibition of host DNA replication before cell death. A retronphage 'killer' gene, ORF336, when overexpressed from a plasmid, causes similar effects without SOS induction. PhiR86 is not detectably u.v.-inducible in recA+ strains.