Isolation and characterization of herC, a mutation of Escherichia coli affecting maintenance of ColE1.

Two modes of ColE1 DNA replication are known, one dependent on RNase H, and the other RNase H independent. The cer114 mutant of the ColE1 replicon is defective in both modes and carries a single base pair alteration 95 bp upstream of the replication origin. An Escherichia coli mutant which restored maintenance of the cer114 replicon was isolated. This host suppressor mutant is defective in RNase H and carries a herC mutation located at 62 min of the E. coli chromosome. The herC mutation is recessive to its wild-type allele and supports maintenance of the mutant replicon in the absence of RNase H. The herC mutation alone conferred cold-sensitive growth, suggesting that the herC gene product is essential for cell growth. The 1832 bp E. coli DNA fragment, containing the wild-type allele of the herC mutation, was cloned and an open reading frame for the HerC protein was determined.