RNase H confers specificity in the dnaA-dependent initiation of replication at the unique origin of the Escherichia coli chromosome in vivo and in vitro.

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Abstract
E. coli rnh mutants defective in RNase H activity display the features of previously described sdrA (stable DNA replication) and dasF (dnaA suppressor) mutants: sustained DNA replication in the absence of protein synthesis, lack of requirement for dnaA protein and the origin of replication (oriC) and sensitivity of growth to a rich medium. The sdrA mutants (selected for continued DNA replication in the absence of protein synthesis) and the dasF mutants (selected as dnaA suppressors) are defective in RNase H activity, measured in vitro. A 760-base-pair fragment containing the rnh+ structural gene complements the phenotype of each of the rnh, sdrA and dasF mutants, indicative of a single gene. One function of RNase H in vivo is in the initiation of a cycle of DNA replication at oriC dependent on dnaA+. In keeping with these results, RNase H contributes to the specificity of dnaA protein-dependent replication initiated at oriC in a partially purified enzyme system.