Escherichia coli rnh mutants lacking ribonuclease H (RNase H) activity can tolerate deletion of the origin of DNA Replication (delta oriC) and transposon-insertional inactivation of an initiator gene (dnaA::Tn10). Introduction of the recA200 allele encoding a thermolabile RecA protein into rnh- dnaA::Tn10 and rnh- delta oriC mutants strains rendered DNA synthesis and colony formation of these mutants temperature sensitive. The temperature sensitivity and the broth sensitivity (Srm-) of the rnh- dnaA::Tn10 recA200 strain was suppressed by the presence of plasmids (pBR322 derivatives) carrying dnaA+ only when the intact oriC site was present on the chromosome VSports手机版. Lack of RNase H activity neither promoted replication of minichromosomes (pOC24 and p lambda asn20) in the absence of required DnaA+ protein nor inhibited dnaA+-dependent minichromosome replication. These results led to the conclusion that RNase H is not directly involved in the events leading to initiation of DNA replication at oriC. Rather, it functions as a specificity factor by eliminating certain forms of RNA-DNA hybrids which could otherwise be used to prime DNA replication at sites other than oriC. .