Abstract
Escherichia coli ribonuclease H was purified to near-homogeneity and identified as the only additional factor required for initiation of in vitro Co1E1 DNA replication from the unique origin by RNA polymerase and DNA polymerase I. Both ribonuclease H activity and stimulating activity for Co1E1 DNA synthesis comigrate with the single protein band in gel electrophoresis. These two activities coincide throughout the process of purification. Some DNA synthesis takes place on covalently closed-circular DNA molecules other than Co1E1 DNA with the three purified enzymes. This DNA synthesis is suppressed by an Escherichia coli single-strand DNA binding protein and/or a high concentration of ribonuclease H. Negative superhelicity of template DNA is required for efficient primer formation. No evidence that supports involvement of ribonuclease III in initiation of Co1E1 DNA replication or its regulation was found.