Possible Involvement of DNA Polymerase I in Excision of RNA from Col E₁ DNA in vivo

Abstract

THE small colicinogenic plasmid E¹ (Col E₁) has a unique requirement for DNA polymerase I for its replication and/or maintenance in Escherichia coli^1,2. There is evidence that RNA polymerase is involved in the replication of this and other DNAs^3–6 and supercoiled Col E¹ DNA, containing a segment of RNA, has been isolated from E. coli cells, treated with chloramphenicol⁷. Several recent reports suggest that RNA, to which the growing DNA chain is covalently linked^5,6, may serve as a primer for the DNA replicase. The short RNA chains are excised under normal conditions during the further replication process. Several enzymes which are able to degrade RNA in DNA-RNA hybrids have been reported^8–10. Very little, however, is known about the enzyme(s) involved in the excision process in vivo. Here we present evidence that Col E¹ DNA, isolated from a mutant strain with a temperature sensitive DNA polymerase I activity (polA12), consists of a substantial fraction of supercoiled DNA containing RNA. This suggests that DNA polymerase I may be involved in the degradation of RNA during Col E¹ DNA synthesis.