Function and Properties of RP4 DNA Primase

Abstract

Plasmid-specific DNA primase activity was detected by an in vitro assay for the conversion of single-stranded circular DNA of small phages to its duplex form (1). A survey of plasmids representative of most of the known incompatibility groups (2) revealed that several of these plasmids specify this activity (Table 1). Existence of a plasmid-encoded DNA primase explains the discovery that IncI plasmids can partially suppress the effect of temperature-sensitive dnaG mutations in Escherichia coli (3). All the plasmids listed in Table 1 specifying primase activity suppress the dnaG3 mutation as measured by colony-forming ability at the nonpermissive temperature, thus providing a potent in vivo assay for this class of enzymes. It is also known that expression with plasmids derepressed for functions of conjugal DNA transfer and for pilus synthesis leads to higher levels of primase than with the corresponding wild-type plasmids (1, 2, 5, 7). This correlation suggests that the enzyme may play a role in conjugal DNA synthesis. Studies with the IncIα plasmid ColIb have indicated that plasmid primase is required to initiate efficient synthesis of DNA complementary to the transferred strand in the recipient cell, with the protein being supplied by the donor parent and probably transmitted between the mating cells (11).