DnaA- and PriA-dependent primosomes: two distinct replication complexes for replication of Escherichia coli chromosome.

Enzymatic analyses of primosome assembly at chromosomal and plasmid origins as well as that at single-stranded replication origins revealed the presence of two distinct primosomes in Escherichia coli for primer RNA synthesis and duplex unwinding. A DnaA-dependent primosome is assembled at oriC, the chromosomal origin of Escherichia coli, as well as at the A site, a single-stranded DNA hairpin containing a dnaA box sequence within its stem. In contrast, PriA protein recognizes a hairpin, called n'-pas (primosome assembly site), and initiates assembly of the phiX174-type PriA-dependent primosome in conjunction with other prepriming proteins. Genetic analyses of the prepriming proteins required specifically for the latter primosome strongly suggested that it is responsible for RecA-dependent, DnaA/oriC-independent replication of the Escherichia coli chromosome. Furthermore, primosome assembly in replication of various plasmids may also be classified into either DnaA-dependent or PriA-dependent type. We propose that Escherichia coli possesses two distinct, mutually exclusive primosomes which are differentially utilized by the chromosome as well as by the plasmids. PriA protein appears to be conserved in a wide range of prokaryotic species, and we will also discuss possible biological function of the PriA-dependent primosome in the process of responses to DNA damages.