Helicase-deficient cysteine to glycine substitution mutants of Escherichia coli replication protein PriA retain single-stranded DNA-dependent ATPase activity. Zn2+ stimulation of mutant PriA helicase and primosome assembly activities.

The PriA replication protein of Escherichia coli guides the ordered assembly of the primosome, the mobile, multiprotein, bidirectional, DNA replication priming/helicase complex of which it is an integral part. Although the PriA protein is not essential for viability, primosome assembly via a PriA-dependent pathway is required for normal cellular replication and growth. The PriA protein itself is multifunctional. In addition to its role in directing primosome assembly, PriA is a site-specific, single-stranded DNA-dependent ATPase (dATPase) and a 3'-->5' DNA translocase and helicase. In an attempt to assess how each individual PriA activity is related to the others (i.e. can one activity function independently of the others or are they intrinsically coupled?), a series of site-directed mutations within priA have been created. priA encodes a cysteine-rich motif, the sequence of which suggests that this region of the protein may be involved in metal binding. Biochemical characterization of three purified cysteine to glycine substitution mutant PriA proteins revealed that these mutant proteins retained their site-specific single-stranded DNA-dependent ATPase activity. However, two of the three mutant proteins were completely incapable of any helicase activity. Residual helicase activity of the third mutant PriA protein could be stimulated 3-fold by the presence of low concentrations of Zn2+ ions. Primosomes assembled with the mutant PriA proteins were also defective in both their ability to act as bidirectional helicase complexes, as well as their ability to synthesize primers for extension by the DNA polymerase III holoenzyme. The results presented here suggest that the cysteine-rich region of PriA is indeed involved in metal binding and that single cysteine to glycine substitutions within this region result in the uncoupling of the ATPase activity of PriA from both its helicase activity and its ability to interact correctly with the DNA template and the six other primosomal proteins.