Two modes of PriA binding to DNA

The role of PriA, required for the assembly of the phi X174-type primosome on DNA, in cellular DNA replication has been unclear since its discovery. R ecent evidence, based on the phenotypes of strains carrying priA null mutat ions, has led to proposals that the primosome assembly activity of PriA was required to load replication forks at intermediates such as D loops during homologous recombination. McGlynn et al. (McGlynn, P., Al-Deib, A. A., Liu , J., Marians, K. J., and Lloyd, R. G, (1997) J. Mol. Biol. 270, 212-221) d emonstrated that PriA could, in fact, bind D loops. We show here that there are two modes of stable binding of PriA to DNA One mode, in which the enzy me binds 3'-single-stranded extensions from duplex DNAs, presumably reflect s the 3' --> 5' DNA helicase activity of PriA. The D loop DNA binding activ ity of PriA can be accounted for by the second mode, where the enzyme binds bent DNA at three strand junctions.