RepB_primase is a DNA-primase produced by P4-like phages. It is a zinc-independent primase unlike Pri-type primases. It takes up a dumbbell shaped consisting of an N-terminal catalytic domain separated by a long alpha-helix plus tether and a C-terminal helical-bundle domain. Primases are necessary for phage replication. RepBprime primases such as in this family recognise both ssiA and ssiB, ie only 1 single-stranded primase initiation site on each strand, independently of each other and then synthesise primers that are elongated by DNA polymerase III. The phage is thus replicated exclusively in leading strand mode [1].

1: Structure and function of primase RepB' encoded by broad-host-range plasmid RSF1010 that replicates exclusively in leading-strand mode. Geibel S, Banchenko S, Engel M, Lanka E, Saenger W; Proc Natl Acad Sci U S A. 2009;106:7810-7815. PMID:19416864