BACTERIOPHAGE-P4 DNA-REPLICATION

Replication of satellite phage P4 of Escherichia call is dependent on three phage-encoded elements: the origin (ori), a cis replication elem ent (crr), and the product of the alpha gene, gp alpha. In vitro P4 re plication is origin-specific resulting in monomeric form I DNA. DNA sy nthesis requires chromosomally encoded proteins DNA polymerase III hol oenzyme, SSB, DNA gyrase and probably topoisomerase I; host-encoded in itiation and priming functions are dispensable. The alpha protein is m ultifunctional in P4 replication, combining three activities in a sing le polypeptide chain. First, the protein complexes specifically with t ype I repeats at ori and err. Second, the helicase activity associated with gp alpha unwinds DNA with 3' --> 5' polarity. Third, the primase activity results in the synthesis of RNA primers. Defined sequence mo tifs in gp alpha correlate with the helicase and primase activities wh ich are arranged in distinct, separable domains. Primase activity is a ssociated with the N-terminal half of the protein, ori/crr binding wit h the C-terminal portion. A model for the initiation mechanism of P4 r eplication which resembles that of mammalian simian virus 40 is discus sed.