The role of gene A protein and E. coli rep protein in the replication of phi X 174 replicative form DNA

The gene A protein of bacteriophage phi X 174 initiates replication of super-twisted RFI DNA by cleaving the viral (+) strand at the origin of replication and binding to the 5' end. Upon addition of E. coli rep protein (single-stranded DNA dependent ATPase), E. coli single-stranded DNA binding protein and ATP, complete unwinding of the two strands occurs. Electron microscopic analyses of intermediates in the reaction reveal that the unwinding occurs by movement of the 5' end into the duplex, displacing the viral strand in the form of a single-stranded loop. Since unwinding will not occur in the absence of either gene A protein or rep protein, it is presumed that the rep protein interacts to form a complex with the bound gene A protein. Single-stranded DNA binding protein facilitates the unwinding by binding to the exposed single-stranded DNA. Further addition of the four deoxyribotriphosphates and DNA polymerase III holoenzyme to the reaction results in synthesis of viral (+) single-stranded circles in amounts exceeding that of the input template. A model describing the role of gene A protein and rep protein in duplex DNA replication is presented and other properties of gene A protein discussed.