PHAGE-P4 ALPHA-PROTEIN IS MULTIFUNCTIONAL WITH ORIGIN RECOGNITION, HELICASE AND PRIMASE ACTIVITIES

Alpha protein of satellite phage P4 of Escherichia coli is multifunctional in P4 replication with three activities. First, the protein (subunit M(r) = 84 900) complexes specifically the P4 origin and the cis replication region required for replication. Alpha protein interacts with all six type I repeats (TGTTCACC) present in the origin. Second, associated with the alpha protein is a DNA helicase activity that is fueled by hydrolysis of a nucleoside 5' triphosphate. All common NTPs except UTP and dTTP can serve as cofactors. Strand separation of partial duplexes containing tailed ends that resemble a replication fork is preferred, although a preformed fork is not absolutely required for the enzyme to invade and unwind duplex DNA. Alpha protein catalyzes unwinding in the 3'-->5' direction with respect to the strand it has bound. Finally, the primase activity already demonstrated for alpha protein is due to synthesis of RNA primers. In vitro, alpha protein generates di- to pentaribonucleotides on single-stranded phage fd DNA. The predominant product is the dimer pppApG, on which most of the longer oligoribo-nucleotides are based. Using DNA oligonucleotides of defined sequence as templates, synthesis of pppApG was also detectable. To date, among prokaryotic and eukaryotic replication systems, gpalpha is the only protein known that combines three activities on one single polypeptide chain.