DNA Synthesis on a Double-stranded DNA Template by the T4 Bacteriophage DNA Polymerase and the T4 Gene 32 DNA Unwinding Protein

T4 DNA polymerase cannot use a nicked duplex DNA molecule as a template-primer for DNA synthesis, apparently because it is unable to displace the 5’ end of the strand paired to the strand serving as the template. Addition of the T4 phage gene 32 DNA unwinding protein (32-protein) facilities strand displacement and allows the T4 DNA polymerase to synthesize DNA using nicked duplex T7 DNA as the template-primer. The first product of this reaction is a rapidly renaturable DNA which is found to contain a few large branches when examined by electron microscopy. Later in the reaction there is also synthesis of a product containing predominantly alternating residues of dAMP and dTMP.