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Nuclease Activities Involved in DNA Replication

David R. Brown, Jerard Hurwitz, Danny Reinberg, Stephen L. Zipursky


The genomes of both the icosahedral and filamentous single-stranded DNA bacteriophages code for site-specific endonucleases essential for duplex phage DNA replication in vivo. Two prototypic and extensively studied endonucleases are the φX174 gene-A protein and the fd gene-2 protein. The proteins function at equivalent points in the closely related life cycles of these two phage types. They initiate duplex DNA synthesis by introducing a single specific nick in the viral (+) strand of supercoiled replicative form (RFI) DNA, and terminate viral strand synthesis such that unit-length circular progeny molecules are produced. Early attempts to understand the properties and functions of these endonucleases involved the analysis of effects of mutations in the φX gene A and the fd gene 2 on specific aspects of the phage life cycles. More recently, emphasis has been placed on the characterization of activities associated with the purified gene products. The roles played by the gene-A protein and the gene-2 protein in the life cycles of icosahedral and filamentous phages, respectively, will be described below, followed by a review of the currently characterized enzymatic activities. In addition, the φX174 A* protein, a related but poorly understood endonuclease, will be discussed. Table 1 summarizes the properties of the φ X gene-A and -A* proteins and the fd gene-2 protein, and serves as an outline of the information to follow.

Role of Site-specific Endonucleases in Phage Life Cycles
The viral DNA of single-stranded circular (SS[C]) DNA phages replicates by way of a...

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