Phosphorylation of Replication Protein A: A Role for cdc2 Kinase in G1/S Regulation

Among the various phases of the cell cycle, we have concentrated on the regulation of the S phase. Our approach is to define biochemically the components of the DNA replication apparatus and then to analyze how the various components are regulated as cells enter, execute, and exit the S phase. Working from the substrate, namely the DNA replication apparatus, backward to the regulatory molecules would complement the more common genetic approach, which first uncovers the regulatory molecules and then works forward to the regulated substrates. Fusion of animal cells at various stages in the cell cycle has shown that S-phase nuclei contain a positive activator of DNA replication which is absent in G1 cells and which could hasten the onset of S phase in G1 nuclei (Rao and Johnson 1970). Hence, one of our goals is to identify biochemically this positive activator, tentatively designated S-phase promoting factor (SPF). Furthermore, once...