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 comple-ment the more common genetic approach, which first uncovers the regulatory molecules and then works for-ward 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 G, cells and which could hasten the onset of S phase in G