DNA replication in Drosophila embryos: Proteins at the fork.

Drosophila embryos provide a rich source of replicative enzymes. Also, the duration of 2 hour embryo DNA synthesis phase of the cell cycle is approximately 6-fold shorter than the more regulated 9 hour embryo S phase. Thus, Drosophila embryos are a good system in which to explore the mechanisms and regulation of DNA replication. Early stage, 2 hour embryos contain at least two distinct DNA polymerases, DNA polymerase α and δ, as determined by associated enzymatic activities (DNA primase and 3'-5' exonuclease), inhibitor studies, immunologic reactivity, and processivity measurements. The observation that a δ-type enzyme with an inherent 3'-5' exonuclease activity is present in Drosophila embryos is a novel observation, and may have important implications for maintaining the fidelity of embryonic DNA synthesis. Both 2 hour and 9 hour embryos contained similar replicative activities. The enzymes which copurified with 2 hour and 9 hour DNA polymerases include a DNA primase activity with DNA polymerase α; and a 3'-5' exonuclease, 5'-3' exonuclease, and DNA ligase activities with DNA polymerase δ. The association of these activities suggests that DNA polymerase α-associated enzymes may initiate Okazaki fragments, which would then be elongated and ligated by the DNA polymerase δ-associated group