Structure andexpression during development of Drosophila melanogaster geneforDNApolymerase oc

TheDrosophila melanogaster geneandcDNAwhich spantheentire openreading frame forDNApolymerase a,werecloned, andtheir nucleotide sequences were determined. Thegeneconsists of6exonsseparated by5short introns. Themajor transcription initiation site waslocalized 85bpupstream fromtheinitiation codon. Thenucleotide sequence oftheopenreading frame revealed apolypeptide of1,505aminoacidresidues withamolecular weight of170,796. Theaminoacid sequence ofthepolypeptide was37%homologous withthatofthecatalytic subunit ofhumanDNA polymerase a.Thissequence contains sixregions, the orders andaminoacidsequences ofwhicharehighly conserved among a numberofotherviral and eukaryotic DNApolymerases. Wefound7aminoacid residues intheregion between the639thand758th positions, identical tothose essential fortheactive site ofEscherichia coil DNA polymerase I-associated 3'-5'exonuclease. Thus, theexonuclease activity may beassociated withDrosophila DNA polymerase a. Levels oftheDNApolymerase a mRNAwerehighin unfertilized eggsandearly embryos, relatively highin adult female flies andsecond-instar larva, andlowin bodies atother stages ofdevelopment. Thisfeature of theexpression issimilar tothat oftheproliferating cell nuclear antigen (anauxiliary protein ofDNApolymerase 6)andseemstocoincide withtheproportions of proliferating cells invarious developmental stages. As thehalflife ofthemRNA forDNApolymerase a in cultured Drosophila Kccells was15min,expression oftheDNA polymerase a geneisprobably strictly regulated atthestepoftranscription.