Identification and characterization of human DNA polymerase β2, a DNA polymerase β-related enzyme

The BRCA1 COOH terminus (BRCT) motif is present in many nuclear proteins that contribute to cell cycle regulation or DNA repair. Polymerase chain reaction-based screening with degenerate primers targeted to the BRCT motif resulted in the isolation of a human cDNA for a previously unidentified DNA polymerase (designated DNA polymerase beta 2) that is closely related to DNA polymerase beta (Pol beta). The predicted Pol beta 2 protein contains a BRCT motif in its NH2-terminal region; its COOH-terminal region exhibits 33% sequence identity to a corresponding region of human Pol beta. The Pol beta 2 gene is expressed in a tissue-specific manner, with transcripts being most abundant in testis. A fusion con struct comprising Pol beta 2 and green fluorescent protein exhibited a predominantly nuclear localization in transfected HeLa cells. Recombinant human Pol beta 2 from insect cells exhibited substantial DNA polymerase activity, but it did not possess terminal deoxyribonucleotidyl transferase activity. A truncated Pd beta 2 mutant lacking the BRCT motif retained substantial DNA polymerase activity, whereas a mutant Pol beta 2 with two alanine point mutations within the DNA polymerase active site did not. These results indicate that Pol beta 2 is a Pol beta-related DNA polymerase with a BRCT motif that is dispensable for its polymerase activity.