A mammalian protein complex that repairs double-strand breaks and deletions by recombination

Abstract

We have purified a high molecular weight complex (RC-1) from calf thymus nuclei that catalyzes a recombinational repair of double-strand gaps and deletions in DNA by gene conversion as well as cross-over events leading to cointegrant products. These have been detected by polymerase chain reaction analysis using oligonucleotide primer pairs that detect joined sequences originally present on only one or the other of the recombination substrates. RC-1 has an apparent molecular mass of about 550-600 kDa and contains at least five polypeptide chains: molecular masses about 230, 210, 160, 130, and 40 kDa. RC-1 contains a DNA polymerase, identified as DNA polymerase epsilon, that co-purifies with RC-1. A DNA ligase, most likely mammalian DNA ligase III, and a 5'-3' exonuclease also copurify with the RC-1. Most preparations of RC-1 contain low levels of a double-strand endonuclease, 3'-5' exonuclease and single-strand nuclease activities. However, DNA helicase, terminal deoxynucleotidyl transferase, or DNA topoisomerase I and II were not detected in RC-1. The DNA polymerase and DNA ligase in RC-1 can act in concert to repair a multiply gapped DNA to a covalently repaired duplex. The bovine single-strand-binding protein stimulates the formation of the recombination products and the repair reaction mentioned above about 4-fold.