BRCT domain of DNA polymerase μ has DNA-binding activity and promotes the DNA polymerization activity

DNA polymerase mu (pol mu) catalyzes nonhomologous end-joining in DNA double-stranded break repair. Pol mu consists of an amino-terminal BRCA1 carboxyl-terminal homology (BRCT) domain and a pol beta-like region, which contains the catalytic site. By DNA cellulose column chromatography, using full-length pol mu and five different deletion mutants, we found that the amino-terminal region has double-stranded DNA (dsDNA)-binding activity. Pol mu without BRCT domain reduces the DNA polymerization activity when compared to full-length pol mu. Observation by atomic force microscopy showed that full-length pol mu binds to the ends and middle part of dsDNA. Pol mu lacking the amino-terminal region or with a mutation within the BRCT domain bound only to DNA ends, whereas the amino-terminal region with the BRCT domain bound to both the ends and the middle part of dsDNA (mpdDNA). Terminal deoxynucleotidyltransferase, which, like pol mu, belongs to the X family DNA polymerases, also bound to mpdDNA through its amino-terminal region.