Two novel human and mouse DNA polymerases of the polX family

We describe here two novel mouse and human DNA polymerases: one (pol lambda) has homology with DNA polymerase beta while the other one (pol mu) is closer to terminal deoxynucleotidyltransferase. However both have DNA polymerase activity in vitro and share similar structural organization, including a BRCT domain, helix-loop-helix DNA-binding motifs and polymerase X domain. mRNA expression of pol lambda is highest in testis and fetal liver, while expression of pol mu is more lymphoid, with highest expression both in thymus and tonsillar B cells. An unusually large number of splice variants is observed for the pol mu gene, most of which affect the polymerase domain. Expression of mRNA of both polymerases is down-regulated upon treatment by DNA damaging agents (UV light, gamma-rays or H(2)O(2)). This suggests that their biological function may differ from DNA translesion synthesis, for which several DNA polymerase activities have been recently described. Possible functions are discussed.