We describe here two novel mouse and human DNA polymerases: one (pol λ) has homology with DNA polymerase β while the other one (pol µ) 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 λ is highest in testis and fetal liver, while expression of pol µ 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 µ gene, most of which affect the polymerase domain. Expression of mRNA of both polymerases is downregulated upon treatment by DNA damaging agents (UV light, γ-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.

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This paper discusses two aspects of immunoglobulin (Ig) gene hypermutation. In the ¢rst approach, a transcription termination signal is introduced in an Ig light chain transgene acting as a mutation substrate, and transgenic lines are generated with control and mutant transgenes integrated in tandem. Analysis of transcription levels and mutation frequencies between mutant and control transgenes clearly dissociates transcription elongation and mutation, and therefore argues against models whereby speci¢c pausing of the RNA polymerase during V gene transcription would trigger an error-prone repair process. The second part reports the identi¢cation of two novel b-like DNA polymerases named Pol l and Pol m, one of which (Pol m) represents a good candidate for the Ig mutase due to its higher lymphoid expression and its similarity with the lymphoid enzyme terminal deoxynucleotidyl transferase. Peculiar features of the expression of this gene, including an unusual splicing variability and a splicing inhibition in response to DNA-damaging agents, are discussed.