The 40.6-kDa subunit of DNA polymerase III of Escherichia coli is a sliding DNA clamp responsible for tethering the polymerase to DNA and endowing it with high processivity (Stukenberg, P. T., Studwell-Vaughan, P. S., and O'Donnell, M.(1991) J. Biol. Chem. 266, 11328-11334). UV irradiation of E. coli induces a smaller 26-kDa form of the subunit, termed *, that, when overproduced from a plasmid, increases UV resistance of E. coli (Skaliter, R., Paz-Elizur, T., and Livneh, Z.(1996) J. Biol. Chem. 271, 2478-2481). Here we show that this protein is synthesized from a UV-inducible internal gene, termed dnaN*, that is located in-frame inside the coding region of dnaN, encoding the subunit. The initiation codon and the Shine-Dalgarno sequence of dnaN* were identified by site-directed mutagenesis. The dnaN* transcript was shown to be induced upon treatment with nalidixic acid, and transcriptional dnaN*-cat gene fusions were UV inducible, suggesting induction of dnaN* at the transcriptional level. Analysis of translational dnaN*-lacZ gene fusions revealed that UV induction was abolished in strains carrying the recA56, lexA3, or rpoH mutations, indicating involvement of both SOS and heat shock stress responses in the induction process. Expression of dnaN* represents a strategy of producing several proteins with related functional domains from a single gene.

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