β subunit of DNA polymerase III holoenzyme is induced upon ultraviolet irradiation or nalidixic acid treatment of Escherichia coli

Exposure of Escherichia coli to UV irradiation of nalidixic acid, which induce both the SOS and heat shock responses, led to a 3–4-fold increase in the amount of the β subunit of DNA polymerase III holoenzyme, as assayed by Western blot analysis using anti-β antibodies. Such an induction was observed also in a Δ rpoH mutant lacking the heat shock-specific σ 32 subunit of RNA polymerase, but it was not observed in aecA13 or lexA3 mutants, in which the SOS response cannot be induced. Mapping of transcription initiation sites of the dnaN gene, encoding the β subunit, using the S1 nuclease protection assay showed essentially no induction of transcription upon UV irradiation, indicating that induction is regulated primarily at the post-transcriptional level. Analysis of translational gene fusions of the dnaN gene, encoding the β subunit, to the lacZ reporter gene showed induction of β-galactosidase activity upon UV irradiation of cells harboring the fusion plasmids. Elimination of a 5′ flanking DNA sequence in which the dnaN promoters P1 and P2 were located, did not affect the UV inducibility of the gene fusions. Thus, element(s) present from P3 downstream were sufficient for the UV induction. The induction of the dnaN-lacZ gene fusions was dependent on the recA and lexA gene products, but not on the rpoH gene product, in agreement with the immunoblot analysis. The dependence of dnaN induction on the SOS regulators was not mediated via classical repression by the LexA repressor, since the dnaN promoter does not contain a sequence homologous to the LexA binding site, and dnaN mRNA was not inducible by UV light. This suggests that SOS control may be imposed indirectly, by a post-transcriptional mechanism. The increased amount of the β subunit is needed, most likely, for increased replication and repair activities in cells which have been exposed to UV radiation.