Control of UV induction of recA protein.

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Abstract
The basal level of recA protein in Escherichia coli K-12 was estimated by an immunoradiometric assay; it is .apprx. 1200 molecules/wild-type bacteria in midexponential phase of growth, slightly more in an excision-deficient (uvrA) strain, and markedly more in recF mutants. Kinetics of induction after UV irradiation showed a rapid increase of recA protein content, which reached a peak level after 60-90 min (20- to 55-fold amplification) and then decreased by dilution of the protein in the growing population. To obtain an identical extent of induction of recA protein, a 10-fold higher UV dose was necessary in a wild-type strain compared to the uvrA mutant strain. In the uvrA strain, the presence of one or only very few pyrimidine dimers on DNA was accompanied by a measurable increase of the constitutive level of recA protein; however, the unexcised dimers were unable to permanently induce the formation of recA protein. The derepressed promoter of recA gene is one of the strongest in E. coli. Its sequence displays many similarities with that of the strongest early promoters of phage T5. Mutants (umuC uvrB and recF uvrB) unable to carry out W-reactivation produced high levels of recA protein after UV irradiation. Apparently, the recF and umuC genes negatively control the regulation of recA protein level.