| Sumario: | Changes in the intracellular concentration of ATP in Escherichia co/i cells expressing the SOS system after UV-irradiation were studied. Under these conditions, there is a dose-dependent increase in the concentration of this nucleotide which is synthetized by metabolization of the degradation products of the UV-damaged DNA through the activity of RecBC exonuclease. The ATP produced is then hydrolysed in the process of LexA repressor cleavage by the activated RecA protein. Furthermore, results obtained in UV-irradiated cultures growing in the presence of adenine (which increases the cellular pool of ATP), and in several ATP-synthase mutants, show that ATP availability after UV-irradiations is an important factor in the modulation of RecA protease activity, and as a consequence of the SOS genes transcription. Thus, cellular ATP concentration is limiting for the SOS system expression in cells UV-irradiated up to 40 J m 2, and in the recA441 (tta) mutant growing at 42 C. in which the RecA441 protein is spontaneously activated.
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