UV-irradiation of E. coli induces a two fold increase in ATP pool in the first 20 min. Afterwards, in RecA+ strains ATP level drops quickly below values of non irradiated cells. Mutants of E. coli defective in RecA protein or with either RecA protease activity deficient or protease resistant LexA repressor do not present this decrease, showing that it is due to cleavage of LexA repressor by RecA protease. The ATP increase produced in the first 20 min is dependent on RecBC exonuclease activity and it must be due to substrate level phosphorylation since an uncoupler such as dinitrophenol does not affect it.