The amino-terminal fragment of the RecA protein was cloned in pUA27 plasmid and introduced in different repair mutants of F.scherichia coli. The UV-mediated induction of several SOS functions was then studied. Results show that the truncated RecA protein plays an important role in the UV-damaged DNA when the level of the chromosomal RecA protein is low. Furthermore, RecA protein inhibited the protease activity of the wild type RecA protein causing a decrease in the SOS system induction. All these data suggest that hybrid tetramers between both truncated and wild type RecA proteins may be formed, and as a consequence the ATPase capacity and active conformation of the wild type RecA protein are blocked.