The bacteriophage 434 repressor dimer preferentially undergoes autoproteolysis by an intramolecular mechanism.

Inactivation of the lambdoid phage repressor protein is necessary to induce lytic growth of a lambdoid prophage. Activated RecA, the mediator of the host SOS response to DNA damage, causes inactivation of the repressor by stimulating the repressor's nascent autocleavage activity. The repressor of bacteriophage lambda and its homolog, LexA, preferentially undergo RecA-stimulated autocleavage as free monomers, which requires that each monomer mediates its own (intramolecular) cleavage. The cI repressor of bacteriophage 434 preferentially undergoes autocleavage as a dimer specifically bound to DNA, opening the possibility that one 434 repressor subunit may catalyze proteolysis of its partner subunit (intermolecular cleavage) in the DNA-bound dimer. Here, we first identified and mutagenized the residues at the cleavage and active sites of 434 repressor. We utilized the mutant repressors to show that the DNA-bound 434 repressor dimer overwhelmingly prefers to use an intramolecular mechanism of autocleavage. Our data suggest that the 434 repressor cannot be forced to use an intermolecular cleavage mechanism. Based on these data, we propose a model in which the cleavage-competent conformation of the repressor is stabilized by operator binding.