Derepression of bacteriophage mu transposition functions by truncated forms of the immunity repressor

To trigger bacteriophage Mu transposition and replication in response to physiological signals, its immunity repressor must be synchronously inactivated. Two repressor mutants (Vir), which have an altered C-terminal domain and are highly susceptible to degradation by ClpXP protease, confer a dominant negative phenotype by promoting degradation of the wild-type repressor. To search for other modified repressors that can induce Mu derepression in vivo and to determine what part of the inducing repressor molecules are needed to target the unmodified repressor population, repressor peptides with nested deletions starting at the C-terminal end were constructed. Such peptides with a C-terminal ssrA degradation tag promoted a sharp reduction in cellular levels of full-length unmodified repressor, a process largely dependent upon the clpP protease function. Only the repressor DNA-binding domain, located at the N-terminal end, was required in tagged peptides to target unmodified repressor. In addition, some repressor peptides containing the DNA-binding domain promoted derepression without the clpP function, being able to promote repressor inactivation without promoting its degradation. None of the modified repressors could promote derepression if immunity was established by a mutant repressor lacking 18 residues at its C-terminal end. The results indicate that inducing repressor peptides influence the function of the C-terminal domain of the intact repressor, a domain that regulates its degradation and DNA binding. They suggest the possibility that tagged repressor molecules, produced by stalled ribosomes, can be inducers of transposition under starvation conditions.