Modulation of phage Mu repressor DNA binding and degradation by distinct determinants in its C-terminal domain

Rapid degradation of the bacteriophage Mu immunity repressor can be induced in trans by mutant, protease-hypersensitive repressors (Vir) with an altered C-terminal domain (CTD). Genetic and biochemical analysis established that distinct yet overlapping determinants in the wild-type repressor CTD modulate Vir-induced degradation by Escherichia coli CIpXP protease and DNA binding by the N-terminal DNA-binding domain (DBD). Although deletions of the repressor C-terminus resulted in both resistance to CIpXP protease and suppression of a temperature-sensitive DBD mutation (cfs62), some cysteine-replacement mutations in the CTD elicited only one of the two phenotypes. Some CTD mutations prevented degradation induced by Vir and resulted in the loss of intrinsic CIpXP protease sensitivity, characteristic of wild-type repressor, and at least two mutant repressors protected Vir from proteolysis. One protease-resistant mutant became susceptible to Vir-induced degradation when it also contained the cts62 mutation, which weakens DNA binding but apparently facilitates conversion to a protease-sensitive conformation. Conversely, this CTD mutation was able to suppress temperature sensitivity of DNA binding by the cts62 repressor. The results suggest that determinants in the CTD not only provide a cryptic CIpX recognition motif but also direct CTD movement that exposes the motif and modulates DNA binding.