C-TERMINAL DELETIONS CAN SUPPRESS TEMPERATURE-SENSITIVE MUTATIONS ANDCHANGE DOMINANCE IN THE PHAGE-MU-REPRESSOR

Mutations in an N-terminal 70-amino acid domain of bacteriophage Mu's repressor cause temperature-sensitive DNA-binding activity. Surprising ly, amber mutations can conditionally correct the heat-sensitive defec t in three mutant forms of the repressor gene, cts25 (D43-G), cts62 (R 47-Q) and cts71 (M28-1), and in the appropriate bacterial host produce a heat-stable Sts phenotype (for survival of temperature shifts). Sts repressor mutants are heat sensitive when in supE or supF hosts and h eat resistant when in Sup(o) hosts. Mutants with an Sts phenotype have amber mutations at one of three codons, Q179, Q187, or Q190. The Sts phenotype relates to the repressor size: in Sup(o) hosts sts repressor s are shorter by seven, 10, or 18 amino acids compared to repressors i n supE or supF hosts. The truncated form of the sts62-1 repressor, whi ch lacks 18 residues (Q179-V196), binds Mu operator DNA more stably at 42 degrees in vitro compared to its full-length counterpart (cts62 re pressor). In addition to influencing temperature sensitivity, the C-te rminus appears to control the susceptibility to in vivo Clp proteolysi s by influencing the multimeric structure of repressor.