A point mutation at the C-terminal half of the repressor of temperate mycobacteriophage L1 affects its binding to the operator DNA

Abstract

The wild-type repressor CI of temperate mycobacteriophage Ll and the temperature-sensitive (ts) repressor CIts391 of a mutant Ll phage, L1cIts391, have been separately overexpressed in E. coli. Both these repressors were observed to specifically bind with the same cognate operator DNA. The operator-binding activity of CIts391 was shown to differ significantly than that of the CI at 32 to 42degreesC. While 40-95% operator-binding activity was shown to be retained at 35 to 42degreesC in CI, more than 75% operator-binding activity was lost in CIts391 at 35 to 38degreesC, although the latter showed only 10% less binding compared to that of the former at 32degreesC. The CIts391 showed almost no binding at 42degreesC. An in vivo study showed that the CI repressor inhibited the growth of a clear plaque former mutant of the Ll phage more strongly than that of the CIts391 repressor at both 32 and 42degreesC. The half-life of the CIts391-operator complex was found to be about 8 times less than that of the Cl-operator complex at 32degreesC. Interestingly, the repressor-operator complexes preformed at WC have shown varying degrees of resistance to dissociation at the temperatures which inhibit the formation of these complexes are inhibited. The CI repressor, but not that of CIts391, regains most of the DNA-binding activity on cooling to 32degreesC after preincubation at 42 to 52degreesC. All these data suggest that the 131(st) proline residue at the C-terminal half of CI, which changed to leucine in the CIts391, plays a crucial rote in binding the Ll repressor to the cognate operator DNA, although the helix-turn-helix DNA-binding motif of the Ll repressor is located at its N-terminal end.