Biochemical characterization of L1 repressor mutants with altered operator DNA binding activity

A mycobacteriophage-specific repressor with the enhanced operator DNA binding activity at 32°C and no activity at 42°C has not been generated yet though it has potential in developing a temperature-controlled expression vector for mycobacterial system. To create such an invaluable repressor, here we have characterized four substitution mutants of mycobacteriophage L1 repressor by various probes. The W69C repressor mutant displayed no operator DNA binding activity, whereas, P131L repressor mutant exhibited very little DNA binding at 32°C. In contrast, both E36K and E39Q repressor mutants showed significantly higher DNA binding activity at 32°C, particularly, under in vivo conditions. Various mutations also had different effects on the structure, stability and the dimerization ability of L1 repressor. While the W69C mutant possessed a distorted tertiary structure, the P131L mutant dimerized poorly in solution at 32°C. Interestingly, both these mutants lost their two-domain structure and aggregated rapidly at 42°C. Of the native and mutant L1 repressor proteins, W69C and E36K mutants appeared to be the least stable at 32°C. Studies together suggest that the mutants, particularly P131L and E39Q mutants, could be used for creating a high affinity temperature-sensitive repressor in the future.