Sequence-specific DNA-binding protein required for the establishment and maintenance of lysogeny. This protein recognizes the sequence 5'-ATTTATTAGAGCA[AT]T-3'. Binding to the operator DNA of c1 repressor is sensitive to N-ethylmaleimide. Thermolability of the mutant P1c1.100 seems to be due to an alteration in the C-terminal half of the repressor.