The mutant recA 441 and recA 430 proteins from Escherichia coli have single amino acid substitutions which distinguish each of them from the wild type recA protein. recA 441 mutants express SOS functions at 42 degrees C even without DNA damage whereas recA 430 mutants are totally unable to express SOS functions at any temperature. Both mutant recA proteins have altered protease functions. However, the amino acid substitutions in each protein are located at a considerable linear distance from one another in their respective polypeptide chains, the recA 441 protein having a lysine in place of a glutamic acid residue at position 68, while the recA 430 protein has a serine in place of a glycine at position 204.