Abstract
The Cro protein specified by bacteriophage lambda is a repressor essential for normal lytic growth of the virus, thus having a physiological role distinct from that of cI, the repressor that maintains lysogeny. We have purified a lambda-specific DNA-binding protein with the requirements for synthesis and biochemical activities expected for Cro protein from studies in vivo. As isolated, the protein appears to be a dimer of molecular weight approximately 18,000 with DNA-binding properties that are very similar, but not identical, to those of the cI protein. We infer that bacteriophage lambda uses the same regulatory region of DNA for two different DNA-binding repressor proteins with subtle differences in binding activity specialized for different physiological roles.