Homologous interactions of lambda repressor and lambda Cro with the lambda operator.

Lambda repressor and lambda Cro bind to the same six sites on the phage chromosome but with different relative affinities. Nucleotides at certain positions in the operator are conserved in all sites, as are amino acids at certain positions in the recognition alpha-helices of repressor and Cro. Here we focus on one of the conserved amino acids, a serine found at position 2 of each recognition helix. We show that, contrary to a previous model, both serines contact the same conserved position in the operator, position 4. We suggest a simplified view of how repressor and Cro recognized similar operator sites but distinguish differently among them.