An unusual RNA polymerase binding site in the immunity region of phage lambda

RNA polymerase binds very tightly at a site called B_rex in the λ immunity region, to the left of the rex gene and about 600 nucleotides to the right of P_L. The complex formed is resistant to L M NaCl in the absence of nucleotide triphosphate. While in vitro little or no transcription is observed from B_rex, in vivo, when inserted in a plasmid vector which allows detection of its activity, it acts as an efficient promoter. We have mapped the site protected by RNA polymerase against DNase and determined its sequence which is abnormal compared that of an average promoter.