Genetic study of a membrane protein: DNA sequence alterations due to 17 lamB point mutations affecting adsorption of phage lambda.

Gene lamB encodes the outer membrane receptor for phage lambda in Escherichia coli K12. We have determined the DNA sequence alterations of 17 lamB point mutations which result in resistance to phage lambda h+. The mutations correspond to four phenotypic classes according to the pattern of growth of three phages which use the lambda receptor: lambda h (a one-step host-range derivative of lambda h+), lambda hh* (a two-step host-range derivative of lambda h+) and K10 (another lambdoid phage). Fourteen mutations are of the missense type and correspond to Gly to Asp changes distributed as follows. One class I mutation is at position 382 of the mature lambda receptor. Seven class I* mutations, four of which at least are independent, are at position 401. Six independent class II mutations are at position 151. The three other (class III) mutations are of the nonsense type. They change codons TGG (Trp) into TAG (amber) at positions 120 (two mutations) and 351 (one mutation). Implications of these results for the topological organization of the lambda receptor as well as possible reasons for the limited number of altered sites detected are discussed.