Adsorption of bacteriophage lambda on the LamB protein of Escherichia coli K-12: point mutations in gene J of lambda responsible for extended host range.

摘要

LamB is the cell surface receptor for bacteriophage lambda. LamB missense mutations yielding resistance to lambda group in two classes. Class I mutants block the growth of lambda with the wild-type host range (lambda h+) but support the growth of one-step host range mutants (lambda h). Class II mutants block lambda h but support the growth of two-step host range mutant (lambda hh*) phages. To identify amino acid residues in the J protein (the tail fiber of phage lambda) responsible for the extended host range phenotype of mutants of phage (lambda h+), we selected a series of one-step (lambda h) and two-step (lambda hh*) host range mutants and analyzed their corresponding J genes. Three different class I LamB missense mutants (mutations at sites 247, 245, and 148) were used to select 11 independent, new, one-step host range mutants (lambda h phages). DNA sequence analysis revealed a single-amino-acid change in each case. The 11 alterations affected only three residues in the distal part of J, corresponding to a Val-->Ala change at site 1077 in five cases, a Thr-->Met change at site 1040 in three cases, and a Leu-->Pro change at site 1127 in three cases. Recombination experiments confirmed that in the cases tested, the mutations identified were indeed responsible for the extended host range phenotype. The class II LamB mutant (Gly-->Asp at site 151) was used to select two-step extended host range mutants (lambda hh* phages) from three new lambda h phages, corresponding to different amino acid modifications in the J protein (at sites 1040, 1077, and 1127). The new lambda hh* phages analyzed corresponded to either double or triple point mutations located at the distal end of the J protein. In all, seven residues involved in the extended host range properties of lambda mutants were identified in the distal part of the J protein, suggesting that the last C-terminal portion of the J protein participates directly in the adsorption of the phage onto LamB. In agreement with the fact that the lambda h mutants (and the lambda hh* mutants) could grow on all of the lamB class I mutations tested, we found tha the nature of the J mutations did not depend on the LamB class I mutant used to select them. This is interpreted as meaning that the mutated residues in the J protein and in the LamB mutants are not involved in allele-specific protein-protein interactions. Rather, the LamB mutations would block a step in phage adsorption, and this block would be overcome by the mutations in the J protein.