A ROLE FOR RESIDUE-151 OF LAMB IN BACTERIOPHAGE-LAMBDA ADSORPTION - POSSIBLE STERIC EFFECT OF AMINO-ACID SUBSTITUTIONS

LamB is the cell surface receptor for bacteriophage lambda. LamB misse nse mutations yielding resistance to h have been previously grouped in two classes. Class I mutants block growth of lambda with mild-type ho st range (lambda h(+)) but support growth of one-step extended-host-ra nge mutants (lambda h). Class II mutants block lambda h but support gr owth of two-step extended host range mutants (lambda hh). While Class I mutations occur at 11 different amino acid sites, in five distinct portions of LamB, all the Class II mutations analyzed previously corre spond to the same G-to-D change at amino acid 151. We generated by in vitro mutagenesis four different new substitutions at site 151 (to S, V, R, and C). Two of the mutants (G-151 --> V [G151V] and G151R) were of Class II, while the two others (G151S and G151C) were of Class I, d emonstrating that not only the site but also the nature of the substit utions at residue 151 was critical for the phage sensitivity phenotype s. The introduction of a negatively charged, a positively charged, or an aliphatic nonpolar residue at site 151 of LamB prevented both lambd a h(+) and lambda h adsorption, indicating that the block is not due t o a charge effect. In contrast to G151D, which was sensitive to all th e lambda hh phages, G151V and G151R conferred sensitivity to only fou r Of the five lambda hh phages. Thus, G151V and G151R represent a new subclass of Class II LamB mutations that is more restrictive with res pect to the growth of lambda hh. Our results agree with the hypothesi s that residue 151 belongs to an accessibility gate controlling the ac cess to the phage tight-binding site and that substitutions at this re sidue affect the access of the phage to the binding site in relation t o the size of the substitute side chain (surface area): the most restr ictive changes are G151V and G151R, followed to a lesser extent by G15 1D and then by G151S and G151C.