DETERMINANTS OF RECEPTOR SPECIFICITY OF COLIPHAGES OF THE T4 FAMILY -A CHAPERONE ALTERS THE HOST-RANGE

E. coli phages of the T4 family (T4, TuIa, TuIb) recognize their cellu lar receptors with a C-terminal region of protein 37. This protein, co mmon to all three phages, is present as a dimer located at the distal part of the long tail fibers and possesses a C-terminal domain consist ing of 40 to 70 highly conserved C-terminal residues, followed by a va riable region of 50 to 80 residues which is again followed by a highly conserved area. Protein 38, not being a component of the mature virio n, is required for dimerization of protein 37; this represents a non-c ovalent association of a structural protein. Seven host range mutants of TuIa or TuIb were analyzed which were able to use proteinaceous rec eptors other than those recognized by their parents. All had suffered amino acid substitutions within the variable region. It is concluded t hat in all probability it is this region which interacts directly with the cellular receptors. Conditional mutants of T4 are known which, wh en propagated at the non-permissive temperature (42 degrees C), yield phage of normal morphology but these are more or less unable to adsorb to cells. The causative amino acid substitutions were found both down stream and upstream from the variable area. Distortion of it in the mu tants could suggest a ''snap-back'' conformation of the tail fiber; th e conserved C-terminal region may fold back and expose the variable re gion as a loop at the tip of the fiber. One of the phage mutants (L93) , when grown at the permissive temperature, had lost the ability to us e the OmpC porin (a receptor for T4) as a receptor. A secondary mutant , able to do so, was isolated. An additional mutation, leading to one amino acid substitution, had occurred in gene 38. This mutant gene act ed in trans and caused a much enhanced temperature-sensitivity of infe ctivity without conferring temperature-sensitivity per se, i.e. the mu tant protein 38 apparently altered the conformation of the receptor-re cognizing area of the dimer of protein 37. A gene from phage lambda, a bout 40% identical to gene 38 of T4, complements gene 38 amber mutants . The corresponding protein also restored the ability of L93 to recogn ize OmpC but did not cause any such temperature-sensitivity. Hence, pr otein 38, classifying as a chaperone, appears to act instructively in conveying steric information to the target polypeptide.