Receptor-recognizing proteins of T-even type bacteriophages. The receptor-recognizing area of proteins 37 of phages T4 TuIa and TuIb.

Escherichia coli phages of the T4 family (T4, TuIa, TuIb) recognize their cellular receptors by means of a C-terminal region of protein 37; a dimer of this polypeptide (1026 residues in T4) is located at the distal part of the long tail fibers. Virions of the T2 family use protein 38 (which is attached to the free end of protein 37) for this purpose. The corresponding areas of genes 37 belonging to TuIa and TuIb were cloned and sequenced. Comparison of the deduced protein primary structures, including those of T4 and lambda Stf (Stf most likely representing a subunit of the side tail fibers of phage lambda) showed that an area of 70 to 100 residues is characterized by very variable sequences, while the sequences of the adjacent 43 to 44 C-terminal residues as well as those upstream from the variable region are highly homologous. The variable regions are flanked and interrupted seven or eight times by the motif His-x-His-y, with x and y most often being Ser or Thr; furthermore, the locations of these repeated tetrapeptides are conserved. Using hybrid phages obtained by recombination of one phage with cloned fragments of gene 37 of another, it could be shown that the area of this gene encoding receptor specificity includes the variable area. The situation is analogous to the known receptor-recognizing region of proteins 38 belonging to the T2-type family, except that the repeating sequence is of a different nature. In T4, receptor specificity is coded for by 382 base-pairs of the 3'-end of the gene, starting exactly at the variable area. It was found that T4 can use the outer membrane protein OmpC or lipopolysaccharide as receptors with the same efficiency, and it is proposed that the 70 residues of the variable part of the protein serve to bind to both ligands.