Tail fiber protein located at the distal ends of the fibers that binds to the adhesion receptors on the host surface, thereby determining the host range. The phage can alter its tropism by modifying this protein. Variants are expressed through a diversity-generating retroelement (DGR) that creates mutant copies of a template repeat and replaces the end of the tail fiber receptor-binding protein with these sequences, thus changing the host range. Milliards of variants of the fiber receptor-binding protein can be created with this system. Homotrimer. Part of the tail fibers. The C-terminus contains a variable sequence in which nucleotide subtitutions can be introduced at 23 sites by the DGR.