Chaperone involved in tail fiber assembly. Expressed alternatively with tail fiber assembly protein U'. Remains associated to the tail fiber and participates in the host receptor binding. Two alternate tail fiber assembly proteins U and U' are encoded extending the host range of the virus. Tail fiber. Expressed alternatively with tail fiber assembly protein U'. The switch from S-U to S'-U' is performed through inversion of a DNA segment called C by the phage invertase protein Cin. Belongs to the tfa family.