Chaperone involved in tail fiber assembly. Remains associated to the tail fiber and participates in the host receptor binding. Binds to the primary receptor (Probable). Two alternate tail fiber assembly proteins U and U' are encoded extending the host range of the virus. Tail fiber. Expressed in the late phase of the viral replicative cycle. Expression of late genes is activated by the viral late transcription activator C. 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 G by the phage invertase protein Gin. Viral particles lack tail fibers in inversion-defective gin mutants (blocked in G+ orientation; no switch to S'-U'). The orientation of the G segment is defined as G+ and G-. G+ orientation provides S-U fibers whereas G- provides S'-U' fibers. S-U and S'-U' dont have the same host range (e.g. respectively E.coli and C.freundii). Belongs to the tfa family.