Arrangement of the gp18 domains in the extended (A) and the contracted (B) tail reprinted from . Domains I, II and III of gp18M are colored blue, olive green and orange red, respectively. The same color scheme is used in (C) the linear sequence diagram of the full-length gp18 and on the ribbon diagram of the gp18M structure. In (B) a part of the domain II from the next disk that becomes inserted between the subunits is shown in bright green. In both extended and contracted sheaths the additional density corresponds to domain IV of gp18 and the tail tube.