Figure 38-3: The unusual structures of the Bxb1 capsid and major tail proteins. A. The capsid (major head subunit) of Bxb1 is similar to L5 and D29 capsids but contains a C-terminal extension that is related to a similar extension at the C-terminus of the Bxb1 major tail subunit. A similar element is also present at the C-termini of Rosebush gp15 and gp21; these may correspond to the major head and tail subunits of Rosebush, but the large N-terminal regions do not have database matches. Related segments are also found in a putative Bxz1 structural protein (gp124) and in the middle of both Bxb1 gp23 and Omega gp35 (not shown). The degree of amino acid sequence identity is shown for segments of vertically adjacent pairs. B. Amino acid sequence alignment of C-terminal extensions of Bxb1 proteins. Sequences were aligned using ClustalX. Residues present in all sequences are indicated by an asterisk above the alignment. Amino acid segments shown are as follows: Bxz2 gp27, 267-342; Bxb1 gp23, 393-448; Bxz1 gp124, 55-119; Omega gp35, 250-302; Bxb1 gp14, 303-397; Bxb1 gp19, 190-283; Rosebush gp15, 580-675; Rosebush gp21, 260-359. The function of this sequence motif is not known.