Structure of normal and contracted tail sheaths of T4 bacteriophage.

The structural arrangement of protein subunits in extended and contracted tail sheaths of T4 bacteriophage has been studied by optical diffrsctometry of electron micrographs. The analysis of such diffraction patterns shows that the extended sheath consists of annuli of six subunits, these being arranged in a helix of close to seven annuli in two turns. The annuhrs repeat in the helix direction is 38-l A. For the contracted sheath the axial periodicity is found to be 32 A, which must arise from twice the repeat of the extended sheath. The symmetry of the contracted sheath diffmction pattern is consistent with a helix of five annuli (of 12 subunits) in one turn. Such a structure is seen to arise simply from the extended sheath by the merging of pairs of annuli. The dimensional changes suggested by the above structures indicate that contraction is accompanied by a conformational change jn the subunit. The pairing of ammli may have to be a progressive one in order not to isolate single annuli, thus being an example of what could be called a “domino reaction”.