STRUCTURE OF NORMAL AND CONTRACTED TAIL SHEATHS OF T4 BACTERIOPHAGE

[摘要] The structural arrangement of protein subunits in extended and contracted tail sheaths of T4 bacteriophage were studied by optical diffractometry of electron micrographs. The analysis of such diffraction patterns shows that the extended sheath consists of annuli of 6 subunits, these being arranged in a helic of close to seven annuli in 2 turns. The annulus repeat in the helix direction is 38.1 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 diffraction pattern is consistent with a helix of 5 annuli (of 12 subunits) in 1 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 in the subunit. The pairing of annuli 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.