Structure of the sheath of bacteriophage T4. I. Structure of the contracted sheath and polysheath

Contracted sheath and polysheath of bacteriophage T4 were examined in the electron microscope by the negative-staining method.The prominent helical grooves of long pitch, which have frequently been noted in contracted sheath, are also found in polysheath, where their number is even, and between 10 and 14. In contracted sheath their number (12) and sense (right-handed) can be easily determined because of a fortunate artifact: parts of the sheath that are only shallowly embedded in the negative stain undergo, on drying, a contraction of up to 40%. A sheath standing on end therefore assumes a conical shape, and the “long-pitch” helical grooves show in projection as spirals. The same contraction also affects specimens of sheath lying flat on the supporting film, and it is suggested that this effect might provide a general method for determining the absolute sense of helical structures.Another set of helical grooves (of short pitch), also previously noted in contracted sheath, is clearly visible in polysheath. They are found to be left-handed and, in combination with the “long-pitch” grooves, define the positions of the subunits at the sheath surface, i.e. its surface lattice. Optical diffraction patterns have been formed from the electron micrographs to measure the parameters of the surface lattices of polysheath and contracted sheath. The effect on the diffraction pattern of flattening and other distortions of the helix is discussed in Appendix 1. A detailed analysis of the diffraction patterns of contracted sheath and polysheath, based on this discussion, follows in Appendix 2. Polysheath is found to have an n₂-fold rotation axis, where n₂ is the number of “short-pitch” grooves and is half the number of “long-pitch” grooves. The ratio of the pitches of the two sets of grooves is also quite similar in contracted sheath and polysheath.In confirmation of Kellenberger &; Boy de la Tour's (1964) suggestion that contracted sheath and polysheath have the same structure, their structural parameters agree to within the accuracy of measurement. Supposing that they do have the same structure, this is determined by combining the data on the surface lattice of polysheath with the observation that there are 12 “longpitch” grooves in contracted sheath. The common structure has a 6-fold rotation axis, and six left-handed basic helices, each containing 13.2 morphological units per turn.The implications of this structure are discussed, especially with regard to the helical disintegration of polysheath. This disintegration process indicates that the strongest bonds lie along the directions of the “short-pitch” grooves.