The shape of the T-even bacteriophage head

Abstract

Electron micrographs of T-even heads have provided no valid reason for believing that the head shape must be a bipyramidal hexagonal prism, as other polyhedra also give hexagonal profiles similar to those shown by T-even heads. Comparison of the structure of the normal head with various defective head forms shows that (1) the molecules are probably hexagonally packed in the head membrane and (2) the local packing of head protein molecules is the same round a terminal and a nonterminal vertex. Of four hypotheses which are described, only two are consistent with this evidence; both require the normal head to have the structure of an isometric capsid with icosahedral symmetry, in which the middle portion has been extended. Of these two hypotheses, the head shape is most satisfactorily accounted for by a hypothesis according to which the normal head has 52 symmetry and has a shape derived from a regular icosahedron by the extension of its middle section (‘prolate icosahedron’). It is shown that it is possible to accommodate the 5-fold axis of this head with the 6-fold symmetry of the base plate.