The morphogenetic core of bacteriophage T4 is a transient structure which exists inside an early head precursor, but which is later proteolytically destroyed at the time of DNA packaging. This structure is of interest because genetic evidence shows that P22 (the major core protein and product of gene 22) is involved in determining the length and shape of the head and in selecting the position of tail attachment. P22 may also be involved in initiation of head assembly and in DNA packaging.

We have isolated T4 polyheads with intact cores for study by negative staining in the electron microscope. Polyheads were used because their elongated structure makes them suitable for optical diffraction and computer analysis of electron microscope images. All polyheads (except those from cells defective in gene 22) were found to have highly ordered cylindrical cores consisting of several helical chains wound about a hollow center.

Cores of l-canavanine-induced polyheads are about 460 Å in diameter and appear to consist of six helical chains spaced about 115 Å apart axially, and with a pitch angle of about 30°. These polyheads probably have the same core structure as an elongated version of a true head precursor, since giant τ-particles were found to have the same structure. Moreover, canavanine polyheads have the surface structural parameters expected of an elongated prohead. They possess hemispherical caps and the core extends into the end of the particle and has a cap of its own. Since the T4 head surface is thought to have a 5-fold axis of rotational symmetry, our data indicate a symmetry mismatch between core and surface. We suggest that head assembly may be initiated with the formation of a connector between core and surface, which later becomes the point of tail attachment.

Polyheads induced by mutations in genes 20, 24 or 40 have cores about 350 Å in diameter consisting of five or six helical chains spaced 131 Å apart axially and wound with a pitch angle of about 40°. Polyheads lacking the internal proteins have a similar, but less stable, core while polyheads induced by an amber mutation in gene 22 apparently have no core at all.

Since head length is determined by an interaction between the core (P22) and the surface (P23), we examined polyheads containing proteins altered by mutations which affect head length. In all cases where petite phage particles arise, we observed either weakened core-surface interactions, unstable cores, or an altered intrinsic curvature of the surface subunit P23.