Fibrous projections from the core of a bacteriophage T7 procapsid.

A cylindrical core previously demonstrated in a bacteriophage T7 procapsid (capsid I) has been further examined by electron microscopy. Fibrous extensions of the core have been observed; these fibers appear to connect the core to the capsid I envelope. After infection of a nonpermissive host with bacteriophage T7 amber mutant in any gene coding for a core protein, the resulting lysates contained more noncapsid assemblies of capsid envelope protein than did wild-type lysates; these assemblies had a mass two to at least 500 times greater than the mass of capsid I. This suggests that the internal core and fibers assist the assembly of subunits in the envelope of capsid I.