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Investigation of bacteriophage T4 by atomic force microscopy


Bacteriophage T4 was visualized using atomic force microscopy (AFM). The images were consistent with, and complementary to electron microscopy images. Head heights of dried particles containing DNA were about 75 nm in length and 60 nm in width, or about 100 nm and 85 nm respectively when scanned in fluid. The diameter of hydrated tail assemblies was 28 nm and their lengths about 130 nm. Seven to eight pronounced, right-handed helical turns with a pitch of 15 nm were evident on the tail assemblies. At the distal end of the tail was a knob shaped mass, presumably the baseplate. The opposite end, where the tail assembly joins the head, was tapered and connected to the portal complex, which was also visible. Phage that had ejected their DNA revealed the internal injection tube of the tail assembly. Heads disrupted by osmotic shock yielded boluses of closely packed DNA that unraveled slowly to expose threads composed of multiple twisted strands of nucleic acid. Assembly errors resulted in the appearance of several percent of the phage exhibiting two rather than one tail assemblies that were consistently oriented at about 72° to one another. No pattern of capsomeres was visible on native T4 heads. A mutant that is negative for the surface proteins hoc and soc, however, clearly revealed the icosahedral arrangement of ring shaped capsomeres on the surface. The hexameric rings have an outside diameter of about 14 nm, a pronounced central depression, and a center-to-center distance of 15 nm. Phage collapsed on cell surfaces appeared to be dissolving, possibly into the cell membrane.

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