A detailed analysis of the T4 long tail fibers has been completed using a combination of computer-enhanced electron microscopy and sequence-based structural predictions. Some generalizations seem to be emerging from this and previous studies of T4 short tail fibers and T7 tail fibers. Mature tail fibers are trimeric. They are segmented molecules composed of rigid rod-like segments separated by hinges. Conformations of different segments are based on triple coiled-coil `-helices or a 3-stranded structure that is rich in -strands and turns (which appears to represent a new fibrous protein conformation). The N-terminal domain binds the tail-fiber to the virion, while the receptor-binding site is at or near the C-terminus. In the course of this work, it appeared that negatively stained STEM images directly showed structural detail that could only be seen by extensive averaging of negatively stained conventional TEM images. A systematic comparison of the signal-to-noise ratio of negatively stained specimens in the STEM and the CTEM is underway.
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