EVIDENCE FOR TILTED SMECTIC LIQUID-CRYSTALLINE PACKING OF FD INOVIRUSFROM X-RAY FIBER DIFFRACTION

Fibers of the fd strain of Inovirus (filamentous bacteriophage) prepar ed above its isoelectric point (about pH 4) give X-ray diffraction pat terns, ''higher pH patterns'', that differ from the patterns of fd fib ers below the isoelectric point, ''lower pH patterns''. The overall di stribution of intensity on higher and lower pH patterns is substantial ly the same, indicating that the virion structure is substantially the same, but the crystalline reflections that define the packing of the virions in crystallites are different. For the lower pH patterns, the crystalline reflections can be indexed on a conventional hexagonal lat tice. However, for the higher pH patterns, the crystalline reflections on the equator and first layer line are spread out in a broad, angula r, but specific fashion (''layer-line fanning''), unlike disorientatio n spreading. We interpret this observation to mean that the crystallit es in the higher pH fiber are tilted with respect to the fiber axis. T his interpretation is supported by simulated fiber diffraction pattern s calculated from models of the packing of tilted virions. The diffrac tion patterns from fibers of some mutants of fd, and from other wild-t ype strains of Inovirus (M13, IKe, If1), do not show layer-line fannin g. We propose that the tilting of crystallites that gives rise to laye r-line fanning in fd fibers is due both to a specific pattern of charg e on the surface of the fd virion at higher pH and to the presence of a specific inter-subunit hydrogen bond. Apparently these features of t he fd virion structure require a strict 2-fold screw axis along the vi rion axis that affects the packing between neighboring virions, result ing in the formation of a tilted smectic liquid crystal phase.