Structure and formation of ribosome crystals in hypothermic chick embryo cells

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The structure of hypothermic ribosome crystals in chick embryo cells was studied by thin-section electron microscopy. Analysis of both serial sections and tilted sections has revealed that the sheet-like crystal, of the p4 plane group, occurs in but one form relative to its surface characteristics. Each ribosome in the crystal therefore shows the same geometric relationship to others in the crystal and to the dense and light matrices bounding the sheet. Variation of cooling rates before fixation produces specific differences in crystal morphology without altering the basic surface lattice. Under conditions of very slow cooling, the large sheet thus produced always curls about a specific axis until its free edges fuse together, forming a complete cylindrical surface. These cylinders frequently aggregate into parallel arrays. At intermediate cooling rates, interphase cells form small curved sheets; mitotic cells, however, form stacks of sheets. The sheets in these stacks alternate successively in polarity, that is, in the direction faced by the dense surface. Rapid cooling of embryos explanted from their eggs yields no crystals in interphase cells, but stacked crystals occur in mitotic cells. This indicates that crystallization within interphase cells must be preceded by the release of ribosomes from polyribosomes during gradual cooling. Free ribosomes occur naturally, however, in mitotic cells, and form stacked sheets under rapid cooling. This stack of sheets is frequently quite tightly compressed, in which case the unit cells of the p4 surface lattice become aligned along axes normal to the surfaces of the sheets, thereby producing a three-dimensional crystal of the p422 configuration.