THE FINE STRUCTURE OF NORMAL AND IRRADIATED YEAST CELLS AND YEAST RIBOSOMES (thesis)

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Abstract
An electron microscope study of the internal organization of the baker's yeast (Saccharomyccs cerevisiae) is presented with particular emphasis on the structural changes brought about by nitrogen starvation and x irradiation. The general characteristics of normal growing yeast cells fixed with KMnO/sub 4/ were a dense granular cytoplasm devoid of most cytomembranes and mitochondria, and a well defined--usually homogeneous-- nucleus located adjacent to the central vacuole. On the other hand, starving cells displayed a progressively less dense cytoplasmic matrix, resulting in enhancement of the visualization of membranous components. The nuclear membrane was apparently a triple structure in growing cells, but had a "classical" double-membrane appearance in starved cells. Intranuclear objects believed to be chromosomes are described and their unusual staibing properties discussed. A structure thought to correspond to the centriole apparently had an intravacuolar location that may be appropriate to the unusual mode of nuclear division in yeast. Cells irradiated with 100,000 r of x rays underwent only slight changes during the first hours of postirradiation incubation, but after two days underwent drastic alterations suggestive of lysis. Unusual cell forms, referred to as ameboid, also appeared during these late stages of radiation damage. In order to obtain information on themore » fine structure of yeast at a higher stage of refinement, ribosomes were isolated from growing, starving, and irradiated cells; and subjected to parallel analysis with the ultracentrifuge and electron microscope. Most striking was the discovery of a 65S ribosome present only in rapidly growing yeast cultures and absent from starving or stationary cells. This finding is discussed together with other recent data on the possible role of such a particle in protein synthesis. The sedimentation constants and relative percent abundances of the ribosomes varied in characteristic ways that were substantiated by size measurements made from electron microscope pictures of similar particles. Radiation apparently had little immediate effect on the properties of the ribosomes studied in this work. (auth) « less

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