Recrystallized pancreatic ribonuclease releases more than half of the ultraviolet-absorbing cellular constituents of Candida utilis and Saccharomyces cerevisiae. The presence of salts and buffer interferes with the action of the enzyme on the cytoplasmic membrane, and intracellular constituents show a similar inhibitory effect. Polymerized ribonuclease and conformational isomers cannot penetrate the pores of the yeast cell wall, but they are effective on the membrane of spheroplasts. All forms of the enzyme interfere strongly with the viability of the cells.

and properties of spheroplasts of the yeast Candida utilis, produced by digestion of the cell walls with snail gut juice (Helix pomnatia) in isotonic medium, were studied by l)hase, interference, and ultraviolet microscopy, and tracer

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Ultraviolet microscopy of purine compounds in the yeast vacuole. J. Bacteriol. 85:399-409. 1962.-Yeast cells (Candida utilis and Saccharomyces cerevisiae) suspended in nitrogen-free medium were exposed to various ultravioletabsorbing biological compounds, particularly nucleic acid constituents. Ultraviolet photomicrography was used to locate these substances in the cells. Purines were taken up readily and concentrated in the vacuoles of C. utilis but not of S. cerevisiae. Crystallization occurred, as observed earlier by other techniques. Neither organism assimilated pyrimidine bases, or purine or pyrimidine nucleosides, at a detectable rate. From the selective uptake and release of some purine derivatives, it can be concluded that the properties of the vacuolar membrane and the cytoplasmic membrane differ in several respects. MATERIALS AND METHODS C. utilis ATCC 9950 and S. cerevisiae ATCC 7752 were cultured as specified earlier (Svihla, Schlenk, and Dainko, 1960, 1961). The washed cells were stored at 4 C until use. For experiments with the purines, pyrimidines, and other ultraviolet-absorbing compounds, the same salt medium was used without ammonium sulfate. Glu-399

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