Fine cytochemical localization of polyphosphates in the yeast Saccharomyces cerevisiae

A late exponential culture, cultivated in the absence of phosphates, and a similar culture supplied with phosphate (phosphate overcompensation conditions) were prepared from an industrial strain of Saccharomyces cerevisiae. For the cytochemical staining, the cellular phosphates were transformed into polymeric metal-phosphate complexes by Ca2+ and Mg2+ ions, added to the fixative. The fixative contained 3% glutaraldehyde, buffered by 100 mM Tris-HCl to pH 6.0, plus 100 mM MgCl2 and 100 mM CaCl2. Staining with lead acetate was followed by OsO4 post-fixation. In cells cultivated in the absence of phosphates lead deposits were found in vacuoles only. In the late exponential culture the staining was observed on the surface of the plasmalemma, on the membranes of the endoplasmic reticulum, in mitochondria, in the cell nucleus, and in vacuoles. As a rule, extensive polyphosphate deposits (metachromatic granules) were found in vacuoles. Two hours after phosphate overcompensation, a high quantity of polyphosphate as found also in the cell wall, e.g., in the isthmus of budding cells (scar ring), in the secondary septa of mother and daughter cells, and in the growth apex. When divalent cations were omitted from the fixative, the staining of polyphosphates was limited to the cell wall and large vacuolar granules. The results of cytochemical staining were compared with the biochemical analysis of polyphosphate content in the cultures under study.