3-DIMENSIONAL STRUCTURE OF TUBULAR NETWORKS, PRESUMABLY GOLGI IN NATURE, IN VARIOUS YEAST STRAINS - A COMPARATIVE-STUDY

Background: In the yeast Saccharomyces cerevisiae, the Golgi apparatus consists of discrete units distributed throughout the cytoplasm. When such units are examined in three dimensions, in relatively thick sect ions prepared for the electron microscope, they usually appear as smal l tubular networks with a stained material accumulating in dilations l ocated at the junctions of membranous tubules. To see whether such tub ular networks are observed in other yeast species, the three-dimension al structure of organelles in eight additional yeast strains, endowed with diverse biological properties, are examined. Methods: Yeast strai ns were grown at 24 degrees C in YPD medium (2% Bactopeptone, 1% Bacto yeast extract, and 2% glucose). Cells that were examined by electron m icroscopy came from exponentially growing cultures grown in a shaking water bath and maintained at a OD 600 (optical density at 600 nm) of 0 .5. Cells were fixed in a fixative containing 2% glutaraldehyde in 0.1 M cacodylate buffer pH 7.4 and 0.8 M sorbitol. They were then treated for 15 min in 1% sodium metaperiodate and postfixed for 1 hr in potas sium ferrocyanide-osmic acid. They were preembedded in agarose prior t o dehydration and finally embedded in Epon, In these conditions, the p reservation of cell organelles was improved and the cytoplasmic retrac tion from the cell wall was minimized. Photographs of sections tilted at +/- 15 degrees from the 0 degrees position of the goniometric stage were used to prepare stereopairs from which the three-dimensional con figuration of the organelles was visualized. Results: In all yeast str ains, tubular networks appeared as separate elements or units disperse d throughout the cytoplasm. Each unit consisted of anastomosed membran ous tubules. In some strains such as Saccharomyces cerevisiae, Zygosac charomyces rouxii, or Saccharomyces pombe, such units appeared mainly as polygonal networks of intensely stained membranous tubules. Along t hese networks, distensions filled with stained material were similar i n size to nearby secretory granules, suggesting that the latter formed by fragmentation of the tubular networks. In Hansenula polymorpha, Pi chia pastoris, and Debaryomyces hansenii, networks of anastomosed tubu les were closely superposed to each other and formed parallel arrays r eminiscent of the stacks of Golgi saccules seen in mammalian cells. Ho wever, in contrast to what is usually found in the latter, the layers making up the parallel arrays in yeasts, were clearly continuous to ea ch other. In other strains, i.e., Kluyveromyces lactis, Candida albica ns, and Candida parapsilosis, the situation was intermediate and their cytoplasm contained only arrays of small size with two or at most thr ee superposed layers of membranous tubules. Small vesicles in the 30-5 0 nm. range were rarely encountered in most yeast strains. Conclusions : It is therefore concluded that tubular networks, presumably Golgi in nature, are present in all yeasts examined so far. Yet, in some strai ns, these tubular networks may be arranged in parallel arrays or stack s. (C) 1995 Wiley-Liss, Inc.