The effects of various carbon sources and of antibiotics on the morphology of hypha cells of the fungus Ustilago cynodontis is described. Nonfermentable substrates promote readily reversible yeastlike colonies from hypha cells: all the hypha cells spread on these substrates give rise to yeastlike colonies that revert to the mycelial phenotype when transferred to glucose medium. Among the antibiotics tested, chloramphenicol (CAP) is found to promote, under certain circumstances, a long-lasting, even permanent modification on the morphology of the colonies: the colonies developed on CAP-glucose media are yeastlike, and a percentage of them give rise to colonies whose morphology remains yeastlike even on drug-free media; this effect is also obtained with cells cultivated in liquid medium. This permanent morphological modification is accompanied by a change of metabolic properties. Similar permanent effects are obtained with ethidium bromide, suggesting that mitochondrial functioning is involved in these modifications.Ustilago cynodontis, an obligate aerobic fungus, has two morphologically different cell types: mycelial and yeastlike which, on solid 4% glucose medium, give rise to colonies that are morphologically different also (Fig. 1). Yeastlike strains are derived from mycelial cells (Nozeran and Chevalier, 1965) and it has been shown that mycelial cells of the 4001 strain (M1) are prototrophic whereas yeastlike cells (L1) which are derived from this M1 mycelial strain require lysine and arginine to grow (Talou and Tavlitzki, 1969),In the present paper, reversible changes and long-lasting, even permanent changes in the morphology of mycelial cells and colonies of the 4001 strain are studied.It is observed that as in many other fungi the morphology of the colonies obtained from mycelial hyphae is dependent on the nature of the carbon source in the growth medium: yeastlike development is promoted on media containing a nonfermentable substrate as the carbon source, but this state is reversible, that is, the colonies remain yeastlike as long as the nonfermentable substrate is present in the growth medium and the phenotypic change reverts to the mycelial form when the cells are transferred to a fermentable growth medium.On the other hand, the results reported in this paper show that chloramphenicol (CAP) and ethidium bromide (EB) can initiate a long-lasting or permanent modification of the mycelial cells,

~Comparison of the pools of glutamic acid and glutamine and of the specific activities of glutamine synthetase and glutamate dehydrogenases in sporulating a/cc and non-sporulating a/cc cells of Saccharomyces cerevisiae revealed a difference in their nitrogen metabolism.Glutamhe synthetase and glutamine appeared to be necessary for the sporulation process, glutamine playing, at least, a catabolic role. However, exogenous glutamine as well as ammoniainhibited sporulationwhile glutamic acid did not. Glutamine seemed to act through its amino group. Both inhibitors had at least two sites of action, one effective early in sporulation and related to DNA synthesis and the other acting later and not related to it. I N T R O D U C T I O NStarvation of the yeast Saccharomyces eerevisiae induces a differentiation process leading to meiosis and sporulation of diploid ( a / a ) cells. This process is inhibited by ammonia and some other nitrogen compounds (Miller, 1963; Piiion, 1977) but very little is known about the metabolism of ammonia in sporulating cells. In this paper, we report our studies on the activities of enzymes involved in ammonia assimilation (glutamhe synthetase, NAD-and NADP-linked glutamate dehydrogenases), the effect of the substrates of these enzymes on the sporulation process, and the variation in the sizes of pools of free glutamine and glutsmic acid during the first hours after the transfer of cells to the sporulation medium with or without ammonia. METHODS Yeast strains, growth and sporulation. Diploid cells of Saccharomyces cerevisiae 532 a / a and 533 ala,isogenic except for their mating type (Pellucuer, 19731, were used. The a / a cells were grown and allowed to sporulate as described by Durieu-Trautmann & Delavier-Klutchko (1977). The alee cells, which are unable to sporulate, were used as a control.Enzyme activities. Extracts were prepared at 4 "C by passing suspensions of cells in 100 mwpotassium phosphate buffer pH 7-8, containing 1 mhl-2-mercaptoethano1, through a French press at 100 MPa. Cell debris was removed by centrifuging for 30 min at 30000 g and the supernatant was dialysed overnight at 4 "C against buffer similar to that used for extraction.Activities of NAD-linked glutamate dehydrogenase (NAD-Gdh; EC 1 .4.1.2) and of NADP-linked glutamate dehydrogenase (NADP-Gdh; EC 1.4.1.4) were estimated by determining the rate of NADH or NADPH oxidation at 25 "C (Boehringer, 1968). The rate of reaction with NAD-Gdh was corrected by subtracting the rate of NADH oxidation observed in the absence of 2-oxoglutarate.The activity of glutamine synthetase (GS; EC 6.3.1 .2) was measured by assaying transferase activity (Woolfolk et al., 1966).Protein was determined by the Folin method of Lowry, using bovine serum albumin as standard.

The mycelial cells of Ustilago cynodontis possess at least two electron transport systems: a cyanide-sensitive cytochrome pathway, which represents the major route for electron transport, and an alternative cyanide-insensitive pathway, inhibited by salicylhydroxamic acid. In the presence of chloramphenicol in the culture medium, mycelial cells respire only by the alternative chain. The stable induced yeast-like cells, obtained by prolonged chloramphenicol treatment of the mycelial cells, respire as the untreated mycelial cells; this result indicates that the phenotypic change induced by chloramphenicol is not related to a modification of the respiratory system.

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