Glucose metabolism and dimorphism in Mucor

Abstract

Mucor racemosus fermented glucose to ethanol, carbon dioxide, and glycerol. When this fungus was grown anaerobically in either the yeast or mycelial form, the catabolism ofglucose was very similar. Yeast cells shifted to aerobic conditions maintained a high flux of glucose carbon through the glycolytic and pentose phosphate pathways. Mycelial cells grown aerobically catabolized glucose in a manner consistent with a respiratory metabolism. Although there was no consistent pattern of glucose metabolism in the mycelial form of Mucor, growth in the yeast form consistently was correlated with a high flux of glucose carbon through the catabolic pathways.Mucor racenosus is a dimorphic phycomycete that can grow either as budding yeasts or as filamentous mycelia. Although the morphology of this fungus appears to be influenced by a variety of environmental factors (4, 14), the presence of a fermentable hexose is essential for the development of the yeast form (2). Larsen and Sypherd (8) showed, for example, that, although dibutyryl cyclic AMP (dbcAMP) induced yeast development in air, the presence of glucose was necessary to maintain the yeast form. Depending on the concentration of glucose in the growth medium, the morphology of M. rouxii, grown under anaerobic conditions (100% N2), varied from the mycelial form to the yeast form (2). Mooney and Sypherd (10) reported, however, that effects of glucose concentration on morphology were not observed with M. racemosus if one carefully regulated the flow rate of nitrogen through the culture. When M. genevensis yeast cells were grown in continuous culture with excess glucose, mycelial cells were not formed even at high oxygen concentrations (13). Indeed, these aerobic yeast cells continued to produce ethanol and carbon dioxide at a rate indicative of a fermentative metabolism. These workers (13) concluded that respiratory capacity was not correlated with morphology. In addition, Paznokas and Sypherd (12) measured the respiratory capacity of aerobic mycelia and aerobic dbcAMP-treated yeast and concluded that morphology was independent of respiratory capacity. The present study was designed to determine the nature of the relationship between the patterns of glucose carbon metabolism and the morphology of Mucor. Changes in the distribution or flux of carbon into the glucose catabolic pathways could reveal changes in metabolic intermediates, enzymes, or end products important to the regulation of morphogenesis. MATEIUALS AND METHODSOrganism and culture conditions. Mucor racemosus (M. huitanicus) ATCC 1216B was used in all experiments. Sporangiospores were prepared as described by Paznokas and Sypherd (12). Cells were grown in either a complex medium consisting of 0.3% (wt/vol) yeast extract (Difco), 1% (wt/vol) peptone (Difco), and 2% (wt/vol) D-glucose; or a minimal medium consisting of 0.05% (wt/vol) yeast nitrogen base (Difco), 2% (wt/vol) D-glucose, 10 mM L-glutamate, 10 mM L-alanine, 10 mM L-aspartate, and 10 mM NH4Cl (J. Peters and P. S. Sypherd, J. Gen. Microbiol., in press)....

Respiratory-competent conditional developmental mutant of Mucor racemosus

“…Many studies had been reported concerning the relationship between the metabolic mode and morphogenesis in Mucor species (3,6,10,13,18,22). The reports are in agreement that yeast development invariably correlates with an increased capacity for fermentative metabolism.…”

Physiological control of phosphorylation ribosomal protein S6 in Mucor racemosus

“…Fermentative metabolisn and lower P.'s predominate during the later growth of these cells. Low Ps's were observed in cells growing in C02 and were correlated with the lowered glycolytic rates evident in these cells (10). The extent of ribosome phosphorylation, therefore, may be positively correlated with the rate of ATP production and negatively correlated with the rate of energy comsumption.…”

Effect of oxygen on morphogenesis and polypeptide expression by Mucor racemosus

“…1). In either anaerobic yeasts or microaerobic mycelia and in C02-grown yeasts polypeptides 4,5,6,10,13,14,18,20,22,23,25,27,28,29,32,34,36,41,49, and 52 were exclusively present or present in greater amounts in the oxygen-depleted cells (CO2 sparged, microaerobic or anaerobic) when compared with the aerobically grown cells. Thus, considerable qualitative and quantitative differences exist between O2-deprived cells and aerobic cells irrespective of the morphology of the cells.…”

“…C02-grown yeast cells had a doubling time of 4 h, while aerobic cells doubled every 1.5 h (data not shown). In addition, it has been shown that these cells differ in carbon catabolism, in terms of flux and distribution through fermentative and oxidative metabolic pathways (5). Therefore, comparison of these two figures cannot differentiate polypeptides unique to one morphology from those that reflect metabolicand growth-dependent differences between the cells.…”

“…The numbered proteins are discussed in the text. laboratories (5,10,12). Other metabolic events requiring molecular oxygen might be important to mycelial development.…”