Effect of L-amino acids on Mucor rouxii dimorphism

Abstract

Mucor rouxii organisms growing aerobically and exponentially on a well-defined minimal medium are able to differentiate as yeasts or as mycelia, depending on the amino acid used as the nitrogen source. When certain amino acids were used as the nitrogen source, spores differentiated only as hyphae, whereas other amino acids gave rise to other morphological forms having different ratios of yeasts to hyphae. In both hyphal and yeast cultures, an aerobic metabolism was predominant, as shown by determining several metabolic parameters such as oxygen tension, glucose consumption, ethanol production, and CO2 release. A complete conversion of yeasts to hyphae was obtained by the appropriate change in the amino acid used as nitrogen source. By preparing spheroplasts from mycelial cultures and transferring them to media with amino acids that induce yeast formation, a 50% yield in the reverse transformation was achieved. A correlation between the change in pH of the medium and cell morphology was observed in different growth conditions. Decrease in the pH of the medium preceded the appearance of hyphae. Also, when the initial pH of the medium was increased, aspartate-containing cultures developed mainly as mycelia, instead of yeasts, with a corresponding decrease in the final pH.Mucor rouxii organisms are able to grow in yeastlike or in filamentous forms, depending on the culture conditions. The growth requirements for the yeast phase of Mucor spp. are considerably more stringent and complex than are those for the hyphal phase (1). In addition, yeast forms are only found under anaerobic conditions with an excess of CO2 (1, 5). Although there are numerous studies on the carbon sources, atmosphere, and other environmental conditions that result in the formation of either yeast or hyphal tubes (1,5,18), there is very little work on the effect of the nitrogen sources on Mucor spp. dimorphism (15).The physiology and regulation of nitrogen assimilation in the fungus Neurospora crassa, and the enzymes involved have been extensively studied (4,(8)(9)(10)19). In our study on the enzymes responsible for ammonium assimilation in M. rouxii, we found unexpectedly that the appearance of different morphologies of M. rouxii depended on the L-amino acid used as the nitrogen source in the medium.In this paper, we show that the capacity of M. rouxii to differentiate as yeast or hyphae (or both) in aerobic cultures depended on the amino acid present as nitrogen source. We report our findings on (i) the growth rate, germination, and morphologies found, (ii) the interconversion of one cellular morphology into another, and (iii) the relationship between the morphological events induced by amino acids and the change of pH in the medium. MATERIALS AND METHODSStrain and media. M. rouxii IM80 was used throughout this study. The solid medium used was yeast extract peptone broth, supplemented with 112 mM glucose (YPG). The liquid medium was Vogel minimal medium (20) supplemented with 28 mM glucose. The nitrogen source used was either NH4Cl (25 mM) or ...

Mucor dimorphism.

“…The influence of L-amino acids on dimorphism and sporangiospore germination was examined in M. rouxii (126) and M. racemosus (225). Certain amino acids were more effective than others in stimulating germination when offered as the sole carbon source (225).…”

“…Some amino acids favored growth in the yeast morphology, whereas others favored growth in the hyphal form when offered as the sole nitrogen source (126). The pH influenced both morphological responses (126,225). There is presently no single theory that satisfactorily explains the diverse effects of amino acids on Mucor morphogenesis.…”