Respiratory-competent conditional developmental mutant of Mucor racemosus

Abstract

A conditional developmental mutant of Mucor racemosus which is capable of oxidative energy metabolism is described. Unlike the wild-type strain the mutant was highly fermentative and exhibited the yeast morphology when grown aerobically in glucose-containing media. The high fermentative activity and yeast morphology under these conditions correlated well with maximal expression of glycolytic enzymes and with expression of some polypeptides' characteristic of anaerobic growth. Aerobic growth of the mutant on amino acids as the sole carbon source resulted in growth in the' mycelial morphology. The mnutant was fully capable'of oxidative metabolism as judged by its ability to grow on amino acids, respiratory capacity, and complement of tricarboxylic acid cycle enzymes. The results support the hypothesis that oxygen controls both the expression of glycolytic enzymes and the expression of proteins involved in morphogenesis. Moreover, they suggest that there are common regulatory elements in the control of these two classes of gene products. Abnormally high levels of aconitase and isocitrate dehydrogenase in the mutant are consistent with the proposal that pool sizes of citrate may act as a regulator of genes responsive to environmental oxygen concentration.

Mucor dimorphism.

“…However, the reported mutants were apparently pleiotropic and were characterized under only a limited set of environmental conditions. Furthermore, the existence of conditional yeast mutants (28,212) with undiminished respiratory competence offers one argument against morphogenetic control at this site. There are several others.…”

“…Borgia et al (28) have isolated a respiratory-competent developmental mutant of M. racemosus that behaves like the wild type in terms of its morphology and energy-yielding catabolic pathways except in the presence of glucose. Under this condition, the organism is highly fermentative and grows as a yeast form whether or not oxygen is present.…”

“…Inderlied et al (102) asserted, in support of the stated hypothesis, that mutant PB104 of M. racemosus is respiratory deficient, possesses defective mitochondria, cannot convert to the hyphal morphology, and displays low NADlinked GDH activity even under an aerobic atmosphere. However, Borgia et al (28) have more recently characterized this mutant and indicated that it is, in fact, a respiratorycompetent, conditional developmental mutant possessing functional mitochondria. It grows in the hyphal form on minimal medium with glutamate as the sole carbon and energy source.…”

“…Ruiz-Herrera and co-workers (196) succeeded in isolating NTG-induced mutants of M. bacilliformis without using a selective enrichment step. Borgia and his colleagues (28) have enriched for NTGinduced morphology mutants of M. racemosus by filtering cells through Miracloth, which passes yeasts while trapping hyphae. The use of one or another of these selection procedures has made it possible to isolate mutants that are auxotrophic (173,187), monomorphic (196), conditionally monomorphic (28,213), respiratory deficient (207), or refractory to the normal effects of exogenous cyclic AMP on morphology (171).…”

Effect of oxygen on morphogenesis and polypeptide expression by Mucor racemosus

“…Yeast morphogenesis only occurs when oxygen is rigorously excluded. When compared with microaerobic mycelia, the yeast morphology of anaerobic cells correlates with alterations in the synthesis of a relatively small number of polypeptides and not with any major change in metabolic pattern or with the levels of glycolytic enzymes (2). This suggests that these polypeptides are involved in the morphogenesis.…”