Respiratory capacity, cyclic adenosine 3',5'-monophosphate, and morphogenesis of Mucor racemosus

Zygomycetes in human disease

The Zygomycetes represent relatively uncommon isolates in the clinical laboratory, reflecting either environmental contaminants or, less commonly, a clinical disease called zygomycosis. There are two orders of Zygomycetes containing organisms that cause human disease, the Mucorales and the Entomophthorales. The majority of human illness is caused by the Mucorales. While disease is most commonly linked to Rhizopus spp., other organisms are also associated with human infection, including Mucor, Rhizomucor, Absidia, Apophysomyces, Saksenaea, Cunninghamella, Cokeromyces, and Syncephalastrum spp. Although Mortierella spp. do cause disease in animals, there is no longer sufficient evidence to suggest that they are true human pathogens. The spores from these molds are transmitted by inhalation, via a variety of percutaneous routes, or by ingestion of spores. Human zygomycosis caused by the Mucorales generally occurs in immunocompromised hosts as opportunistic infections. Host risk factors include diabetes mellitus, neutropenia, sustained immunosuppressive therapy, chronic prednisone use, iron chelation therapy, broad-spectrum antibiotic use, severe malnutrition, and primary breakdown in the integrity of the cutaneous barrier such as trauma, surgical wounds, needle sticks, or burns. Zygomycosis occurs only rarely in immunocompetent hosts. The disease manifestations reflect the mode of transmission, with rhinocerebral and pulmonary diseases being the most common manifestations. Cutaneous, gastrointestinal, and allergic diseases are also seen. The Mucorales are associated with angioinvasive disease, often leading to thrombosis, infarction of involved tissues, and tissue destruction mediated by a number of fungal proteases, lipases, and mycotoxins. If the diagnosis is not made early, dissemination often occurs. Therapy, if it is to be effective, must be started early and requires combinations of antifungal drugs, surgical intervention, and reversal of the underlying risk factors. The Entomophthorales are closely related to the Mucorales on the basis of sexual growth by production of zygospores and by the production of coenocytic hyphae. Despite these similarities, the Entomophthorales and Mucorales have dramatically different gross morphologies, asexual reproductive characteristics, and disease manifestations. In comparison to the floccose aerial mycelium of the Mucorales, the Entomophthorales produce a compact, glabrous mycelium. The asexually produced spores of the Entomophthorales may be passively released or actively expelled into the environment. Human disease with these organisms occurs predominantly in tropical regions, with transmission occurring by implantation of spores via minor trauma such as insect bites or by inhalation of spores into the sinuses. Conidiobolus typically infects mucocutaneous sites to produce sinusitis disease, while Basidiobolus infections occur as subcutaneous mycosis of the trunk and extremities. The Entomophthorales are true pathogens, infecting primarily immunocompetent hosts. They generally do not invade blood vessels and rarely disseminate. Occasional cases of disseminated and angioinvasive disease have recently been described, primarily in immunocompromised patients, suggesting a possible emerging role for this organism as an opportunist.

Zygomycetes in human disease

The Zygomycetes represent relatively uncommon isolates in the clinical laboratory, reflecting either environmental contaminants or, less commonly, a clinical disease called zygomycosis. There are two orders of Zygomycetes containing organisms that cause human disease, the Mucorales and the Entomophthorales. The majority of human illness is caused by the Mucorales. While disease is most commonly linked to Rhizopus spp., other organisms are also associated with human infection, including Mucor, Rhizomucor, Absidia, Apophysomyces, Saksenaea, Cunninghamella, Cokeromyces, and Syncephalastrum spp. Although Mortierella spp. do cause disease in animals, there is no longer sufficient evidence to suggest that they are true human pathogens. The spores from these molds are transmitted by inhalation, via a variety of percutaneous routes, or by ingestion of spores. Human zygomycosis caused by the Mucorales generally occurs in immunocompromised hosts as opportunistic infections. Host risk factors include diabetes mellitus, neutropenia, sustained immunosuppressive therapy, chronic prednisone use, iron chelation therapy, broad-spectrum antibiotic use, severe malnutrition, and primary breakdown in the integrity of the cutaneous barrier such as trauma, surgical wounds, needle sticks, or burns. Zygomycosis occurs only rarely in immunocompetent hosts. The disease manifestations reflect the mode of transmission, with rhinocerebral and pulmonary diseases being the most common manifestations. Cutaneous, gastrointestinal, and allergic diseases are also seen. The Mucorales are associated with angioinvasive disease, often leading to thrombosis, infarction of involved tissues, and tissue destruction mediated by a number of fungal proteases, lipases, and mycotoxins. If the diagnosis is not made early, dissemination often occurs. Therapy, if it is to be effective, must be started early and requires combinations of antifungal drugs, surgical intervention, and reversal of the underlying risk factors. The Entomophthorales are closely related to the Mucorales on the basis of sexual growth by production of zygospores and by the production of coenocytic hyphae. Despite these similarities, the Entomophthorales and Mucorales have dramatically different gross morphologies, asexual reproductive characteristics, and disease manifestations. In comparison to the floccose aerial mycelium of the Mucorales, the Entomophthorales produce a compact, glabrous mycelium. The asexually produced spores of the Entomophthorales may be passively released or actively expelled into the environment. Human disease with these organisms occurs predominantly in tropical regions, with transmission occurring by implantation of spores via minor trauma such as insect bites or by inhalation of spores into the sinuses. Conidiobolus typically infects mucocutaneous sites to produce sinusitis disease, while Basidiobolus infections occur as subcutaneous mycosis of the trunk and extremities. The Entomophthorales are true pathogens, infecting primarily immunocompetent hosts. They generally do not invade blood vessels and rarely disseminate. Occasional cases of disseminated and angioinvasive disease have recently been described, primarily in immunocompromised patients, suggesting a possible emerging role for this organism as an opportunist.

Evidence that MRas1 and MRas3 proteins are associated with distinct cellular functions during growth and morphogenesis in the fungus Mucor racemosus

The filamentous fungus Mucor racemosus provides a simple and unique model system for defining the function of individual ras genes in a gene family which is closely related to mammalian ras genes. The current study was designed to investigate the role of Mras1 and Mras3 in different stages of fungal morphogenesis, including sporangiospore germination, sporulation, and dimorphic transitions. The overall patterns of Mras1 and Mras3 transcript and protein accumulation were markedly different but, in general, transcripts and proteins were present at low levels during spherical growth and their accumulated level increased severalfold during polar growth (germ tube emergence and elongation). In contrast to Mras1, relatively high levels of Mras3 transcript accumulated during sporulation and MRas3 protein accumulated in sporangiospores. Transformation of M. racemosus with an activated allele of Mras3 reduced growth rate during aerobic sporangiospore germination, while a dominant-negative allele of Mras3 caused a 40% decrease in viable asexual spores. An activated allele of Mras1 increased growth rate during sporangiospore germination but neither activated nor dominant-negative alleles of Mras1 affected total number of asexual spores. Expression of MRas3 and MRas1 proteins appear to be subject to different regulatory mechanisms: exogenous dibutyryl-cAMP and fusidienol caused a strong repression of the level of MRas3 protein (but not MRas1) concurrent with the inhibition of polar growth. Differential posttranslational modification and intracellular localization of MRas1 and MRas3 proteins were also observed. The data strongly suggest that Mras3 and Mras1 play different roles in regulation of cell growth and morphogenesis in Mucor.

Yeast pseudohyphal growth is regulated by GPA2, a G protein alpha homolog

Pseudohyphal differentiation, a filamentous growth form of the budding yeast Saccharomyces cerevisiae, is induced by nitrogen starvation. The mechanisms by which nitrogen limitation regulates this process are currently unknown. We have found that GPA2, one of the two heterotrimeric G protein alpha subunit homologs in yeast, regulates pseudohyphal differentiation. Deltagpa2/Deltagpa2 mutant strains have a defect in pseudohyphal growth. In contrast, a constitutively active allele of GPA2 stimulates filamentation, even on nitrogen-rich media. Moreover, a dominant negative GPA2 allele inhibits filamentation of wild-type strains. Several findings, including epistasis analysis and reporter gene studies, indicate that GPA2 does not regulate the MAP kinase cascade known to regulate filamentous growth. Previous studies have implicated GPA2 in the control of intracellular cAMP levels; we find that expression of the dominant RAS2(Gly19Val) mutant or exogenous cAMP suppresses the Deltagpa2 pseudohyphal defect. cAMP also stimulates filamentation in strains lacking the cAMP phosphodiesterase PDE2, even in the absence of nitrogen starvation. Our findings suggest that GPA2 is an element of the nitrogen sensing machinery that regulates pseudohyphal differentiation by modulating cAMP levels.

Mucor dimorphism

Mucor dimorphism has interested microbiologists since the time of Pasteur. When deprived of oxygen, these fungi grow as spherical, multipolar budding yeasts. In the presence of oxygen, they propagate as branching coenocytic hyphae. The ease with which these morphologies can be manipulated in the laboratory, the diverse array of morphopoietic agents available, and the alternative developmental fates that can be elicited from a single cell type (the sporangiospore) make Mucor spp. a highly propitious system in which to study eukaryotic cellular morphogenesis. The composition and organization of the cell wall differ greatly in Mucor yeasts and hyphae. The deposition of new wall polymers is isodiametric in yeasts and apically polarized in hyphae. Current research has focused on the identity and control of enzymes participating in wall synthesis. An understanding of how the chitosome interacts with appropriate effectors, specific enzymes, and the plasma membrane to assemble chitin-chitosan microfibrils and to deposit them at the proper sites on the cell exterior will be critical to elucidating dimorphism. Several biochemical and physiological parameters have been reported to fluctuate in a manner that correlates with Mucor morphogenesis. The literature describing these has been reviewed critically with the intent of distinguishing between causal and casual connections. The advancement of molecular genetics has afforded powerful new tools that researchers have begun to exploit in the study of Mucor dimorphism. Several genes, some encoding products known to correlate with development in Mucor spp. or other fungi, have been cloned, sequenced, and examined for transcriptional activity during morphogenesis. Most have appeared in multiple copies displaying independent transcriptional control. Selective translation of stored mRNA molecules occurs during sporangiospore germination. Many other correlates of Mucor morphogenesis, presently described but not yet explained, should prove amenable to analysis by the emerging molecular technology.

Mitochondrial activity and development: energy carrier selection in a controlled reaction mechanism

There is evidence of an important role carried out by mitochondria and by proteins of mitochondrial origin during key moments of development. We assume that it is possible to represent the energy activity of the cell through the level of ATP production and we propose a simple model system for the ATP/ADP cycle. The model takes into account the basic aspect of ATP dynamics in macromolecular synthesis and involves the possibility of selection between states of high and low protein production. A simple extension of the model leads to the creation of a mechanism enabling the establishment of self-sustained oscillations.

Hyperoxidant states cause microbial cell differentiation by cell isolation from dioxygen

A general theory giving an explanation of microbial cell differentiation is presented. Based on experimental results, an unstable hyperoxidant state is postulated to trigger differentiation. Simple rules, involving the reduction of dioxygen and the isolation from dioxygen by diverse mechanisms, are proposed to govern transitions between the growth state and the differentiated states. With this view, common features of microbial differentiation processes, dimorphic growth, cell differentiation in dioxygen evolving phototrophs and in anaerobes are analyzed. The theory could have implications for understanding cell differentiation in higher organisms.

Role of lysine methylation in the activities of elongation factor 1 alpha

Previous work in our laboratory has demonstrated that 19% of the lysine residues in the protein synthesis elongation factor (EF-1 alpha) are methylated when the factor is purified from the mycelial form of the fungus Mucor racemosus. However, the same factor, when purified from spores of M. racemosus, is largely unmethylated. Despite its wide-spread occurrence in a great number of basic proteins, the functional significance of lysine N-methylation remains poorly understood. Spore and mycelial forms of EF-1 alpha were therefore compared in a series of assays to determine their relative affinities for various substrates and cofactors known to interact with the factor during the elongation cycle. The results suggested that hypomethylated and fully methylated EF-1 alpha had equal affinities for GTP, aminoacyl-tRNA, and ribosomes. Also, methylation did not appear to affect the accuracy of translation in an in vitro system. However, experiments did suggest that methylation may affect the ability of the factor to form complexes with other subunits (EF-1 beta gamma) which are known to enhance the overall rate of protein synthesis.

Three genes for the elongation factor EF-1 alpha in Mucor racemosus

We cloned three genes from Mucor racemosus coding for protein synthesis elongation factor 1 alpha (EF-1 alpha). A 110-base-pair (bp) EF-1 alpha-specific cDNA clone was identified by hybrid-selected translation. The nucleotide sequence of the cDNA showed significant homology to a region of the Saccharomyces cerevisiae genes for EF-1 alpha (TEF1 and TEF2). The cDNA was used to isolate an 850-bp EcoRI genomic DNA fragment containing a portion of the EF-1 alpha gene. Screening of a lambda/M. racemosus genomic DNA bank with the 850-bp EcoRI probe resulted in the identification of three DNA fragments containing a common 850-bp EcoRI fragment within a short overlapping region. S1 nuclease analysis of the three EF-1 alpha DNA fragments showed that the EF-1 alpha transcript covered the short overlapping region in the clones. Restriction fragments purified from flanking regions in each clone were used to probe a HindIII digest of M. racemosus genomic DNA. Each flanking probe hybridized to one of three DNA fragments which hybridized to the 850-bp EF-1 alpha-specific probe. Nucleotide sequence data from two random "shotgun clones" of one of the three genes show good homology to two regions of S. cerevisiae TEF1. The data indicate the presence of three genes for EF-1 alpha in M. racemosus located at unique sites in the genome.

Expression of the adenylate cyclase gene during cell elongation in Escherichia coli K-12

Expression of the adenylate cyclase gene (cya) in synchronized Escherichia coli cells was investigated by using the cya-lacZ protein and operon fusion plasmids. The regulation of cya expression during the cell cycle is characterized as follows: cya is expressed during cell elongation; expression is repressed during cell division; regulation is exerted at the transcriptional level. To test cya expression during cell elongation, we constructed a plasmid (pLCR1) in which the lacUV5 promoter operator was fused to the structural gene of cya and investigated the effect of cya expression by isopropyl-beta-D-thiogalactopyranoside (IPTG) on the cell division of cells containing pLCR1. By the addition of IPTG, cell division was inhibited and filaments were formed. Such an inhibitory effect was antagonized by adding cyclic GMP to the culture medium and was not observed in the crp mutant.

Three genes for the elongation factor EF-1 alpha in Mucor racemosus

We cloned three genes from Mucor racemosus coding for protein synthesis elongation factor 1 alpha (EF-1 alpha). A 110-base-pair (bp) EF-1 alpha-specific cDNA clone was identified by hybrid-selected translation. The nucleotide sequence of the cDNA showed significant homology to a region of the Saccharomyces cerevisiae genes for EF-1 alpha (TEF1 and TEF2). The cDNA was used to isolate an 850-bp EcoRI genomic DNA fragment containing a portion of the EF-1 alpha gene. Screening of a lambda/M. racemosus genomic DNA bank with the 850-bp EcoRI probe resulted in the identification of three DNA fragments containing a common 850-bp EcoRI fragment within a short overlapping region. S1 nuclease analysis of the three EF-1 alpha DNA fragments showed that the EF-1 alpha transcript covered the short overlapping region in the clones. Restriction fragments purified from flanking regions in each clone were used to probe a HindIII digest of M. racemosus genomic DNA. Each flanking probe hybridized to one of three DNA fragments which hybridized to the 850-bp EF-1 alpha-specific probe. Nucleotide sequence data from two random "shotgun clones" of one of the three genes show good homology to two regions of S. cerevisiae TEF1. The data indicate the presence of three genes for EF-1 alpha in M. racemosus located at unique sites in the genome.

Effect of oxygen on morphogenesis and polypeptide expression by Mucor racemosus

The morphology of Mucor racemosus in cultures continuously sparged with nitrogen gas was investigated. When appropriate precautions were taken to prevent oxygen from entering the cultures, the morphology of the cells was uniformly yeastlike irrespective of the N2 flow rate. When small amounts of oxygen entered the cultures the resulting microaerobic conditions evoked mycelial development. Polypeptides synthesized by aerobic mycelia, microaerobic mycelia, anaerobic yeasts, and yeasts grown in a CO2 atmosphere were compared by two-dimensional gel electrophoresis. The results indicated that a large number of differences in polypeptide expression exist when microaerobic mycelia or anaerobic yeasts are compared with aerobic mycelia and that these alterations correlate with a change from an oxidative to a fermentative metabolic mode. Relatively few differences in polypeptide composition exist when microaerobic cells are compared with anaerobic cells, but these changes correlate with a change from the mycelial to the yeast morphology. We hypothesize that oxygen regulates the expression of polypeptides involved in both the metabolic mode and in morphogenesis.

Respiratory-competent conditional developmental mutant of Mucor racemosus

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 mutant 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.

Regulation of protein synthesis factor EF-1 alpha in Mucor racemosus

The protein synthesis elongation factor EF-1 alpha of Mucor racemosus hyphae contained eight or nine methylated amino acids per molecule, whereas the factor from sporangiospores was nonmethylated. During the course of spore germination, the specific activity of the factor in crude extracts increased sixfold. This increase in activity was accompanied by a constant level of EF-1 alpha-specific mRNA and a constant level of EF-1 alpha protein. Methylation of the protein, however, accelerated during the germination process, in parallel with the increase in specific activity of the factor. We propose that the activity of EF-1 alpha is regulated during germination through methylation of the protein and does not involve transcriptional regulation.

Regulation of protein synthesis factor EF-1 alpha in Mucor racemosus

The protein synthesis elongation factor EF-1 alpha of Mucor racemosus hyphae contained eight or nine methylated amino acids per molecule, whereas the factor from sporangiospores was nonmethylated. During the course of spore germination, the specific activity of the factor in crude extracts increased sixfold. This increase in activity was accompanied by a constant level of EF-1 alpha-specific mRNA and a constant level of EF-1 alpha protein. Methylation of the protein, however, accelerated during the germination process, in parallel with the increase in specific activity of the factor. We propose that the activity of EF-1 alpha is regulated during germination through methylation of the protein and does not involve transcriptional regulation.

Trehalase activity and cyclic AMP content during early development of Mucor rouxii spores

Incubation of Mucor rouxii sporangiospores in complex medium under aerobic conditions resulted in a transient 20-fold increase in trehalase activity. Maximum activity was reached after 15 min. Simultaneously, the cyclic AMP (cAMP) content increased approximately eightfold, reaching a maximum within 10 min. Increases in trehalase activity and cAMP content were also observed under anaerobic conditions (CO2). The extent of trehalase activation and the changes in cAMP content, during both aerobic and anaerobic incubation, varied with the medium used. Trehalase was activated in vitro by a cAMP- and ATP-dependent process. An even faster activation was obtained when cAMP was replaced by the catalytic subunit of beef heart protein kinase. The coincidence of, and the correlation between, increased cAMP contents and trehalase activities support the involvement of a cAMP-dependent phosphorylation in the in vivo regulation of trehalase activity.

Molecular aspects of fungal dimorphism

The approach to this article will be the review of experimental developments achieved in recent years in the fields of biochemistry, molecular biology, ultrastructure, etc. and that have suggested models and hypothesis in order to understand the regulation of the molecular mechanisms involved in fungal differentiation. Since fungal dimorphism has been seen by many investigators as a useful model of differentiation in eukaryotic systems, and also because of the prevalence of dimorphism among human pathogenic fungi, we have considered relevant to review the work done in Mucor, but also in dimorphic pathogenic fungi such as Paracoccidioides brasiliensis, Histoplasma capsulatum, Candida albicans, Blastomyces dermatitidis, and Sporothrix Schenckii. From this point of view, the involvements of cyclic nucleotides, nucleic acid polymerases, synthesis of macromolecules, synthesis of cell wall and less studied factors will be reviewed.

Isolation and biochemical analysis of Mucor bacilliformis monomorphic mutants

Fourteen stable mutants of Mucor bacilliformis which grew yeastlike under both aerobic and anaerobic conditions were isolated after treatment of growing mycelium with N-methyl-N'-nitro-N-nitrosoguanidine. Biochemical characterization of the mutants included determination of growth in different carbon and nitrogen sources, determination of sensitivity of respiration to cyanide and salicylhydroxamate, analysis of cytochrome spectra, determination of glutamate dehydrogenases, glutamine synthase, and ornithine decarboxylase activities, and measurement of cyclic AMP levels. Data showed that all mutants were defective in some aspect of oxidative metabolism and had low levels of ornithine decarboxylase, whereas other characters were variable. It was concluded that morphological transition in M. bacilliformis is probably associated with mitochondrial functions and expression of ornithine decarboxylase, but may be independent of cyclic AMP and glutamate dehydrogenase levels. The importance of genetic studies in the analysis of dimorphism is stressed.

Isolation and characterization of yeast mutants deficient in adenylate cyclase and cAMP-dependent protein kinase

Mutants of Saccharomyces cerevisiae that require cAMP for growth have been isolated. Some of the mutants isolated were deficient in adenylate cyclase activity and mapped at a locus, cyr1, located near the centromere of chromosome X. Growth of cells carrying the cyr1 mutation was arrested at the G1 phase of the yeast cell cycle in the absence of cAMP. The cyr1 mutation was suppressed by a secondary mutation designated bcy1. The bcy1 mutation bypassed the need for cAMP for growth. The bcy1 mutants had extremely low levels of cAMP-binding protein and cAMP-dependent protein kinase but produced a high level of cAMP-independent protein kinase. The results indicate that cAMP is an essential factor for yeast cells to proceed through the cell cycle via the activation of protein kinase.

Anomalous cell wall synthesis in Mucor mucedo (L.) Fres. induced by some fungicides and other compounds related to the problem of dimorphism

An anomalous cell wall thickening in Mucor mucedo is induced already within 60-120 min by some fungicides (etridiazol, chloroneb, pentachloronitrobenzene, dicloran, drazoxolon, biphenyl) as well as with a N2-atmosphere or high concentrations of glucose, but not with 2,4-dinitrophenol, chlorinated phenols, dichlofluanid and antimycin A. This effect seems to be identical to the change from the mycelial (M-) to the yeast (Y-) form in dimorphic fungi, which can be achieved by culture conditions as well as by addition of chemicals. The cause seems to be a specific, complex change in the metabolic state. A scheme of regulation is presented which explains most of the experimental results described till now.

Temperature- and cyclic nucleotide-induced phase transitions of Histoplasma capsulatum

The transition from yeast to mycelia of Histoplasma capsulatum could be accomplished by shifting the temperature of incubation from 37 to 25 degrees C. It was accompanied by many changes in cellular metabolism, including changes in respiration, intracellular cyclic adenosine 3',5'-monophosphate (cAMP) levels, and activities of two enzymes specific for the yeast phase, cystine reductase (EC 1.6.4.1) and cysteine oxidase (EC 1.13.11.20). Even at 37 degrees C, the yeast to mycelial transition could be induced by cAMP and agents which raise the intracellular levels of cAMP (theophylline, acetylsalicylic acid, prostaglandin E1, and nerve growth factor). During this morphogenesis the same pattern of changes occurred as in the temperature-induced transition. Therefore, these changes were not simply dependent on a shift in temperature, but rather were part of the process of the phase transition.

Regulation of dimorphism in Histoplasma capsulatum by cyclic adenosine 3',5'-monophosphate

During temperature-induced transition of the dimorphic pathogenic fungus Histoplasma capsulatum from the single yeast cell form to the multicellular mycelial form, there was an increase in intracellular cyclic adenosine 3',5'-monophosphate (cAMP) levels as well as a striking accumulation of cAMP in the medium. cAMP levels also changed during the reverse mycelium-to-yeast transition.

Control of adenosine 3',5'-monophosphate level and protein phosphorylation by depolarizing agents in Coprinus macrorhizus

The treatment of mycelial cells with membrane-active antibiotics, uncouplers of oxidative phosphorylation and KCl leads to a transient increase in adenosine 3',-5'-monophosphate (cyclic AMP) levels in Coprinus macrorhizus. The maximal values and duration of increase in the cyclic AMP level depended on the kind and amount of these drugs. The treatment with these drugs simultaneously resulted in a rapid increase in the phosphorylation of three cellular proteins. The levels and time course of phosphorylation of these proteins were paralleled with the increase of cyclic AMP level in response to the drugs used. Thus, the treatment of these drugs causes the transient increase of cyclic AMP level and cyclic AMP stimulates the phosphorylation of particular proteins by activating protein kinases.

Glyoxylate cycle in Mucor racemosus

The dimorphic phycomycete Mucor racemosus was grown in media containing acetate, glutamate, and peptone as carbon sources. The component enzymes of the glyoxylate bypass, isocitrate lyase and malate synthase, were present under these conditions throughout the growth cycles. Highest specific activities for each enzyme were found in media with acetate as the carbon source. In an enriched peptone medium containing glucose, neither activity was detected until glucose was exhausted from the medium. Treatment of acetate-grown cells with glucose resulted in a rapid decline in the specific activities of both enzymes. The importance of this cycle in acetate-grown cells was indicated by the ability of itaconic acid (100 mM) to inhibit the growth of M. racemosus in acetate but not glutamate media. Itaconate was also shown to be a potent inhibitor of isocitrate lyase activity in vitro.

Cyclic adenosine 3',5'-monophosphate and germination of sporangiospores from the fungus Mucor

Cyclic adenosine 3',5'-monophosphate (cAMP) metabolism was examined in germinating sporangiospores of Mucor genevensis and Mucor mucedo. Exogenous cAMP prevented normal hyphal development from sporangiospores. Internal pools of cAMP fluctuated profoundly during development. Spherical growth of the spores was characterized by large pools of cAMP whereas germ tube emergence and hyphal elongation were characterized by small pools of cAMP. These observations suggest a possible role for cAMP in sporangiospore germination. Adenylate cyclase activities fluctuated significantly during germination with maximum values attained during spherical growth. In contrast, cAMP phosphodiesterase activities remained constant throughout germination. Internal cAMP levels may therefore be regulated by adjustment of adenylate cyclase activities. The binding of cAMP by soluble cell proteins was measured. cAMP-binding activity changed greatly during germination. Dormant and spherically growing spores possessed the highest activities. Developing hyphae contained the lowest activities. Use of the photoaffinity label, 8-azido-[32P]cAMP, in conjunction with sodium dodecyl sulfate-polyacrylamide-gel electrophoresis allowed the identification of a small population of morphogenetic-stage-specific proteins which bind cAMP and may be of regulatory significance to development.

Changes in cyclic nucleotide levels and dimorphic transition in Candida albicans

The relationship between the levels of cyclic nucleotides and dimorphic transition in Candida albicans was examined. The results showed that cells of this pathogenic fungus contained both cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP), the concentration of the latter being about one-tenth that of the former in stationary-phase cells of the yeast form. Our results further indicated that germ tube formation induced by incubation at 40 degrees C followed a rise in cAMP concentration in the cell with no accompanying change in cGMP content. Cysteine, which suppressed germination, also reversed the increase in intracellular cAMP concentration. Dibutyryl cAMP (1 MM) significantly promoted germination in proline medium at temperatures of 32 to 34 degrees C. These results suggested that cAMP was one of the controlling factors in the morphological transition in Candida albicans.

Effect of hexoses on the levels of pyruvate decarboxylase in Mucor rouxii

Pyruvate decarboxylase activity in the dimorphic fungus Mucor rouxii increased 25- to 35-fold in yeastlike and mycelial cells grown in the presence of glucose as compared to the activity observed in mycelial cultures grown in the absence of glucose.

S-adenosylmethionine levels and protein methylation during morphogenesis of Mucor racemosus

The intracellular level of S-adenosylmethionine increased as the yeast-phase cells of Mucor racemosus were induced to convert to hyphae. This increase correlated well with time course of the conversion in cell type and was independent of the metabolic changes caused by the shift to aerobic conditions. There was no significant change in the intracellular level of spermidine, a polyamine synthesized from putrescine and the propylamine group of S-adenosylmethionine. Spermine was not detected. An examination of protein methylation revealed an increase in the methylation of total protein during the shift in cell type and possible qualitative as well as quantitative changes in specific base proteins.

Differential synthesis of polypeptides during morphogenesis of Mucor

The extent of differential gene expression during morphogenesis of Mucor racemosus was investigated by two-dimensional polyacrylamide gel electrophoresis of neutral and acidic polypeptides. Cellular proteins were labeled with [35S]methionine in cells growing in either the yeast or hyphal form, or in yeast cells undergoing the transition of hyphae. The results showed that of the 400 to 500 polypeptides resolved by electrophoresis, relatively few were specific to one or the other morphological form. The major change in the patterns of proteins synthesized during morphogenesis was a change in rates of synthesis of individual polypeptides. Experiments in which morphogenesis was affected under aerobic or anaerobic conditions showed that the majority of changes in the protein patterns were associated with morphogenesis and were not a specific response to O2.

Regulation of ornithine decarboxylase during morphogenesis of Mucor racemosus

During the yeast-to-hyphae transition of the dimorphic phycomycete Mucor racemosus, there was a 30- to 50-fold increase in the activity of ornithine decarboxylase. Increased enzyme activity preceded the emergence of germ tubes and reached a maximum before conversion was completed. Subsequently, enzyme levels rapidly declined, despite the continuation of mycelial growth. Both putrescine and spermidine blocked the enzyme activity response. Protein synthesis was required for the increase in enzyme activity during morphogenesis. A combination of actinomycin D and netropsin inhibited ribonucleic acid synthesis but failed to inhibit the increase in ornithine decarboxylase activity. There was a twofold increase in the enzyme half-life during morphogenesis with either trichodermin or verrucarin to inhibit protein synthesis.

Physiological control of phosphorylation ribosomal protein S6 in Mucor racemosus

The level of phosphorylation of ribosomal protein S6 increased with accelerating rates of growth and protein synthesis in Mucor racemosus. Lowered levels of phosphorylation were seen under conditions of metabolic shift-down or the onset of stationary phase, and no phosphorylation was detected in sporangiospores. Changing metabolic states, changing intracellular levels of adenosine triphosphatase, and the level of phosphorylation of protein S6 were correlated in M. racemosus.

Changing pattern of cyclic AMP-binding proteins during germination of Mucor racemosus sporangiospores

Interation of cyclic AMP with a profoundly changing pattern of specific binding proteins was shown during aerobic germination of sporangiospores from the fungus Mucor racemosus. 32P-labeled 8-azido-cycli AMP, an analogue of cyclic AMP that forms a covalent linkage with the binding proteins under u.v. light, was used as the ligand. Binding proteins carrying this photoaffinity label were separated by polyacrylamide-gel electrophoresis and identified by radioautography. Equibiltrium dissociation constants (Kd) and binding-response curves in the presence of competing nucleotides were identical for both 8-azido-cyclic [32P]AMP and cyclic [3H]AMP. A quantitative binding assay with both 8-azido-cyclic [32P]AMP and cyclic [3H]AMP over the time course of sporangiospore germination indicated a parallel relationship between cyclic AMP-binding capacity and the intracellular concentrations of cyclic AMP reported in a previous study [Paznokas & Sypherd (1975) J. Bacteriol. 124, 134--139]. Both of these parameters attained transient high values at a time of development when addition of exogenous cyclic AMP prevents hyphal-germ-tube emergence. The measured Kd values did not change during sport germination.

Control of cyclic adenosine 3',5'-monophosphate levels by depolarizing agents in fungi

It has been reported that diverse treatments which depolarize the plasma membrane of Neurospora crassa produce rapid increases in cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels. In the current study, membrane active antibiotics, which are known or putative depolarizing agents, were found to produce similar cyclic AMP increases, not only in N. crassa, but also in the distantly related fungi Saccharomyces cerevisiae and Mucor racemosus. Uncouplers of oxidative phosphorylation, which have been found to depolarize Neurospora, also produced cyclic AMP increases in all three fungi. The time course of the cyclic AMP response to these various treatments was similar in all three fungi. The fungal studies and studies on depolarized central nervous tissue suggest that cyclic AMP increases may be produced in response to plasma membrane depolarization in diverse eucaryotic cells. A model is proposed for eucaryotic microorganisms in which membrane depolarization serves as a signal of breakdown of the plasma membrane integrity. The subsequent cyclic AMP increase, in turn, may mediate cellular response to help protect the plasma membrane from chemical and mechanical threats to its integrity.

Morphology-associated expression nicotinamide adenine dinucleotide-dependent glutamate dehydrogenase in Mucorracemosus

The in vivo regulation of glutamate dehydrogenase (GDH) was studied in Mucor racemosus as a function of nutritional conditions and morphological state. Both nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP)-dependent GDH activities were found. The effect of carbon and nitrogen source on the specific activity of the NAD-dependent GDH suggests that its role is primarily catabolic. The NAD-dependent activity was generally an order of magnitude greater in mycelial cells than in yeast-phase cells grown on the same medium. During yeast-to-hyphal morphogenesis the increase in NAD-dependent activity preceded the appearance of hyphal cells both under aerobic and anaerobic conditions. Exogenous dibutyryl-cyclic AMP prevented the increase in NAD-dependent GDH concomitantly with the suppression of morphological differentiation. The NADP-dependent activity did not change appreciably during morphogenesis.

Cyclic AMP and cyclic GMP in germinating conidia of Neurospora crassa

A new method for obtaining synchronous germination allowed accurate time-course studies of endogenous levels of cyclic adenosine 3',5'-monophosphate and cyclic guanosine 3',5'-phosphate in germinating conidia of Neurospora crassa. The levels of both cyclic nucleotides remained constant throughout germination, showing that they neither signal nor respond to any of the biochemical changes which are known to occur in the germination process. Conidal germination was approximately normal in three cr-1 mutants of Neurospora which have been shown to be deficient in adenylate cyclase activity. Cyclic AMP levels in the mycelia of cr-1 mutants were low, but surprisingly, the levels in conidia were normal.

Regulation of macromolecular synthesis during hyphal germ tube emergence from Mucor racemosus sporangiospores

Protein and RNA syntheses were examined during hyphal germ tube emergence from sporangiospores of a dimorphic phycomycete, Mucor racemosus. Both classes of macromolecules were synthesized immediately upon introduction of the dormant sporangiospores into nutrient medium. The specific rates of synthesis of both protein and RNA accelerated during initial germ tube emergence and reached a maximum when the emergence of new germ tubes ended. The specific rates of synthesis later decreased during further hyphal elongation. The distribution of ribosomes between active polysomes and monosomes and inactive subunits was determined by sucrose density gradient centrifugation, and the rate of amino acid addition to nascent polypeptide chains was calculated throughout the developmental sequence. The results showed that both the percentage of ribosomes active in protein synthesis and the velocity of ribosome movement along the mRNA were continuously adjusted throughout hyphal germ tube development. The free intracellular amino acid pools were measured throughout development. Alanine, glutamate, and aspartate were present at very high concentrations in the dormant spores but were rapidly depleted during hyphal germ tube emergence. The results of these studies are discussed in relation to hyphal germ tube development from yeast cells of Mucor and dormant spores of other fungal species.

Glucose metabolism and dimorphism in Mucor

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 of glucose 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.