Mechanism of interaction of an endofungal bacterium Serratia marcescens D1 with its host and non-host fungi

Association of bacteria with fungi is a major area of research in infection biology, however, very few strains of bacteria have been reported that can invade and reside within fungal hyphae. Here, we report the characterization of an endofungal bacterium Serratia marcescens D1 from Mucor irregularis SS7 hyphae. Upon re-inoculation, colonization of the endobacterium S. marcescens D1 in the hyphae of Mucor irregularis SS7 was demonstrated using stereo microscopy. However, S. marcescens D1 failed to invade into the hyphae of the tested Ascomycetes (except Fusarium oxysporum) and Basidiomycetes. Remarkably, Serratia marcescens D1 could invade and spread over the culture of F. oxysporum that resulted in mycelial death. Prodigiosin, the red pigment produced by the Serratia marcescens D1, helps the bacterium to invade fungal hyphae as revealed by the increasing permeability in fungal cell membrane. On the other hand, genes encoding the type VI secretion system (T6SS) assembly protein TssJ and an outer membrane associated murein lipoprotein also showed significant up-regulation during the interaction process, suggesting the involvement of T6SS in the invasion process.

Heterotrimeric G-alpha subunits Gpa11 and Gpa12 define a transduction pathway that control spore size and virulence in Mucor circinelloides

Mucor circinelloides is one of the causal agents of mucormycosis, an emerging and high mortality rate fungal infection produced by asexual spores (sporangiospores) of fungi that belong to the order Mucorales. M. circinelloides has served as a model genetic system to understand the virulence mechanism of this infection. Although the G-protein signaling cascade plays crucial roles in virulence in many pathogenic fungi, its roles in Mucorales are yet to be elucidated. Previous study found that sporangiospore size and calcineurin are related to the virulence in Mucor, in which larger spores are more virulent in an animal mucormycosis model and loss of a calcineurin A catalytic subunit CnaA results in larger spore production and virulent phenotype. The M. circinelloides genome is known to harbor twelve gpa (gpa1 to gpa12) encoding G-protein alpha subunits and the transcripts of the gpa11 and gpa12 comprise nearly 72% of all twelve gpa genes transcript in spores. In this study we demonstrated that loss of function of Gpa11 and Gpa12 led to larger spore size associated with reduced activation of the calcineurin pathway. Interestingly, we found lower levels of the cnaA mRNAs in sporangiospores from the Δgpa12 and double Δgpa11/Δgpa12 mutant strains compared to wild-type and the ΔcnaA mutant had significantly lower gpa11 and gpa12 mRNA levels compared to wild-type. However, in contrast to the high virulence showed by the large spores of ΔcnaA, the spores from Δgpa11/Δgpa12 were avirulent and produced lower tissue invasion and cellular damage, suggesting that the gpa11 and gpa12 define a signal pathway with two branches. One of the branches controls spore size through regulation of calcineurin pathway, whereas virulences is controlled by an independent pathway. This virulence-related regulatory pathway could control the expression of genes involved in cellular responses important for virulence, since sporangiospores of Δgpa11/Δgpa12 were less resistant to oxidative stress and phagocytosis by macrophages than the ΔcnaA and wild-type strains. The characterization of this pathway could contribute to decipher the signals and mechanism used by Mucorales to produce mucormycosis.

Functional complementarity of ancient plant-fungal mutualisms: contrasting nitrogen, phosphorus and carbon exchanges between Mucoromycotina and Glomeromycotina fungal symbionts of liverworts

Liverworts, which are amongst the earliest divergent plant lineages and important ecosystem pioneers, often form nutritional mutualisms with arbuscular mycorrhiza-forming Glomeromycotina and fine-root endophytic Mucoromycotina fungi, both of which coevolved with early land plants. Some liverworts, in common with many later divergent plants, harbour both fungal groups, suggesting these fungi may complementarily improve plant access to different soil nutrients. We tested this hypothesis by growing liverworts in single and dual fungal partnerships under a modern atmosphere and under 1500 ppm [CO₂ ], as experienced by early land plants. Access to soil nutrients via fungal partners was investigated with ¹⁵ N-labelled algal necromass and ³³ P orthophosphate. Photosynthate allocation to fungi was traced using ¹⁴ CO₂ . Only Mucoromycotina fungal partners provided liverworts with substantial access to algal ¹⁵ N, irrespective of atmospheric CO₂ concentration. Both symbionts increased ³³ P uptake, but Glomeromycotina were often more effective. Dual partnerships showed complementarity of nutrient pool use and greatest photosynthate allocation to symbiotic fungi. We show there are important functional differences between the plant-fungal symbioses tested, providing new insights into the functional biology of Glomeromycotina and Mucoromycotina fungal groups that form symbioses with plants. This may explain the persistence of the two fungal lineages in symbioses across the evolution of land plants.

Gastrointestinal microbiota alteration induced by Mucor circinelloides in a murine model

Mucor circinelloides is a pathogenic fungus and etiologic agent of mucormycosis. In 2013, cases of gastrointestinal illness after yogurt consumption were reported to the US FDA, and the producer found that its products were contaminated with Mucor. A previous study found that the Mucor strain isolated from an open contaminated yogurt exhibited virulence in a murine systemic infection model and showed that this strain is capable of surviving passage through the gastrointestinal tract of mice. In this study, we isolated another Mucor strain from an unopened yogurt that is closely related but distinct from the first Mucor strain and subsequently examined if Mucor alters the gut microbiota in a murine host model. DNA extracted from a ten-day course of stool samples was used to analyze the microbiota in the gastrointestinal tracts of mice exposed via ingestion of Mucor spores. The bacterial 16S rRNA gene and fungal ITS1 sequences obtained were used to identify taxa of each kingdom. Linear regressions revealed that there are changes in bacterial and fungal abundance in the gastrointestinal tracts of mice which ingested Mucor. Furthermore, we found an increased abundance of the bacterial genus Bacteroides and a decreased abundance of the bacteria Akkermansia muciniphila in the gastrointestinal tracts of exposed mice. Measurements of abundances show shifts in relative levels of multiple bacterial and fungal taxa between mouse groups. These findings suggest that exposure of the gastrointestinal tract to Mucor can alter the microbiota and, more importantly, illustrate an interaction between the intestinal mycobiota and bacteriota. In addition, Mucor was able to induce increased permeability in epithelial cell monolayers in vitro, which might be indicative of unstable intestinal barriers. Understanding how the gut microbiota is shaped is important to understand the basis of potential methods of treatment for gastrointestinal illness. How the gut microbiota changes in response to exposure, even by pathogens not considered to be causative agents of food-borne illness, may be important to how commercial food producers prevent and respond to contamination of products aimed at the public. This study provides evidence that the fungal microbiota, though understudied, may play an important role in diseases of the human gut.

Biochemical mechanism of phytoremediation process of lead and cadmium pollution with Mucor circinelloides and Trichoderma asperellum

This study focused on the bioremediation mechanisms of lead (0, 100, 500, 1000 mg kg^-1) and cadmium (0,10,50,100 mg kg^-1) contaminated soil using two indigenous fungi selected from mine tailings as the phytostimulation of Arabidopsis thaliana. The two fungal strains were characterized as Mucor circinelloides (MC) and Trichoderma asperellum (TA) by internal transcribed spacer sequencing at the genetic levels. Our research revealed that Cadmium was more toxic to plant growth than lead and meanwhile, MC and TA can strengthen A. thaliana tolerance to cadmium and lead with 40.19-117.50% higher root length and 58.31-154.14% shoot fresh weight of plant compared to non-inoculation. In this study, TA exhibited a higher potential to the inactivation of cadmium; however, MC was more effective in lead passivation. There was a direct correlation between the type of fungi, heavy metal content, heavy metal type and oxidative damage in plant. Both lead and cadmium induced oxidative damage as indicated by increased superoxide dismutase and catalase activities, while the antioxidant levels were significantly higher in fungal inoculated plants compared with those non-inoculated. The analysis of soil enzyme activity and taxonomic richness uncovered that the dominant structures of soil microbial community were altered by exogenous microbial agents. MC enhanced higher microbial diversity and soil enzyme activity than TA. The two indigenous fungi lessened several limiting factors with respect to phytoremediation technology, such as soil chemistry, contamination level and transformation, and metal solubility.

Does co-inoculation of Lactuca serriola with endophytic and arbuscular mycorrhizal fungi improve plant growth in a polluted environment?

Phytoremediation of polluted sites can be improved by co-inoculation with mycorrhizal and endophytic fungi. In this study, the effects of single- and co-inoculation of Lactuca serriola with an arbuscular mycorrhizal (AM) fungus, Rhizoglomus intraradices, and endophytic fungi, Mucor sp. or Trichoderma asperellum, on plant growth, vitality, toxic metal accumulation, sesquiterpene lactone production and flavonoid concentration in the presence of toxic metals were evaluated. Inoculation with the AM fungus increased biomass yield of the plants grown on non-polluted and polluted substrate. Co-inoculation with the AM fungus and Mucor sp. resulted in increased biomass yield of plants cultivated on the polluted substrate, whereas co-inoculation with T. asperellum and the AM fungus increased plant biomass on the non-polluted substrate. In the presence of Mucor sp., mycorrhizal colonization and arbuscule richness were increased in the non-polluted substrate. Co-inoculation with the AM fungus and Mucor sp. increased Zn concentration in leaves and roots. The concentration of sesquiterpene lactones in plant leaves was decreased by AM fungus inoculation in both substrates. Despite enhanced host plant costs caused by maintaining symbiosis with numerous microorganisms, interaction of wild lettuce with both mycorrhizal and endophytic fungi was more beneficial than that with a single fungus. The study shows the potential of double inoculation in unfavourable environments, including agricultural areas and toxic metal-polluted areas.

A protein complex bearing an oxidase with napthalene dihydrodiol dehydrogenase activity is induced in Mucor circinelloides strain YR-1 during growth on polycyclic aromatic compounds

Fungi are organisms capable of growing in a myriad of conditions and respond to counteract environmental cues. Several locations in the world are polluted with oil and its derivatives, and some microorganisms tolerant to these compounds have been isolated. Some fungi can grow in the presence of molecules such as polycyclic aromatic hydrocarbons as sole carbon sources. In this report, we further characterized the induced enzymes with phenanthrene from Mucor circinelloides YR-1 strain, isolated from a polluted field near a petrochemical facility in México. We identified a putative oxidase that is induced when growth with phenanthrene as sole carbon source at a pH of 8.5 and is NADP⁺ dependent. We show that this enzyme bears naphthalene dihydrodiol dehydrogenase activity with substrate preference for the cis-naphthalene over the trans-naphthalene, with an optimal pH in the range of 8-10. Mass spectrometry analysis revealed that the induced enzyme belongs to the NADP⁺ oxidase family enzymes with the typical Rossmann-fold for NADP⁺ binding. This enzyme seems to form a high molecular weight structure (~ 541 kDa) and with a monomer of 57 kDa, suggesting that the multimer is constituted of 10 subunits. Our findings contribute to understanding of the roles that dihydrodiol dehydrogenases have in organisms exposed to toxic compounds in the environment and can regulate their expression.

Proteomic analysis of the adaptative response of Mucor spp. to cheese environment

In the cheese industry context, Mucor species exhibit an ambivalent behavior as some species are essential "technological" organisms of some cheeses while others can be spoiling agents. Previously, we observed that cheese "technological" species exhibited higher optimal growth rates on cheese related matrices than on synthetic media. This growth pattern combined with morphological differences raise the question of their adaptation to cheese. In this study, using a comparative proteomic approach, we described the metabolic pathways of three Mucor strains considered as "technological" or "contaminant" in the cheese environment (M. lanceolatus UBOCC-A-109153, M. racemosus UBOCC-A-109155, M. circinelloides CBS 277-49) as well as a non-cheese related strain (M. endophyticus CBS 385-95). Overall, 15.8 to 19.0% of the proteomes showed a fold change ≥1.6 in Potato Dextrose Agar (PDA) versus Cheese Agar (CA), a cheese mimicking-medium. The 289 differentially expressed proteins identified by LC MS-MS analysis were mostly assigned to energy and amino-acid metabolisms in PDA whereas a higher diversity of biological processes was observed for cheese related strains in CA. Surprisingly, the vast majority (72.9%) of the over-accumulated proteins were different according to the considered medium and strain. These results strongly suggest that the observed better adaptative response of "technological" strains to cheese environment is mediated by species-specific proteins.

BIOLOGICAL SIGNIFICANCE

The Mucor genus consists of a multitude of poorly known species. In the food context, few species are known for their positive role in the production of various food products, including cheese, while others are spoiling agents. The present study focused on the analysis of morphological and proteome differences of various Mucor spp. representative strains known as either positively (hereafter referred as "technological") or negatively (hereafter referred as "contaminant") associated with cheese or non-related to cheese (endophyte) on two different media, a synthetic medium and a cheese-mimicking medium. The main goal was to assess if adaptative traits of "technological" strains to the cheese environment could be identified. This work was based on observations we did in a recently published physiological study (Morin-Sardin et al., 2016). One of the important innovative aspects lies in the use for the first time of an extensive 2-DE approach to compare proteome variations for 4 strains on two different media. Results obtained offered an insight in the metabolic mechanisms associated with growth on a given medium and showed that adaptation to cheese environment is probably supported by species-specific proteins. The obtained data represent an essential step point for more targeted studies at the genomic and transcriptomic levels.

Fungus-associated bacteriome in charge of their host behavior

Bacterial-fungal interactions are widespread in nature and there is a growing number of studies reporting distinct fungus-associated bacteria. However, little is known so far about how shifts in the fungus-associated bacteriome will affect the fungal host's lifestyle. In the present study, we describe for the first time the bacterial community associated with the saprotrophic fungus Mucor hiemalis, commonly found in soil and rhizosphere. Two broad-spectrum antibiotics that strongly altered the bacterial community associated with the fungus were applied. Our results revealed that the antibiotic treatment did not significantly reduce the amount of bacteria associated to the fungus but rather changed the community composition by shifting from initially dominating Alpha-Proteobacteria to dominance of Gamma-Proteobacteria. A novel approach was applied for the isolation of fungal-associated bacteria which also revealed differences between bacterial isolates obtained from the original and the antibiotic-treated M. hiemalis. The shift in the composition of the fungal-associated bacterial community led to significantly reduced fungal growth, changes in fungal morphology, behavior and secondary-metabolites production. Furthermore, our results showed that the antibiotic-treated isolate was more attractive and susceptible to mycophagous bacteria as compared to the original isolate. Overall, our study highlights the importance of the fungus-associated bacteriome for the host's lifestyle and interactions and indicate that isolation with antibacterials is not sufficient to eradicate the associated bacteria.

Antifungal Rhizosphere Bacteria Can increase as Response to the Presence of Saprotrophic Fungi

Knowledge on the factors that determine the composition of bacterial communities in the vicinity of roots (rhizosphere) is essential to understand plant-soil interactions. Plant species identity, plant growth stage and soil properties have been indicated as major determinants of rhizosphere bacterial community composition. Here we show that the presence of saprotrophic fungi can be an additional factor steering rhizosphere bacterial community composition and functioning. We studied the impact of presence of two common fungal rhizosphere inhabitants (Mucor hiemalis and Trichoderma harzianum) on the composition of cultivable bacterial communities developing in the rhizosphere of Carex arenaria (sand sedge) in sand microcosms. Identification and phenotypic characterization of bacterial isolates revealed clear shifts in the rhizosphere bacterial community composition by the presence of two fungal strains (M. hiemalis BHB1 and T. harzianum PvdG2), whereas another M. hiemalis strain did not show this effect. Presence of both M. hiemalis BHB1 and T. harzianum PvdG2 resulted in a significant increase of chitinolytic and (in vitro) antifungal bacteria. The latter was most pronounced for M. hiemalis BHB1, an isolate from Carex roots, which stimulated the development of the bacterial genera Achromobacter and Stenotrophomonas. In vitro tests showed that these genera were strongly antagonistic against M. hiemalis but also against the plant-pathogenic fungus Rhizoctonia solani. The most likely explanation for fungal-induced shifts in the composition of rhizosphere bacteria is that bacteria are being selected which are successful in competing with fungi for root exudates. Based on the results we propose that measures increasing saprotrophic fungi in agricultural soils should be explored as an alternative approach to enhance natural biocontrol against soil-borne plant-pathogenic fungi, namely by stimulating indigenous antifungal rhizosphere bacteria.

Enhancement of phytoremediation of Cd- and Pb-contaminated soils by self-fusion of protoplasts from endophytic fungus Mucor sp. CBRF59

The aim of this study was to isolate protoplasts from endophytic fungi and to carry out self-fusion of protoplasts for their enhancement of metal resistance. Self-fusant CBRF59T3 with resistance to 25 mM Cd(II) was constructed by self-fusion of inactivated protoplasts from Mucor sp. CBRF59. The inoculation of CBRF59 and CBRF59T3 improved significantly the availability of Cd(II) and Pb(II) in the soil. Compared with CBRF59, CBRF59T3 inoculation increased the content of water-soluble Cd(II) by 24%. The dry weight of rape inoculated with CBRF59 and CBRF59T3 was both higher than that of the uninoculation rape. Inoculation of CBRF59T3 further increased the dry weight of rape by 62% than CBRF59 in the higher Cd(II)-+Pb(II)-contaminated soil. Compared with CBRF59, CBRF59T3 inoculation increased the concentration of Cd(II) in rape shoots by 35-189% in Cd(II)- and Cd(II)-+Pb(II)-contaminated soils. The inoculation of CBRF59T3 also enhanced the translocation of Cd(II) from roots to shoots and increased the amount of extracted Cd(II) by rape. The results indicated that the mutants constructed by protoplast fusion is a feasible and efficient method to improve stress tolerance of uncharacterized fungi for phytoremediation of soils contaminated by heavy metals.

Sporangiospore size dimorphism is linked to virulence of Mucor circinelloides

Mucor circinelloides is a zygomycete fungus and an emerging opportunistic pathogen in immunocompromised patients, especially transplant recipients and in some cases otherwise healthy individuals. We have discovered a novel example of size dimorphism linked to virulence. M. circinelloides is a heterothallic fungus: (+) sex allele encodes SexP and (-) sex allele SexM, both of which are HMG domain protein sex determinants. M. circinelloides f. lusitanicus (Mcl) (-) mating type isolates produce larger asexual sporangiospores that are more virulent in the wax moth host compared to (+) isolates that produce smaller less virulent sporangiospores. The larger sporangiospores germinate inside and lyse macrophages, whereas the smaller sporangiospores do not. sexMΔ mutants are sterile and still produce larger virulent sporangiospores, suggesting that either the sex locus is not involved in virulence/spore size or the sexP allele plays an inhibitory role. Phylogenetic analysis supports that at least three extant subspecies populate the M. circinelloides complex in nature: Mcl, M. circinelloides f. griseocyanus, and M. circinelloides f. circinelloides (Mcc). Mcc was found to be more prevalent among clinical Mucor isolates, and more virulent than Mcl in a diabetic murine model in contrast to the wax moth host. The M. circinelloides sex locus encodes an HMG domain protein (SexP for plus and SexM for minus mating types) flanked by genes encoding triose phosphate transporter (TPT) and RNA helicase homologs. The borders of the sex locus between the three subspecies differ: the Mcg sex locus includes the promoters of both the TPT and the RNA helicase genes, whereas the Mcl and Mcc sex locus includes only the TPT gene promoter. Mating between subspecies was restricted compared to mating within subspecies. These findings demonstrate that spore size dimorphism is linked to virulence of M. circinelloides species and that plasticity of the sex locus and adaptations in pathogenicity have occurred during speciation of the M. circinelloides complex.

Ethanol-induced changes in expression profiles of cell growth, fatty acid and desaturase genes of Mucor rouxii

We investigated the response of Mucor rouxii to ethanolic stress conditions. A differential response was found that was dependent on growth phase and ethanol concentration. 5% (v/v) ethanol showed an inhibitory effect on the mycelial growth of all stages. However, the ethanol sensitivity was specifically observed in active growing phases (12 and 21 h-grown cultures), in which the biomass and ratio of unsaturated/saturated fatty acids (UFA/SFA) decreased greatly after ethanol exposure compared to non-ethanol adding culture. With respect to different ethanol concentrations, M. rouxii was tolerant to low ethanol concentrations (about 1-3%, v/v) such that there was not much change in biomass and UFA/SFA ratio, in contrast to the 5% ethanol-added culture. We also showed the molecular basis of this response mechanism, demonstrating that expression of Delta(9)-, Delta(12)- and Delta(6)-desaturase genes, responsible for fatty acid desaturation in M. rouxii, were coordinately down-regulated upon exposure to ethanol stress.

Optimization of culture variables for improving glucoamylase production by alginate-entrapped Thermomucor indicae-seudaticae using statistical methods

Alginate-entrapped sporangiospores of Thermomucor indicae-seudaticae were used for the production of glucoamylase. The critical variables that affected glucoamylase production were identified by Plackett-Burman design (sucrose, yeast-extract, K(2)HPO(4) and asparagine) and further optimized by using a four factor central composite design (CCD) of response surface methodology (RSM). Immobilized sporangiospores secreted 41% and 60% higher glucoamylase titers in shake flasks and airlift fermenter, respectively, when the variables were used at their optimum levels (sucrose 3.0%, yeast-extract 0.2%, K(2)HPO(4) 0.1% and asparagine 0.35%). Glucoamylase production (26.3 U ml(-1)) in the optimized medium was in good agreement with the values predicted by the quadratic model (26.7 U ml(-1)), thereby confirming its validity. The enzyme production was sustainable in flasks of higher volume and also airlift fermenter, and attained a peak within 32 h in the fermenter as compared to that of 48 h in shake flasks.

Hongos nematófagos de huevos de Toxocara canis en un paseo público de La Plata, Argentina

Fungi have showed a great potential for the biological control of nematodes. However, they have not been evaluated for the control of animal and/or human parasites transmitted by egg contaminated soils. Environmental contamination with Toxocara spp. eggs is a public health problem. Accidental swallowing of Toxocara canis eggs (a nematode of dogs) usually results on a zoonotic infection (toxocarosis). The objectives of this research were: 1) To test the presence of antagonistic fungi against T. canis in the soil in public places of La Plata city, Argentina, infected with eggs of this parasite, 2) To determine the possible association between biotic and abiotic factors of the soil with the presence of fungal parasites of egg nematodes. Soil samples were tested for: textural type, organic matter (%), pH, presence of egg-parasite fungi, of larvae and of nematode eggs, in particular of Toxocara spp. The studied area showed the following characteristics: pH: 6.6-8.0, organic matter: 1.2-70%, with a predominantly loam texture. The following antagonistic fungal genera were identified: Acremonium, Aspergillus, Chrysosporium, Fusarium, Humicola, Mortierella, Paecilomyces and Penicillium. A prevalence of 70% was detected for nematode eggs, of 33% for Toxocara spp. eggs and of 90% for larvae. No association between the presence of egg-parasite fungi and the considered factors was found. More studies are necessary to know the natural antagonism factors to T. canis eggs for its in situ biological control.

Distinct white collar-1 genes control specific light responses in Mucor circinelloides

Light regulates many developmental and physiological processes in a large number of organisms. The best-known light response in the fungus Mucor circinelloides is the biosynthesis of beta-carotene. Here, we show that M. circinelloides sporangiophores also respond to light, exhibiting a positive phototropism. Analysis of both responses to different light wavelengths within the visible spectrum demonstrated that phototropism is induced by green and blue light, whereas carotenogenesis is only induced by blue light. The blue regulation of both responses suggests the existence of blue-light photoreceptors in M. circinelloides. Three white collar-1 genes (mcwc-1a, mcwc-1b and mcwc-1c) coding for proteins showing similarity with the WC-1 photoreceptor of Neurospora crassa have been identified. All three contain a LOV (light, oxygen or voltage) domain, similar to that present in fungal and plant blue-light receptors. When knockout mutants for each mcwc-1 gene were generated to characterize gene functions, only mcwc-1c mutants were defective in light induction of carotene biosynthesis, indicating that mcwc-1c is involved in the light transduction pathway that control carotenogenesis. We have also shown that positive phototropism is controlled by the mcwc-1a gene. It seems therefore that mcwc-1a and mcwc-1c genes control different light transduction pathways, although cross-talk between both pathways probably exists because mcwc-1a is involved in the light regulation of mcwc-1c expression.

Biological control of peach fungal pathogens by commercial products and indigenous yeasts

The potential use of the commercial biocontrol products Serenade (Bacillus subtilis QST-713) and Trichodex (Trichoderma harzianum Rifai strain T39) to inhibit the postharvest pathogenic molds Penicillium crustosum and Mucor circinelloides was investigated. Both products exhibited antagonistic activity in vitro against the pathogens, reducing their growth at different levels. In addition, epiphytic yeasts isolated from peaches were identified as Candida maltosa, Pichia fermentans, and Pichia kluyveri by PCR-restriction fragment length polymorphism of internal transcribed spacer regions and screened for antagonistic activity against the same molds. The efficacy of biocontrol in vitro was dependent on the concentration of the yeast cells. Optimal yeast concentrations were above 10(7) CFU ml(-1). However, C. maltosa and P. fermentans were more effective than P. kluyveri in inhibiting molds. The exclusion of antifungal metabolite production and direct competition for nutrients or space with the pathogens was proposed as the mechanism of biocontrol. Application of biocontrol agents directly on artificially wounded peach fruits significantly reduced the incidence of mold rot during storage at 20 degrees C.

Comparative fatty acid profiling ofMucor rouxiiunder different stress conditions

To understand the relationship between fatty acid metabolism and the growth morphology of Mucor rouxii, fatty acid profiling was studied comparatively in cells grown under conditions which included different atmospheric conditions or the addition of phenethyl alcohol (PEA). The significant difference in fatty acid profiles from M. rouxii grown under aerobic or anaerobic conditions was not found to be directly related to morphological growth. Oxygen limitation, which induced the formation of pure multipolar budding yeasts, led to a decrease in long-chain fatty acids-- particularly unsaturated fatty acids-- and an increase in medium-chain saturated fatty acids, a finding which contrasted with the aerobic cultures, including mycelia and PEA-induced bipolar budding cells. High levels of C18 : 1Delta(9) were found in aerobic yeast cultures with additional PEA when compared to that in the aerobically grown mycelia. The identification of unusual fatty acids in Mucor in response to alcoholic and hypoxic stresses - including odd-numbered fatty acids and 7-hydroxy dodecanoic acid (7-OH C12 : 0) in addition to the more common fatty acids - implied that an important role existed for these unusual fatty acids.

Biological activity of trisporoids and trisporoid analogues in Mucor mucedo (-)

In the course of their sexual interactions, zygomycete fungi communicate via an elaborate series of carotene-derived compounds, namely trisporic acid and its biosynthetic progenitors. A novel building-block strategy allowed the systematic generation of structurally modified trisporoids along with putative early biosynthetic precursors for physiological tests. The impact of discrete structural elements was documented by the ability of individual compounds to induce sexually committed hyphae in Mucor mucedo. The activity screening contributed to establish general structure-function relationships for trisporoid action. Most crucial for activity were the dimension of the longer side chain, the polarity of functional groups at C(4) and C(13), and the number of conjugated double bonds in the side chain. The presence of an oxygen substituent at the cyclohexene ring is not essential for function. The overall biological activity apparently results from the combination of the various structural elements.

Effect of myoinositol on sporangiospore-yeast transformation of Mucor circinelloides Tieghem cultivated in synthetic broth

Mucor species exhibit fungal dimorphism in controlled environments. In this work, we examined the effect of myoinositol supplementation on the growth and morphology of Mucor circinelloides. Using sporangiospores as inoculums, diverse morphologies were induced in synthetic broth incubated at pH 4.5, temp. 20 degrees C, ambient. The morphologies included thallic suptypes (holoblastic-, holothallic-, enterothallic conidia as well as vesicular conidial headgroups), which were determinate in growth, and proliferating yeast forms. Analysis of variance, p<0.05, showed that time had significant impact on growth. A separation of means, l.s.d. 14.34, p<0.05, indicated that myoinositol supplementation at 500 microM supported the least growth, but 2.0-3.0 mM levels had the higher values, and this was followed by the control, 300 microM, 1.0, 2.0, 5.0 mM supplementations. Although the predominant morphology, that is, terminal budding yeast cells was not quantitated, observation showed that it was more preponderant, had optimal size and cell shape became more regular at 2.0 mM myoinositol supplementation.

[Sterols of the fungus Mucor hiemalis sporangiospores]

We earlier reported that the viability and lipid composition of Mucor hiemalis F-1156 sporangiospores, as well as their capacity to develop yeast-like cells, depend on the age of the spore-forming culture [1]. With the increase in the time of the culture growth, sporangiospores exhibit a decrease in the levels of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, cardiolipin, and reserve lipids, as well as in the degree of fatty acid unsaturation, whereas the level of glycolipids increases. However, sterol composition was not been studied. Sterols are involved in morphogenetic processes [2, 3], and therefore we suggested that sporangiospores from senescent cultures, which, upon germination, develop mycelium and yeast-like cells, may exhibit a distinctive sterol pattern, along with the changed composition of fatty acids and polar and neutral lipids.

[Sporangiospores of the fungus Mucor lusitanicus 12M: correlation between lipid composition, viability, and morphology of growth upon germination]

We studied viability of sporangiospores from a surface culture of the fungus Mucor lusitanicus 12M grown on wheat bran. With culture ageing, the sporangiospores exhibited a tendency toward dimorphic growth upon germination and then lost the ability to germinate. This correlated with changes in the sporangiospore lipid composition, which involved a reduction in the total lipid pool and in the levels of reserve lipids and phospholipids in particular. We suggest that lipid catabolism in sporangiospores causes their defectiveness in the senescent culture.

[Changes in the lipid composition of Mucor hiemalis sporangiospores related to the age of the spore-forming culture]

Analysis of sporangiospore lipids of the fungus Mucor hiemalis F-1156 showed that alterations occur in the content of fatty acids and individual classes of lipids during long-term cultivation (for about 20 days). The changes in the lipid composition related to the age of the spore-forming mycelium suggest an important role of sporangiospore lipids in spore germination and in further development of the spherical cells formed in this processes. The M. hiemalis F-1156 sporangiospores with a lipid pool exhausted during long-term cultivation can give rise to both mycelial and yeastlike growth.

In vitro evidence of mycoparasitism of the ectomycorrhizal fungus Laccaria laccata against Mucor hiemalis in the rhizosphere of Pinus sylvestris

Interactions between the ectomycorrhizal fungus Laccaria laccata and the soil fungus Mucor hiemalis f. hiemalis in co-culture, and in the rhizosphere of in vitro-grown Pinus sylvestris seedlings were investigated by light- and scanning electron-microscopy. In co-culture, mycelial growth away from the L. laccata colony reduced the number of aerial hyphae at the contact zone and increased the density and compactness of the mycelium-characterized gross morphology of the saprobic fungus. Although the growth of M. hiemalis was suppressed, no penetration of M. hiemalis hyphae after the colony was entered by L. laccata was observed. Instead, dense coiling of L. laccata hyphae around sporangiophores, overpowering them and causing them to disappear, was quite common. On nonmycorrhizal roots, sporangiospores germinated heavily and formed long hyphae for 2 days post inoculation, whereas their germination was totally inhibited on mycorrhizal roots. At 3 days after inoculation, only sporangia were seen with mycelial mats firmly attached to the roots by the mantle hyphae, whereas some remnants of sporangiophores, ruptured sporangial walls and degraded hyphae of M. hiemalis were overgrown by the mantle hyphae. During the next 3 days, the mantle-hyphae-invading sporangia formed short, thin branches that grew directly towards individual spores, tapering off upon contact.

Morphology and physiology of the dimorphic fungus Mucor circinelloides (syn. M. racemosus) during anaerobic growth

The dimorphic Mucor circinelloides requires an anaerobic atmosphere and the presence of 30% CO2 to grow as a multipolar budding yeast, otherwise hyphal growth predominates. Establishing other means to control the morphology would be a distinct advantage in the development of a fermentation process for this organism for the production of heterologous proteins. Thus, conditions suppressing polarised growth while at the same time abolishing the CO2 requirement were investigated in submerged cultivations. It was found that supplementing cultures with mixtures of ergosterol and Tween 80 resulted in yeast-like growth under 100% N2. Their impact on growth and morphological development was assessed at a range of concentrations. Maximum biomass levels and the specific growth rate decreased at elevated levels of ergosterol and Tween 80. Possible effects of carbon dioxide and the added fatty acid/sterol mixture on supporting yeast growth by influencing the fluidity of the plasma membrane or affecting polarised growth are discussed.

Differential expression of desaturases and changes in fatty acid composition during sporangiospore germination and development in Mucor rouxii

Polyunsaturated fatty acids (PUFAs), namely, oleic (C18:1), linoleic (C18:2), and gamma-linolenic acid (C18:3), constituted the majority in the total fatty acid content (44%) of sporangiospores of Mucor rouxii. At 30 degrees C, the germination begins within 1h at which time spore swelling occurs, followed by germ tube emergence within 3-4h. Throughout germination, an increase in gamma-linolenic acid (GLA) was observed and its content was highest at germ tube emergence. It took longer for sporangiospores of M. rouxii to germinate at sub-optimal temperatures (15 and 35 degrees C). However, the content of GLA was higher at the germ tube initiation than at the mycelial stage at all temperatures, suggesting the association of GLA and germination of sporangiospores. This finding was substantially confirmed by differential expression of delta9-, delta12-, and delta6-desaturase genes measured during spore germination. The expression of three desaturase genes parallels the pattern of GLA synthesis. By using RT-PCR techniques to follow gene expression, we found that mRNA of delta12- and delta6-desaturase genes were translated as soon as the spores were introduced into a fresh medium while the mRNA of delta9-desaturase gene could not be detected until 2h after introduction. A sharp increase in mRNA of delta6-desaturase genes correlated well with an increase in GLA content at germ tube emergence (4h). These results demonstrated that changes in fatty acid composition of sporangiospore of M. rouxii and differential expression of desaturase genes occurred during germination, and that extensive changes in GLA synthesis associated with some events in germination process.

Identification and analysis of genes involved in the control of dimorphism in Mucor circinelloides (syn. racemosus)

Mucor circinelloides (syn. racemosus) is a non-pathogenic dimorphic fungus belonging to the class of zygomycetes. We are developing a novel system for heterologous protein production exploiting the dimorphic growth characteristics of M. circinelloides. In order to identify potential genetic regulators of morphology we have initiated a characterisation of key genes involved in signal transduction in Mucor. We have cloned and characterised pkaR and pkaC encoding the regulatory subunit (PKAR) and the catalytic subunit (PKAC), respectively, of the cAMP-dependent protein kinase A (PKA) of M. circinelloides. In anaerobically grown yeast cells, the levels of expression of both pkaR and pkaC were significantly higher than the levels of expression in aerobically grown mycelium. However, during the dimorphic shift, i.e. during the transition from anaerobic yeast growth to aerobic filamentous growth, the expression of pkaR was found to increase approximately two-fold. These results indicate that regulation of PKA activity is conferred at different levels according to growth and environmental conditions. Overexpression of pkaR resulted in a multi-branched colony phenotype on solid medium indicating that PKAR plays a role in filamentation and branching. Fragments of genes encoding factors of the mitogen-activated protein (MAP) kinase (MAPK) pathway have also been cloned: mpk1 (mitogen-activated protein kinase 1) encoding a MAPK homologue and ste12 encoding a transcription factor.

A gene coding for ornithine decarboxylase (odcA) is differentially expressed during the Mucor circinelloides yeast-to-hypha transition

The differential display technique was used to identify genes from Mucor circinelloides involved in the yeast-to-hypha transition. Using a limited set of primer combinations, cDNA fragments corresponding to mRNAs differentially expressed during the dimorphic transition were isolated. Northern analyses showed that the accumulation of the transcript detected by hybridisation with one of the cDNA fragments increased during the transition and was undetectable at the mycelial stage. Sequence analysis and database searches of this fragment revealed high similarity to ornithine decarboxylase (ODC) encoding genes. The odcA gene of M. circinelloides was isolated from genomic and cDNA libraries and characterised. Electrophoretic karyotyping and hybridisations showed that the odcA gene is single-copy and linked to the leuA and rDNA genes. The single transcript detected (2.1 kb), was considerably longer than the deduced ORF. Through non-radioactive primer extension analysis four transcription initiation sites were mapped to positions -61, -167, -239 and -436 from the start codon. The ODC mRNA levels increased during the yeast-to-hypha transition, reaching a maximum at 120 min, which was accompanied by a rise in ODC enzymatic activity. The expression pattern of the odcA gene showed that in M. circinelloides the ODC levels are transcriptionally regulated, in contrast with other dimorphic fungi in which a post-transcriptional regulation has been proposed.

Growth physiology and dimorphism of Mucor circinelloides (syn. racemosus) during submerged batch cultivation

Mucor circinelloides is being investigated as a possible host for the production of heterologous proteins. Thus, the environmental conditions defining the physiology and morphology of this dimorphic fungus have been investigated in submerged batch cultivation. The optimal conditions for growth of each form have been defined. Pure cultures of the multi-polar budding yeast form could be obtained under anaerobic conditions (with 70% N2/30% CO2 or 100% N2 as the sparge gas and without aeration). The highest maximum specific growth rate (0.30 h(-1)) was obtained in anaerobic cultivation, the yield of biomass on glucose (Y(SX)) was 0.12 (c-mole basis). A high maximum specific growth rate was obtained when the organism grew as the filamentous form under aerobic conditions (0.25 h(-1)), with a Y(SX) of 0.24 (c-mole basis). The maximum specific growth rates achieved are comparable to most industrial filamentous fungi under similar growth conditions. High levels of ethanol were observed with all growth conditions. The overriding effector of morphological development was found to be oxygen. In batch cultures it was therefore possible to induce the dimorphic shift by controlling the influent gas atmosphere. A specific growth rate of 0.19 h(-1) was maintained during the shift from the yeast to the filamentous form.

Biological interaction between soil fungi and Toxocara canis eggs

Fungi from the soil of public areas in La Plata, Argentina were isolated and evaluated for their biological interaction with Toxocara canis eggs in vitro. We isolated and identified two fungal species: Fusarium pallidoroseum and Mucor hiemalis. Each species was co-cultured with T. canis eggs in sterile distilled water. The samples were observed by light and scanning electron microscopy (SEM) at days 4, 7 and 14 post-inoculation. Under the conditions of our experiments, F. pallidoroseum exhibited a high ovicidal activity on T. canis eggs, whereas M. hiemalis exhibited no such effects.

Liquid chromatography study of pyrene degradation by two micromycetes in a freshwater sediment

Pyrene biodegradation in a freshwater sediment without fungi supply, or inoculated with two sediment micromycetes, Mucor racemosus var. sphaerosporus and Phialophora alba was studied after 0, 5, 13, 28, 60 and 90 days. The influence of glucose addition was estimated, and a liquid chromatographic method for simultaneous quantitative determination of residual anthracene, fluoranthene and pyrene in the sediment was developed. Samples with PAHs were extracted in Soxhlet with ethyl acetate, and LC analysis was performed on a 5 microm Supelcosil column (150 x 4.6 mm I.D.) with gradient elution (2 ml min(-1)) of acetonitrile-water and UV detection at 254 nm. Recoveries of anthracene, fluoranthene and pyrene were 90.3%+/-1.1%, 93.2%+/-0.9% and 90.42%+/-1.9%, respectively, without interference. The native sediment microorganisms (with or without glucose added) have shown 35% pyrene degradation and sediment with glucose inoculated by the strains revealed 40%.

Heavy metal removal in a biosorption column by immobilized M. rouxii biomass

Mucor rouxii biomass was immobilized in a polysulfone matrix. The spherical immobilized biomass beads were packed in a column. The biosorption column was able to remove metal ions such as Pb, Cd, Ni and Zn not only from single-component metal solutions but also from multi-component metal solutions. Column kinetics for metal removal were described by the Thomas model. For single-component metal solutions, the metal removal capacities of the beads for Pb, Cd, Ni and Zn were 4.06, 3.76, 0.36 and 1.36 mg/g, respectively. For a multi-component metal solution containing Cd, Ni and Zn, the capacities were 0.36, 0.31 and 0.40 mg/g for Cd, Ni and Zn, respectively. The adsorbed metal ions were easily desorbed from the beads with 0.05N HNO3 solution. After acid desorption and regeneration with deionized water, the beads could be reused to adsorb metal ions at a comparable capacity.

Ethanol-induced dimorphism and lipid composition changes in Mucor fragilis CCMI 142

AIMS

To study the effect of ethanol on morphology, lipid production and fatty acid profile of Mucor fragilis CCMI 142 cultures.

METHODS AND RESULTS

Cell inhibition in shake flask cultures due to alcohol toxicity grew linearly from 0.418 mol x 1(-1) to 0.816 mol x 1(-1) ethanol corresponding to a decrease of specific growth rate. The growth inhibition constant took the value of 2.27 mol x 1(-1). The germination of fungal spores into hyphae is inhibited by concentrations from 0.418 mol x 1(-1) to 0.816 mol x 1(-1) ethanol. In this range, M. fragilis CCMI 142 spores form, exclusively, budding yeast-like cells instead of filaments. Below 0.418 mol 1-1 ethanol the formation of yeast-like cells was stimulated and there was a spore germination delay.

CONCLUSION

The lipid content decreased as the concentration of ethanol increased, and was associated with an increase of unsaturated/saturated fatty acid ratio.

SIGNIFICANCE AND IMPACT OF THE STUDY

The major conclusion of the study is the production of an enriched unsaturated fatty acids final product with particular emphasis to the presence of gamma-linolenic acid (18:3omega6) a biologically active compound with a useful impact in nutraceutical science.

Bioreactors based on immobilized cells

For purification of oil-containing sewage water of oil-proceeding enterprises it has been elaborated by authors the laboratory bioreactor on the basis of immobilized mycelial Mucor miehei fungus cells with periodical and constant supply of purified solution. The purification process is tested on model and real oil-containing liquid wastes. It has been established that under true bioreactor exploitation, 1 g of Mucor miehei immobilized cells enables to purify more than 100 L of sewage water, containing 0.1-0.15% of lipid substances.

Effect of spores of saprophytic fungi on phytoalexin accumulation in seeds of frog-eye leaf spot and stem canker-resistant and -susceptible soybean (Glycine max L.) cultivars

Two saprophytic fungi (Mucor ramosissimus and Rhizopus sp.) were tested for their ability to induce phytoalexin production by seeds of frog-eye leaf spot and stem canker-resistant and -susceptible soybean (Glycine max L.) cultivars. Only M. ramosissimus was shown to elicit a response and qualitative differences in phytoalexin accumulation were found between the susceptible and resistant cultivars. Glyceollins I, II, and III and glycinol were isolated from the susceptible cultivar, whereas Glyceollins I, II, and III, glycinol, glyceocarpin, genistein, isoformononetin, and N-acetyltyramine accumulated in the resistant cultivar in response to the same fungal elicitor. Genistein was found to be an inducibly formed isoflavonoid instead of a constitutive metabolite in the resistant cultivar, whereas N-acetyltyramine is described for the first time as a soybean phytoalexin. All the compounds, except genistein, showed fungitoxic activity against Cladosporium sphaerospermum. Spectral data of the pterocarpan phytoalexins, genistein, and N-acetyltyramine are also given in this work.

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.

Inactivation of Mucor plumbeus by the combined actions of chitinase and high hydrostatic pressure

Sporangiospores were treated with high hydrostatic pressure and/or fungal chitinase in order to study the inhibition of germination and growth of the food spoiling mold Mucor plumbeus. Total fungal inhibition was obtained either at 4.0 kbar or by 10 U/ml of chitinase from Penicillium janthinellum. A pretreatment with 1 U/ml of the same chitinase reduced the pressure necessary to obtain complete spore inhibition to 3 kbar.

DNA amplification polymorphisms of Mucor piriformis

Mucor piriformis can cause postharvest decay in various fruits and vegetables stored at low temperatures. Thirty isolates of this fungus, collected from infected fruit, were subjected to random amplified polymorphic DNA (RAPD) analysis. Seven different 10-bp primers were used to determine the type and extent of intraspecific genetic polymorphisms. Nineteen composite amplification types were identified, indicating a higher degree of variability than found in previous isoenzyme studies. Numerical analysis with the UPGMA technique revealed three clusters, which correlated with the mating competency of the isolates or their place of origin. These results demonstrate that RAPD analysis can identify isolates and subspecific populations of M. piriformis.

Contact-sensing by hyphae of dermatophytic and saprophytic fungi

Contact-sensing or thigmotropism is the directional growth response of cells in relation to topographical guidance cues. Thigmotropism is thought to play a major role in the location of infectable sites on plants by phytopathogenic fungi and has recently been shown to be a property of hyphae in the human pathogenic fungus Candida albicans. Here we show that hyphae of the dermatophytes Epidermophyton floccosum, Microsporum canis and Trichophyton mentagrophytes reorientate their direction of growth in response to grooves and pores of membrane substrata as did hyphae of the saprophytes Mucor mucedo and Neurospora crassa. This suggests that the thigmotropic behaviour of hyphae is not a specific property of pathogens, but rather a general feature of the growth of fungal hyphae that must forage for nutrients on surfaces and within solid materials.

Physiological properties and fatty acid composition in Mucor circinelloides f. circinelloides

Sporangiospores of Mucor circinelloides f. circinelloides CBS108.16 could germinate and grow on a wide variety of carbon sources in synthetic liquid media. Growth was supported by aldoses which have the same configuration at carbon atom number two as glucose. Di- and trisaccharides consisting of D-glucopyranosyl moieties were assimilated, while polysaccharides like inuline and starch were also utilised. Various alcohols and organic acids could be assimilated, while the phenolic compounds tested could not support aerobic growth. The fungus was able to ferment carbohydrates consisting of D-glucopyranosyl moieties, grow in the absence of vitamins and in the presence of cycloheximide. It also liquefied gelatin and produced lipases and cellulolytic enzymes. It was found that the highest percentages polyunsaturated fatty acids were produced when acetic acid, glucose, mannitol, soluble starch or trehalose was used as carbon source. The absence of vitamins in the medium lowered the percentage of these fatty acids.

Cloning and sequence analysis of a putative transcription factor (MTF1) gene from Mucor circinelloides

We describe here the fortuitous cloning of a putative transcription factor gene (MTF1) from the dimorphic fungus Mucor circinelloides. Sequence analysis of MTF1 revealed an open reading frame (ORF) of 1059 nucleotides encoding a protein of M(r) 39601. The deduced amino acid sequence from the ORF imparts two glutamine-rich stretches which are homologous to a number of transcription factors characterized previously from various organisms. A Southern blot analysis of Mucor genomic DNA digested with different restriction endonucleases and probed with the 1.9 kb EcoR1 fragment of the putative transcription factor gene shows a single copy number of the the gene. Northern analysis during morphogenetic changes in Mucor suggested constitutive expression of the gene.

Genetic evidence for independence between fermentative metabolism (ethanol accumulation) and yeast-cell development in the dimorphic fungus Mucor rouxii

Three allyl-alcohol-resistant mutants were isolated in the dimorphic fungus Mucor rouxii and characterized with regard to their alcohol dehydrogenase (ADH) activity in vitro and in vivo as well as their ability to execute the morphological alternatives of dimorphism under different environmental stimuli, either in the absence or in the presence of oxygen. These studies indicated that fermentation and yeast-cell development are independent events and that ADH activity is essential for growth of the fungus in the absence of oxygen. Heterokaryon construction and analysis indicated that in the three mutant strains the corresponding genetic alterations are recessive nuclear mutations which behave as allelic in complementation tests.

Regulation of protein kinase A subunits during germination of Mucor rouxii sporangiospores

Levels of protein kinase A (PKA) subunits and of cAMP have been measured during aerobic germination of the sporangiospores of the dimorphic fungus Mucor rouxii; further, the holoenzyme and its catalytic (C) and regulatory (R) subunits have been visualized through sucrose gradient centrifugation. Sporangiospores contain around 0.06 microM of a dimeric holoenzyme species of 5.5 S and a sixfold excess of a free R subunit of 2.7 S. Both these species are proposed to be derived by proteolysis from the native forms. Enzymic activity at this stage is highly inhibited, as demonstrated with permeabilized cells. Immediately upon germination, and after a transient increase in cAMP concentration from 10 microM to 90 microM, C-subunit levels fall to 30%. After the onset of germination, the specific activity and concentration of both the 5.5 S holoenzyme species and the 2.7 S species of free R subunit decrease in parallel to the increase in total protein and volume. Net synthesis of C and R subunits to form a native holoenzyme species of 8.8 S is apparent 4 h onwards after germination. A significant increase in cellular concentration is observed at 6 h. At 7 h of growth, when germ-tube emission is complete, the holoenzyme concentration is around 0.23 microM; there is almost no free R subunit and the intracellular concentration of cAMP is around 3 microM. A role for PKA during germination and morphogenesis is discussed.

The arrangement of F-actin and microtubules during germination of Mucor rouxii sporangiospores

The distribution of F-actin microfilaments and microtubules was analyzed in germinating sporangiospores of Mucor rouxii by labeling with rhodamine-tagged phalloidin and by immunofluorescence microscopy. The transition from isodiametrical to apical growth was accompanied by a switch from uniform distribution of F-actin patches to a polarized accumulation of F-actin material at the germ tube tips. Immunoblotting of cell-free extracts of M. rouxii with a monoclonal anti-porcine alpha-tubulin antibody (TU-01) disclosed two discrete bands of alpha-tubulin suggesting the existence of two alpha-tubulin genes in this fungus. Immunofluorescence microscopy of germinating cells stained with the same antibody revealed an elaborate network of cytoplasmic microtubules that persisted during the entire germination process and extended into the apex of the germ tube. Although their precise roles remain undetermined, the observed arrangement of cytoskeletal elements during germination is consistent with their presumed involvement in cell wall morphogenesis: the long axial microtubules serving as long-distance conveyors of wall-building vesicles to the apical region while the concentrated F-actin patches mark the participation of microfilaments in the zone of intense vesicle exocytosis at the hyphal apex.

Nikkomycin-resistant mutants of Mucor rouxii: physiological and biochemical properties

We isolated three nikkomycin-resistant mutants of the dimorphic fungus M. rouxii which were physiologically characterized regarding their response to yeast-phase inducing conditions and their sensitivity to bacilysin. Mutant strains G21 and G23, showed a qualitatively normal, though delayed, dimorphic transition and partial cross-resistance to bacilysin. Mutant strain G27 showed an altered dimorphism, producing a high proportion (50%) of hyphal cells, and a wild-type sensitivity to bacilysin. Cell-free extracts from this mutant exhibited an activity of both basal and protease-activated chitin synthetase which was overexpressed as compared with the parental strain and mutants G21 and G23. Results are discussed in terms of the different genetic background of the mutants.

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.

Cultures of separated mating types of Blakeslea trispora make D and E forms of trisporic acids

Trisporic acids are end products of the sex-specific pheromones in mucoraceous fungi. We have found three new trisporic acids in cultures of Blakeslea trispora in which (+) and (-) mating types were separated by a membrane with 0.45-microns pores. Two of the trisporic acids were new compounds; the structure of the third [previously described by Miller and Sutter [(1984) J. Biol. Chem. 259, 6420] as methyl trisporate-E with a hydroxyl group at C-2] was revised. Trisporic acid-E(3R), trisporic acid-E(3S), and trisporic acid-D(2S) were in a 1:1:2 ratio, accounted for 9% of the total trisporic acids, and differed by the position and configuration of a hydroxyl group on the ring at C-2 or C-3, the conformation of the ring, the extent of rotation of the side chain relative to the ring, and either a carbonyl or hydroxyl group on the side chain at C-13. These three compounds accounted for only 0.5% of the total trisporic acids in combined mating type cultures. Since the combined cultures did not metabolize trisporic acid-E(3R), its biosynthesis apparently ceases when opposing mating types contact each other physically. We speculate that B. trispora and Phycomyces blakesleeanus utilize different pheromones to regulate an early event (possibly zygotropism) in sexual development.