Lignocellulolytic enzyme profiles of edible mushroom fungi

One of the most economically-viable processes for the bioconversion of many types of lignocellulosic wastes is represented by edible mushroom cultivation. Lentinula edodes, Volvariella volvacea and Pleurotus sajor-caju are three important commercially cultivated mushrooms which exhibit varying abilities to utilise different lignocellulosics as growth substrate. Examination of the lignocellulolytic enzyme profiles of the three species show this diversity to be reflected in qualitative variations in the major enzymic determinants (i.e. cellulases, ligninases) required for substrate bioconversion. For example, L. edodes, which is cultivated on highly lignified substrates such as wood or sawdust, produces two extracellular enzymes which have been associated with lignin depolymerisation in other fungi, (manganese peroxidase and laccase). Conversely, V. volvacea, which prefers high cellulose-, low lignin-containing substrates produces a family of cellulolytic enzymes including at least five endoglucanases, five cellobiohydrolases and two β-glucosidases, but none of the recognised lignin-degrading enzymes.

Isolation and characterisation of a calnexin homologue, clxA, from Aspergillus niger

We describe the isolation of a gene (clxA) encoding calnexin from laboratory and industrial strains of Aspergillus niger. Calnexin is a chaperone, which specifically recognises monoglucosylated glycoproteins in the endoplasmic reticulum, and is thus an essential component of the process that assesses the folded state of nascent secreted glycoproteins. Manipulation of chaperones has previously been adopted in attempts to overcome some of the problems associated with the secretion of heterologous proteins from filamentous fungi. The A. niger clxA gene encodes a 562-residue protein with strong homology to the calnexin of Schizosaccharomyces pombe. The clxAgene product complements a S. pombe cnx1 mutant. Motifs associated with genes controlled via the Unfolded Protein Response (UPR) were identified by sequence homology in the promoter of clxA. Steady-state levels of clxA mRNA were elevated in a strain expressing bovine prochymosin fused to the catalytic domain of glucoamylase. The ORF is punctuated by four introns, and contains two sets of four repeated peptide motifs that are characteristic of the calnexin family, together with a putative membrane-spanning domain. Deletion studies indicate that clxA is not an essential gene in A. niger.

Effect of tunicamycin on N-acetyl-beta-D-glucosaminidase produced by Trichoderma harzianum

The effect of tunicamycin, an inhibitor of protein N-glycosylation, was studied in non-growing mycelium of Trichoderma harzianum induced to secrete N-acetyl-beta-D-glucosaminidase by the addition of N-acetylglucosamine. Tunicamycin (30 microg ml(-1)) had no significant effect on growth of the fungus, or on the total protein secreted or specific activity of N-acetyl-beta-D-glucosaminidase. However, in the presence of the inhibitor an underglycosylated form of the enzyme was produced. The apparent molecular masses for this and the native enzyme were 110 and 124 kDa, respectively. Both forms of the enzyme showed the same optimum pH and temperature, but the underglycosylated form was more sensitive to inactivation by both high temperature (60 degrees C) and the proteolytic enzyme trypsin.

The effect of pH on glucoamylase production, glycosylation and chemostat evolution of Aspergillus niger

The effect of ambient pH on production and glycosylation of glucoamylase (GAM) and on the generation of a morphological mutant produced by Aspergillus niger strain B1 (a transformant containing an additional 20 copies of the homologous GAM glaA gene) was studied. We have shown that a change in the pH from 4 to 5.4 during continuous cultivation of the A. niger B1 strain instigates or accelerates the spontaneous generation of a morphological mutant (LB). This mutant strain produced approx. 50% less extracellular protein and GAM during both chemostat and batch cultivation compared to another strain with parental-type morphology (PS). The intracellular levels of GAM were also lower in the LB strain. In addition, cultivation of the original parent B1 strain in a batch-pulse bioreactor at pH 5.5 resulted in a 9-fold drop in GAM production and a 5-fold drop in extracellular protein compared to that obtained at pH 4. Glycosylation analysis of the glucoamylases purified from shake-flask cultivation showed that both principal forms of GAM secreted by the LB strain possessed enhanced galactosylation (2-fold), compared to those of the PS. Four diagnostic methods (immunostaining, mild methanolysis, mild acid hydrolysis and beta-galactofuranosidase digestion) provided evidence that the majority of this galactose was of the furanoic conformation. The GAMs produced during batch-pulse cultivation at pH 5.5 similarly showed an approx. 2-fold increase in galactofuranosylation compared to pH 4. Interestingly, in both cases the increased galactofuranosylation appears primarily restricted to the O-linked glycan component. Ambient pH therefore regulates both GAM production and influences its glycosylation.

Galactofuranoic-oligomannose N-linked glycans of alpha-galactosidase A from Aspergillus niger

Extracellular alpha-galactosidase A was purified from the culture filtrate of an over-producing strain of Aspergillus niger containing multiple copies of the encoding aglA gene under the control of the glucoamylase (glaA) promoter. Endoglycosidase digestion followed by SDS/PAGE, lectin and immunoblotting suggested that glycosylation accounted for approximately 25% of the molecular size of the purified protein. Monosaccharide analysis showed that this was composed of N-acetyl glucosamine, mannose and galactose. Mild acid hydrolysis, mild methanolysis, immunoblotting and exoglycosidase digestion indicated that the majority of the galactosyl component was in the furanoic conformation (beta-D-galactofuranose, Galf). At least 20 different N-linked oligosaccharides were fractionated by high-pH anion-exchange chromatography following release from the polypeptide by peptide-N-glycosidase F. The structures of these were subsequently determined by fast atom bombardment mass spectrometry to be a linear series of Hex(7-26)HexHA(c2). Indicating that oligosaccharides from GlcNA(c2)Man(7), increasing in molecular size up to GlcNA(c2)Man(24) were present. Each of these were additionally substituted with up to three beta-Galf residues. Linkage analysis confirmed the presence of mild acid labile terminal hexofuranose residues. These results show that filamentous fungi are capable of producing a heterogeneous mixture of high molecular-size N-linked glycans substituted with galactofuranoic residues, on a secreted glycoprotein.

Beta-galactofuranoside glycoconjugates on conidia and conidiophores of Aspergillus niger

Galactose in the furanoic conformation appears to be limited to bacteria and lower eukaryotes. Galactofuranoic (Galf)-containing glycoconjugates that occur in organisms pathogenic or allergenic to man are frequently antigenic and immunodominant. We have used an immunochemical approach, employing a monoclonal antibody that recognises Galf epitopes, to investigate the presence of Galf-containing glycoconjugates within conidia and conidiophores of Aspergillus niger. ELISA and immunofluorescence microscopy indicated that specific and saturable binding sites were found on both. Inhibition studies confirmed that this binding was to Galf-containing glycoconjugates. Interestingly, the conidiophore heads were particularly rich in these glycoconjugates. Western blotting identified a Galf glycoprotein of 150-200 kDa from disrupted conidia.

An extracellular beta-galactofuranosidase from Aspergillus niger and its use as a tool for glycoconjugate analysis

Aspergillus niger produces an extracellular beta-galactofuranosidase, which can specifically hydrolyse beta-D-galactofuranose (Galf) from glycoconjugates. The production of this enzyme can be induced by the addition of a Galf-containing A. niger mycelial wall extract. However, on other carbon sources accumulation occurred only during the starvation conditions of the late stationary phase. Extracellular glucoamylases from this stage of cultivation possessed significantly lower levels of Galf than those from the earlier exponential growth phase when beta-galactofuranosidase is absent, suggesting in situ beta-galactofuranosidic hydrolysis. The beta-galactofuranosidase responsible was subsequently purified to homogeneity and characterised. It is a glycoprotein of 90 kDa (determined by SDS-PAGE) with activity against beta-linked Galf residues, with a Km of 4 mM against p-nitrophenyl-beta-D-galactofuranoside and a pH optimum of 3-4. The preparation did not contain other contaminating glycosidase activities; p-nitrophenyl-beta-D- and -alpha-D-galactopyranose, and alpha-D-methyl-Galf were not hydrolysed. Results are presented to show that this enzyme could be employed as a useful tool for the analysis of glycoconjugates containing biologically important Galf components.

An unambiguous microassay of galactofuranose residues in glycoconjugates using mild methanolysis and high pH anion-exchange chromatography

An original, unambiguous microassay of galactofuranose (Galf) residues in glycoconjugates is described. The method involves mild acid methanolysis (5 mM HCl) for 3 h at 84 degrees C followed by high pH anion-exchange chromatography using a routine monosaccharide system. The methanolysis products Mealpha-Galf and Mebeta-Galf were characterized chromatographically by comparison with the authentic compounds and by their response to treatment with mild acid and with beta-galactofuranosidase. Testing against p-nitrophenyl-beta-Galf and UDPalpha-Galf showed the method to be applicable to both alpha- and beta- galactofuranosides over the range 10-200 pmol. The results of partial mild methanolysis over shorter periods were consistent with initial inversion of anomeric configuration at methylation followed by anomerization to an equilibrium mixture of alpha- and beta-forms. When applied to a sample of invertase from Aspergillus nidulans, the method indicated that all of the mild acid-labile galactose (78% of the total galactose present) was in the form of a galactofuranoside and that much of this was in the beta-configuration. As expected, when applied to asialofetuin (known to contain galactose only in the pyranoside form, Galp), NPalpha-Galp, NPbeta-Galp, or UDPalpha-Galp, mild acid methanolysis failed to produce any galactofuranoside.

Glucoamylase overexpression and secretion in Aspergillus niger: analysis of glycosylation

We have studied the effects of overexpression and secretion of a homologous model glycoprotein, glucoamylase (GAM-1), on glycosylation in a single gene copy wild-type parent and multiple gene copy transformants of Aspergillus niger. In batch culture the B36 strain, which possess 80 additional copies of the GAM glaA gene, secreted about 5-8-fold more protein and GAM-1 than the parent strain (N402). A comparison of the glycosylation of GAM-1 secreted by the parent strain with that secreted by the multiple copy and hyper-secreting B36 strain showed that both the N-linked and O-linked glycan composition was very similar. Short oligomannose N-linked glycans were found (Man(7-8)GlcNAc(2)). O-Linked glycans were comprised of short (1-3 residues) oligosaccharide chains of mannose and galactose. Evidence is presented that this galactose is present in the novel galactofuranose conformation. This glycan composition of GAM-1 differed from that of a commercially available (A. niger) GAM source. Microsomes prepared from the mycelium showed a 2-3-fold co-ordinated increase in the activity of the dolichol phosphate:glycosyltransferases. Similar results were obtained from strains B1 (20 copies of glaA) and N402 when grown at a low dilution rate in a chemostat, although both the levels of GAM secretion and the activities of the dolichol phosphate:glycosyltransferases were lower than found in batch culture. These data suggest that A. niger is capable of secreting large amounts of a single glycoprotein combined with higher activity levels of the dolichol phosphate:glycosyltransferases without an increase in the heterogeneity of the glycan structures. Thus, from a biotechnological viewpoint, protein glycosylation may not be a bottleneck to enhanced glycoprotein production using A. niger.

Cloning of the cbhI and cbhII genes involved in cellulose utilisation by the straw mushroom Volvariella volvacea

The straw mushroom Volvariella volvacea is cultivated on substrates rich in cellulose and has been shown to produce a family of cellulolytic enzymes. A PCR-based strategy was adopted to clone genes involved in cellulose utilisation, using degenerate primers designed to amplify conserved catalytic domain sequences of cellobiohydrolases (CBHs). PCR with these primers produced two DNA fragments with sequence similarity to the cbhI and cbhII gene families detected in Trichoderma, Phanerochaete and Agaricus species. Full-length clones of these genes were obtained from an EMBL3 genomic library, and RACE-PCR was used to verify the presence of introns. The cbhI homologue has a coding region of 1722 bp, containing two introns, generating a 536 amino acid polypeptide product. The cbhII gene has a coding region of 1693 bp, containing five introns, and gives rise to a 470-amino acid polypeptide product. Northern and PCR analyses were used to study the expression of the genes. These revealed that transcripts of both genes were induced on medium containing cellulose with cbhI being expressed more strongly than cbhII - but were repressed on medium containing glucose.

Extracellular proteins in fungi: a cytological and molecular perspective

Protein secretion is a vital process in fungi. For many, the secretion of hydrolytic enzymes provides a crucial step in their nutrition in nature. However, in recent years the list of different types of secreted proteins that have been discovered has extended significantly. These have been shown to have a diversity of functions including toxic molecule transport and control of desiccation. The majority of secreted proteins are glycosylated and our understanding of this aspect of fungal biochemistry has also extended in recent years. This review addresses the process of protein secretion from the cytological, biochemical and genetical standpoints. Advances in technology in many areas of scientific approach have enabled a better and understanding of this important process in fungi.

Investigation of the glycosyltransferase enzymes involved in the initial stages of the N-linked protein glycosylation pathway in Aspergillus niger

A protocol has been developed to produce functional microsomes from mycelium of Aspergillus niger. Using these preparations, radioactive biochemical assays have been designed and optimised to measure the in vitro activities of three of the enzymes of the N-linked dolichol phosphate (Dol-P) glycosylation pathway: UDP-N-GlcNAc:dolichol-P GlcNAc-1-P transferase (GPT), Dol-P mannose synthase (DPMS) and Dol-P glucose synthase (DPGS). Exogenous Dol-P and Triton X-100 are essential for in vitro activity. All three Dol-P:glycosyl transferases had alkaline pH optima and activity rapidly decreased below neutrality. Characterisation of the glycolipid products by anion-exchange chromatography and TLC showed that they were Dol-P-P-GlcNAc, Dol-P-Glc and Dol-P-Man for GPT, DPGS and DPMS, respectively. The antibiotic tunicamycin completely inhibited GPT activity at a concentration of 100-200 ng ml(-1) and an IC50 of 40 ng ml(-1), but had little effect on the other two enzymes. The ratio of the activity of the three enzymes to each other suggested that DPMS may be involved in other cellular activities and is probably under different control mechanisms than the other two.

The purification and characterization of a Trichoderma harzianum exochitinase

A chitinolytic enzyme was purified from the culture filtrate of T. harzianum (T198) by precipitation with ammonium sulphate followed by affinity binding to swollen chitin and release with 10% (v/v) acetic acid. The molecular weight of the enzyme was calculated to be 28 and 27.5 kD by gel filtration chromatography and SDS-PAGE, respectively. The isoelectric point of the enzyme was 7.4. The pH optimum for activity was 3.5 and maximum activity was obtained at 50 degrees C. The enzyme displayed activity on a wide array of chitin substrates of more than two N-acetylglucosamine units in length. HPLC analysis of hydrolysis products demonstrated that the enzyme was an exochitinase releasing N-acetylglucosamine only.

The molecular biology of secreted enzyme production by fungi

Enzymes from filamentous fungi are already widely exploited, but new applications for known enzymes and new enzymic activities continue to be found. In addition, enzymes from less amenable non-fungal sources require heterologous production and fungi are being used as the production hosts. In each case there is a need to improve production and to ensure quality of product. While conventional, mutagenesis-based, strain improvement methods will continue to be applied to enzyme production from filamentous fungi the application of recombinant DNA techniques is beginning to reveal important information on the molecular basis of fungal enzyme production and this knowledge is now being applied both in the laboratory and commercially. We review the current state of knowledge on the molecular basis of enzyme production by filamentous fungi. We focus on transcriptional and post-transcriptional regulation of protein production, the transit of proteins through the secretory pathway and the structure of the proteins produced including glycosylation.

Secretion of two beta-fructofuranosidases by Aspergillus niger growing in sucrose

Aspergillus niger is induced to secrete two invertases, named SUC 1 and SUC 2, when grown on a minimal medium containing sucrose. Although, both have been classified as beta-D-fructofuranoside fructohydrolases, SUC 2 also possesses inulin hydrolytic activity (sucrose/inulin activity ratio of 4). These activities have been separated from each other and almost completely purified by anion-exchange, lectin affinity chromatography, and chromatofocusing. SUC 1 appeared as a single glycoprotein band on PAGE and SDS-PAGE corresponding in size to 250 and 125 kDa, respectively, compared with a much broader band (suggesting greater glycan heterogeneity) of 210-240 and 90-120 kDa for SUC 2. Therefore, both may be dimers, in their natural conformation. The glycan part of both contained the same monosaccharides: mannose, glucose, galactose, and N-acetylglucosamine; however, SUC 1 had approximately 10-fold more mannose and this was utilized to separate it from SUC 2 by Galanthus nivalis lectin affinity. Both the apparent Km values and the pH activity curves were different; SUC 1 did not show normal Michaelis-Menten kinetics to sucrose and apparent Michaelis constants of 30 and 160 mM were obtained. Activity was observed over a large range of pH 4.5-9 with a maximum at pH 6. In contrast, SUC 2 exhibited a Km of 40 and 1.7 mM to sucrose and inulin, respectively, with a pH optimum of 5.0 for both. Treatment with endo-beta-N-acetylglucosaminidase suggests that in both SUC 1 and SUC 2 some of the glycan present was N-linked glycan but that the differences in enzyme activities were not due to the N-linked moiety.

In vitro assessment of the influence of nutrition, temperature and larval density on trapping of the infective larvae of Heligmosomoides polygyrus by Arthrobotrys oligospora, Duddingtonia flagrans and Monacrosporium megalosporum

The influence of nutrient level, temperature and larval density on the trapping of Heligmosomoides polygyrus L3 by the nematophagous fungi Arthrobotrys oligospora, Duddingtonia flagrans and Monacrosporium megalosporum were investigated by quantification of trapped nematodes. All 3 factors were found to have a significant effect on the number of larvae trapped by A. oligospora and M. megalosporum. Decreased nutrient concentrations resulted in increased trapping for these 2 fungi, but nutrient availability was not found to have a significant effect on trapping by D. flagrans. The 3 fungi were found to have similar responses to temperature, with peak trapping occurring at or near the optimum growth temperatures. Nematode trapping was found to be density dependent for all 3 fungi, with increased percentage trapping at increased larval densities. Comparison in a single experiment of the relative importance of these factors to each fungus showed that nutrient level was the main factor influencing trapping by A. oligospora, whereas D. flagrans and M. megalosporum were more dependent on larval density.

Isolation of Ewingella americana from the cultivated mushroom, Agaricus bisporus

The isolation of Ewingella americana, an unusual Enterobacteriaceae, is reported here for the first time in a non-animal reservoir. Thirty-five strains of E. americana have been recovered from the cultivated mushroom, Agaricus bisporus. The biochemical characteristics of these strains are consistent with previously published descriptions of this species recovered from clinical specimens and from molluscs. DNA reassociation analysis was used to confirm the identity of mushroom-derived E. americana, and restriction fragment length polymorphism was used to reliably differentiate strains that otherwise demonstrated little phenotypic variation.

Purification and partial characterization of the high and low molecular weight form (S- and F-form) of invertase secreted by Aspergillus nidulans

Two forms of secreted invertase have been purified from Aspergillus nidulans by ion-exchange and gel-filtration chromatography. S-invertase gave a single, broad, glycoprotein band on PAGE and SDS-PAGE corresponding in size to 185 and 78 kDa, respectively, compared with 94 and 110 kDa for F-invertase. The carbohydrate of S-invertase contained mainly mannose (14%) and less galactose (5%) whereas the F-form yielded mainly galactose (29%) and less mannose (12%). Three sharp bands of enzymically active glycoprotein for both the S-form (pI 4.9-5.2) and the F-form (pI 3-4.2) were observed after isoelectric focusing. Deglycosylation with Endo H simplified this pattern to one enzymically active protein band (pI 5.2). The aglycoenzymes gave narrow bands on PAGE and SDS-PAGE corresponding to 115 kDa and 60 kDa respectively for both S- and F-forms. The specific activity of S-invertase was three-fold higher than that of F-invertase both before and after deglycosylation. The Km values of the two forms of invertase were very similar. Significant homology existed between the N-terminal amino-acid sequences of S-invertase (and of internal peptides derived from it) and sequences of invertase from other species. It is suggested that the higher carbohydrate content in F-invertase results in the native enzyme existing as a monomer and having a greater negative charge and lower specific enzyme activity compared with the dimeric S-enzyme.

The subtilisins of the invertebrate mycopathogens Verticillium chlamydosporium and Metarhizium anisopliae are serologically and functionally related

The major extracellular proteases from the nematophagous fungus Verticillium chlamydosporium and the entomophagous fungus Metarhizium anisopliae, VCP1 and Pr1, respectively, are closely related both functionally and serologically. Antibodies raised against either enzyme cross-reacted with both antigens, suggesting that they have common epitopes. The VCP1 and Pr1 antisera labelled bovine pancreatic elastase and proteinase K, respectively. Neither antiserum reacted with commercial chymotrypsin. An antiserum to a serine protease from the closely related V. suchlasporium also cross-reacted with VCP1 and Pr1. In contrast, a polyclonal antibody to an isoform of Pr1 exclusive to M. anisopliae isolate ME1 failed to recognize Pr1 from M. anisopliae V245 or VCP1. The N-terminal amino acid sequence of VCP1 revealed similarities with subtilisin-like enzymes from other fungi, but the closest match was with Pr1. The pure enzymes, VCP1 and Pr1, failed to hydrolyse mono-aminoacyl-naphthylamide substrates but demonstrated dipeptidyl peptidase activity against Gly-Pro-beta NA and Leu-Ala-beta NA, respectively. These results are discussed in the context of specificity of invertebrate mycopathogens.

The nematophagous fungus Verticillium chlamydosporium produces a chymoelastase-like protease which hydrolyses host nematode proteins in situ

The nematophagous fungus Verticillium chlamydosporium secreted several proteases in submerged culture in which soya peptone was the sole carbon and nitrogen source. One protease, VCP1 (M(r) 33,000, pI 10.2), was purified 14-fold from culture filtrates to apparent homogeneity using preparative isoelectric focusing in free solution, and shown to rapidly hydrolyse the chymotrypsin substrate Suc-(Ala)2-Pro-Phe-pNA and elastin. VCP1 had a Km for Suc-(Ala)2-Pro-Phe-pNA of 4.3 x 10(-5) M and a kcat of 5.8 s-1. It was highly sensitive to PMSF and TPCK, but only moderately sensitive to chicken egg-white and soya bean trypsin inhibitors. VCP1 degraded a wide range of polymeric substrates, including Azocoll, hide protein, elastin, casein and albumin, and accounted for most of the non-specific protease activity detected in culture filtrates. The purified enzyme hydrolysed proteins in situ from the outer layer of the egg shell of the host nematode Meloidogyne incognita and exposed its chitin layer. VCP1 was secreted by several isolates of V. chlamydosporium and V. lecanii, pathogens of nematodes and insects respectively, but not plant-pathogenic species of Verticillium. These observations suggest that VCP1 or similar enzyme(s) may play a role in the infection of invertebrates.

Protein secretion in filamentous fungi--trying to understand a highly productive black box

Protein secretion is important in all fungi. The majority of proteins by fungi are thought to be glycosylated and many of them are structurally associated with the cell envelope, the plasma membrane and the cell wall. Many of the enzymes secreted by fungi have been incorporated into commercial processes and used in a range of industries. The existence of strains producing very high levels of secreted enzymes stimulated interest in the use of fungi as hosts for the expression of recombinant proteins. Despite the attention that protein secretion in fungi has attracted, and the multifaceted importance of the process, our understanding of the cellular mechanisms involved is still minimal and, for the most part, it is necessary to extrapolate from other eukaryotic organisms. However, current research suggests that protein secretion in filamentous fungi is intimately associated with the process of growth at the hyphal tip. Such unique features merit a detailed study of this important phenomenon.

Purification and characterisation of an Aspergillus niger invertase and its DNA sequence

A secreted invertase was purified 23-fold by ultrafiltration, ion-exchange, and gel filtration chromatography from the culture supernatant of 18 h sucrose-grown cultures of Aspergillus niger. The purified enzyme hydrolysed sucrose and raffinose but there was no detectable hydrolysis of inulin, melezitose or PNPG. Invertase activity was optimal at pH 5.5 and 50 degrees C. The molecular mass of reduced invertase was 115 kDa, as determined by SDS gel electrophoresis. The native molecular weight of between 225 kDa and 250 kDa, estimated by electrophoresis under non-denaturing conditions, suggests that the protein is a dimer of identical subunits. The suc1 gene encoding this protein was completely-sequenced. The translated sequence yields a protein of 566 amino acids with a calculated molecular mass of 61 kDa, suggesting that carbohydrates represent about 50% of the mass of the protein.

Cloning of an Aspergillus niger invertase gene by expression in Trichoderma reesei

The filamentous fungus Aspergillus niger produces two glycosylated forms of the sucrose-hydrolysing enzyme, invertase. In contrast, some Trichoderma species lack invertase and are unable to utilise sucrose as a sole carbon source. Using an A. niger genomic library constructed in a cosmid vector containing the ura5 gene of Podospora anserina as a selectable marker, and the T. reesei ura5- strain as a sucrose-minus recipient strain, an A. niger invertase gene (suc1) has been cloned by a sib selection procedure. PAGE and enzyme analysis confirmed that transformants had acquired invertase activity. The cloned gene contained DNA sequences which were complementary to the amino-acid sequences of tryptic peptides found in invertase purified from A. niger. The suc1 invertase gene can be used as a dominant selectable marker for the transformation of Trichoderma strains.

Analysis of promoter activity by transformation of Acremonium chrysogenum

Promoter activity was examined in the beta-lactam-producing fungus, Acremonium chrysogenum, by assessment of the properties of transformant isolates. Transformation was achieved using plasmid constructs specifying hygromycin B resistance (HyR) linked to the promoter elements of gpdA (the glucose-6-phosphate dehydrogenase-encoding gene of Aspergillus nidulans), and pcbC [the gene encoding the isopenicillin N synthetase (IPNS) enzyme of A. chrysogenum]. Transformation frequency, HyR levels, and Hy phosphotransferase (HPT) levels suggested that the transformants of constructs using the gpdA promoter showed a higher level of expression of the HyR gene than in transformants obtained using the pcbC promoter. The patterns of integration of the transforming DNA also differed in that pcbC promoter construct transformants appeared to have tandem repeats. All integrations of plasmid DNA occurred on a single chromosome which was different in four out of five transformants studied. Multiple copy transformants of constructs using the pcbC promoter did not show the regulated pattern of expression of HPT activity observed with IPNS in untransformed strains.

Regulation of chitinase synthesis in Trichoderma harzianum

The production of chitinase by Trichoderma species is of interest in relation to their use in biocontrol and as a source of mycolytic enzymes. Fourteen isolates of the genus were screened to identify the most effective producer of chitinase. The best strain for chitinase was Trichoderma harzianum 39.1, and this was selected for study of the regulation of enzyme synthesis. Washed mycelium of T. harzianum 39.1 was incubated with a range of carbon sources. Chitinase synthesis was induced on chitin-containing medium, but repressed by glucose and N-acetylglucosamine. Production of the enzyme was optimal at a chitin concentration of 0.5%, at 28 degrees C, pH 6.0 and was independent of the age of the mycelium. The synthesis of chitinase was blocked by both 8-hydroxyquinoline and cycloheximide, inhibitors of RNA and protein synthesis, respectively. The mode of chitinase synthesis in this fungus is discussed.

Chromosome rearrangements in improved cephalosporin C-producing strains of Acremonium chrysogenum

A number of A. chrysogenum strains from a lineage improved in cephalosporin C production were analysed by contoured-clamped homogeneous electric field gel electrophoresis (CHEF). Although antibiotic titre was increased across the lineage, chromosome rearrangements were only observed at two points in it. In one member of the lineage the chromosomal changes included those which altered the size of the chromosome on which the isopenicillin N synthetase gene (pcbC) was located. It is proposed that chromosome changes are a chance event in an industrial strain improvement programme.

Regulation of invertase in Aspergillus nidulans: effect of different carbon sources

Aspergillus nidulans produces an extracellular beta-D-fructofuranoside fructohydrolase (invertase) when grown on a medium containing the beta-fructofuranosides sucrose or raffinose, indicating that synthesis is subject to induction by the substrate. On a growth medium containing sucrose, production was maximal at 15 h in cultures incubated at 28 C degrees. After this time the level of detectable invertase in the cultures declined. A proportion of the enzyme was secreted during the linear growth phase of the fungus. Various sugars were investigated for induction of invertase, but only the two beta-fructofuranosides induced high production levels; with the other sugars, the enzyme was produced only at a low constitutive level. Mycelium grown under repressive conditions (1% glucose), rapidly produced invertase when transferred to sucrose-containing medium. After 80 min the invertase level in these cultures was 26-fold higher than the constitutive level. The repressive effect of other sugars, e.g. glucose and xylose, on invertase production was also demonstrated in this experimental system.

Solubilisation of a cell wall bound invertase in Aspergillus nidulans

Aspergillus nidulans produces an extracellular invertase when incubated in the presence of sucrose and about half of the activity produced was found to be associated with the mycelium. Sixty percent of this mycelial invertase could be solubilised by simple mechanical disruption. Among the agents tested for solubilisation of invertase, proteinase K and dithiothreitol were the most effective.

Regulation of Isopenicillin N Synthetase (IPNS) Gene Expression in Acremonium Chrysogenum

Total RNA was extracted daily from the beta-lactam antibiotic producing fungus A. chrysogenum strain CO728 during a 7 day cephalosporin C fermentation. IPNS mRNA species, with a size of about 1.5 kb, were detected by Northern blotting at high levels between days 2 and 4. The rapid appearance of IPNS mRNA in mycelial extracts up to day 2 suggests that IPNS is regulated at the transcriptional level. Primer extension and S1 endonuclease mapping studies indicate the existence of two major and at least two minor transcription initiation start sites. There was no change in the relative levels of the four transcripts during the period they could be detected. A region upstream of the IPNS structural gene (pcbC) has been sequenced and the transcription initiation sites appear as major and minor pairs on either side of one of the pyrimidine-rich blocks that punctuate the promoter sequence.

Genetic mapping in Streptomyces clavuligerus by protoplast fusion

The development of a protoplast manipulation protocol for the industrially important bacterium Streptomyces clavuligerus, which produces the beta-lactamase inhibitor clavulanic acid, made possible a preliminary genetic mapping study based on protoplast fusion crosses. A preliminary position for 11 markers on the S. clavuligerus genetic map is proposed. Fusion progeny were characterized by random spore analysis because the markers present in the strains were not amenable to the conventional four-on-four selection procedure. Whilst the resulting map is similar to that derived by conjugation for S. clavuligerus and S. coelicolor, further analysis of the markers is required to confirm these observations.

Protoplast isolation and regeneration in Streptomyces clavuligerus

The regeneration of streptomycete protoplasts is a major step following genetic manipulations such as fusion and DNA-mediated transformation. Reports of studies on the regeneration of protoplasts from Streptomyces clavuligerus are limited and for this reason the experiments described in this paper were carried out. An investigation of protoplast formation and cytology was made to gain further insight into the loss of protoplast viability in osmotically stabilized support media. Protoplasts with the highest regeneration frequency were isolated from mycelium, grown in a two-stage culture system (without glycine), using lysozyme dissolved in a sucrose osmoticum containing 1% bovine serum albumin. The latter promoted improved protoplast viability. A systematic survey was made of the components of regeneration medium R5, previously used for S. clavuligerus, and other potentially advantageous components and conditions, in an attempt to raise the regeneration frequency of the protoplasts. An improved regeneration medium (R6) and protocol which supported higher and more consistent levels of regeneration of S. clavuligerus protoplasts resulted from these experiments. These improved procedures for protoplast isolation and regeneration proved to be suitable for other streptomycete species.

Developments in protoplast fusion in fungi

Protoplasts have proved to be valuable tools in fungal genetics. Removal of the cell wall is clearly very significant because this organelle has an important role in cell-cell interactions. Using the molecular 'glue' polyethylene glycol or electrofusion methods, protoplasts can be fused in intraspecies and interspecies and intergeneric combinations. The technique has therefore opened up an important new area of study which has both fundamental and applied importance.

Protoplast fusion as a tool for genetic analysis in Cephalosporium acremonium

Protoplasts of nutritionally complementary strains of Cephalosporium acremonium were fused and plated onto media which supressed the growth of both parents. The regenerating colonies were used for genetic analysis and were found to be of two types, stable haploid recombinants and unstable heterozygotes (aneuploids and/or diploids). Analysis of these colonies provided evidence for eight linkage groups and a relatively high rate of mitotic crossing-over. The gene order for three of the markers on one linkage group was also determined.

Further studies on protoplast fusion and interspecific hybridization within the Aspergillus nidulans group

Hybridization of eight species of the Aspergillus nidulans group was attempted using auxotrophic mutants and protoplast fusion methods. Viable fusion products were obtained from eight crosses. Allodiploid hybrids were recovered from crosses involving A. nidulans with A. rugulosus, A. quadrilineatus, A. nidulans var. echinulatus and A. violaceus, although some mutants only gave heterokaryons. Crosses involving these latter species also gave heterokaryons. Crosses between A. nidulans and A. unguis, A. stellatus and A. heterothallicus were unsuccessful. Fusions involving three parents gave heterokaryons made up of only two of them.

Aspects of genetic interaction in hybrids of Aspergillus nidulans and Aspergillus rugulosus obtained by protoplast fusion

Hybrids were produced by protoplast fusion between strains of Aspergillus rugulosus and mitotic master strains of Aspergillus nidulans with a genetic marker on each linkage group. Analysis of segregants induced by growth on benomyl revealed recombination between every pair of unlinked markers. Parental combinations of markers were often recovered at significantly higher frequencies than expected. This aberrant segregation was not correlated with any particular pair of linkage-groups and was attributed to inter-species incompatibility. The segregation of genetic markers of A. rugulosus from the hybrids suggested that A. nidulans and A. rugulosus may differ in haploid chromosome number and chromosome size. In sexual crosses between A. nidulans and strains containing chromosomes of mixed parental origin recombinants were recovered. The results support the classification of A. nidulans and A. rugulosus as separate species.

Induced segregation in interspecific hybrids of Aspergillus nidulans and Aspergillus rugulosus obtained by protoplast fusion

Interspecific hybrids produced by polyethylene glycol induced fusion of protoplasts from auxotrophic mutants of Aspergillus nidulans and Aspergillus rugulosus were grown in the presence of the recombinogens benomyl and chloral hydrate to stimulate segregation. The A. nidulans parental strains used had a known genetic marker in each linkage group. Hybrids grown on complete medium containing benomyl yielded more segregants. Analysis of the segregants showed that the distribution of A. nidulans linkage groups was random. No specific linkage group appeared in all the segregants. The two parents are closely related taxonomically and the findings from these experiments suggest that a high degree of chromosomal homology may exist between them.

A particulate chitin synthase from Aspergillus flavus Link: the properties, location, and levels of activity in mycelium and regenerating protoplast preparations

Chitin synthase (ED 2.4.1.16) has been characterized in Aspergillus flavus. A K(m) value of 2.5 m(M) was obtained for the substrate UDPGlcNAc. The enzyme had a requirement for GlcNAc, and Mg2+ and activity was increased in the presence of soluble chitodextrins F1 and F2. The optimum activity was obtained using Tris--HCl buffer, pH 7.5, with a secondary peak at pH 6.2 and an incubation temperature of 29.5 degrees C. Distribution patterns of chitin synthase in protoplasts and mycelial material were very similar. The highest specific activity was found in a 200 000 X g fraction. Enzyme levels in growing mycelium increased during the exponential growth phase after which they declined. Activity also increased during the early stages of regeneration of both conidial and mycelial protoplasts, despite an initial lack in net protein synthesis. Chitin synthase levels were also dependent upon the carbon source available during regeneration.

Conditions for induced fusion of fungal protoplasts in polyethylene glycol solutions

Solutions containing polyethylene glycol MW 6000 (PEG) induced fusion of protoplasts of Penicillium chrysogenum. Balanced heterokaryons were formed by fusion of nutritionally complementing protoplasts. Heterokaryotic fusion products were obtained up to a frequency of 4% of the number of protoplasts, surviving the fusion treatment. Investigation of the conditions, necessary to achieve this high fusion frequency, showed that supplementing the PEG solution with Ca++ and adjustment to high pH gave the best results. Mechanisms of fusion of fungal protoplasts by PEG, calcium and alkaline pH are discussed in view of the obtained results.

Chitin synthase in Mortierella vinacea: properties, cellular location and synthesis in growing cultures

Chitin synthase of Mortierella vinacea was present in the "microsomal' fraction (100 000 g precipitate), the 'cell-wall' fraction (2000 g precipitate) and the 'mitochondrial' fraction (10 000 g precipitate). The properties of the 'microsomal' enzyme were investigated. The pH optimum was between 5-8 and 6-2, and the temperature optimum was between 31 and 33 degrees C. The Km for UDP N-acetyl-D-glucosamine was 1.8 mM. The enzyme was stimulated by Mg2+ and a slight stimulation was also effected by N-acetyl-D-glucosamine. Soluble chitodextrins were inhibitory. A pH-dependent, heat-stable inhibitor of chitin synthase activity was present in the soluble cytoplasm from the mycelium. The effects of aeration and glucose concentration on enzyme production in growing cultures were also investigated; maximum specific activity of chitin synthase was associated with the cessation of exponential growth.