Safety of the fungal workhorses of industrial biotechnology: update on the mycotoxin and secondary metabolite potential of Aspergillus niger, Aspergillus oryzae, and Trichoderma reesei.

This review presents an update on the current knowledge of the secondary metabolite potential of the major fungal species used in industrial biotechnology, i.e., Aspergillus niger, Aspergillus oryzae, and Trichoderma reesei. These species have a long history of safe use for enzyme production. Like most microorganisms that exist in a challenging environment in nature, these fungi can produce a large variety and number of secondary metabolites. Many of these compounds present several properties that make them attractive for different industrial and medical applications. A description of all known secondary metabolites produced by these species is presented here. Mycotoxins are a very limited group of secondary metabolites that can be produced by fungi and that pose health hazards in humans and other vertebrates when ingested in small amounts. Some mycotoxins are species-specific. Here, we present scientific basis for (1) the definition of mycotoxins including an update on their toxicity and (2) the clarity on misclassification of species and their mycotoxin potential reported in literature, e.g., A. oryzae has been wrongly reported as an aflatoxin producer, due to misclassification of Aspergillus flavus strains. It is therefore of paramount importance to accurately describe the mycotoxins that can potentially be produced by a fungal species that is to be used as a production organism and to ensure that production strains are not capable of producing mycotoxins during enzyme production. This review is intended as a reference paper for authorities, companies, and researchers dealing with secondary metabolite assessment, risk evaluation for food or feed enzyme production, or considerations on the use of these species as production hosts.

Co-crystal of tramadol-celecoxib (CTC) for acute moderate-to-severe pain.

OBJECTIVE: This narrative review aims to provide a clinical perspective on the potential role of co-crystal of tramadol-celecoxib (CTC) in the management of acute moderate-to-severe pain by synthesizing the available preclinical and clinical data, with emphasis on phase 3 trials. METHODS: A non-systematic literature review was performed using a targeted PubMed search for articles published between January 1, 2000, and May 2, 2023; all publication types were permitted, and selected articles were limited to those published in English. Search results were manually reviewed to identify references based on their preclinical and clinical relevance to CTC and management of acute moderate-to-severe pain. RESULTS: The crystalline structure of CTC alters the physicochemical properties of tramadol and celecoxib, modifying their pharmacokinetics. If taken in a free combination, tramadol reduces absorption of celecoxib. Conversely, administration of CTC slows tramadol absorption and lowers its maximum plasma concentration, while increasing celecoxib plasma concentration through its enhanced release. In clinical studies across models of acute moderate-to-severe pain, CTC demonstrated an early onset of analgesia, with improved efficacy and lower rescue medication use, compared with either agent alone. CTC's safety profile was in line with that expected for the individual components; no additive effects were observed. CTC exhibited tramadol-sparing effects, with efficacy seen at lower daily/cumulative opioid doses vs. tramadol alone. CONCLUSIONS: Results from phase 3 trials suggest that the modified physicochemical properties of tramadol and celecoxib in CTC translate into an improved clinical benefit-risk profile, including fewer opioid-related adverse effects due to lower overall opioid dosing.

Safety of Co-Crystal of Tramadol-Celecoxib (CTC) in Patients with Acute Moderate-to-Severe Pain: Pooled Analysis of Three Phase 3 Randomized Trials.

INTRODUCTION: Multi-modal analgesia is desirable for the management of acute pain since it can provide effective pain relief at lower doses, thereby aiding tolerability. Co-crystal of tramadol-celecoxib (CTC) provides effective analgesia in models of acute pain. Co-crystallization can alter the pharmacokinetics of individual components, potentially improving tolerability. We sought to better understand the safety and tolerability of CTC in patients with acute postoperative pain. METHODS: We conducted a pooled analysis of safety data from three phase 3 randomized controlled trials in adults with acute moderate-to-severe pain following oral surgery, bunionectomy, and elective abdominal hysterectomy. We present data for CTC 200 mg twice daily (BID) and its comparators: tramadol 50 mg four times daily (QID) (one trial), tramadol 100 mg QID (two trials), celecoxib 100 mg BID (two trials), and placebo (three trials). RESULTS: In total, n = 551 patients received CTC 200 mg BID, n = 183 received tramadol 50 mg QID, n = 368 received tramadol 100 mg QID, n = 388 received celecoxib 100 mg BID, and n = 274 received placebo. The prevalence of adverse events (AEs) related to study drug up to 48 h was numerically lower with CTC 200 mg BID (35.9%) than with tramadol 50 mg QID (47.5%) and 100 mg QID (44.8%) but greater than with celecoxib 100 mg BID (12.4%) and placebo (20.4%). The most frequent AEs related to study drug up to 48 h were somnolence, nausea, dizziness, and vomiting, which occurred more frequently in patients receiving tramadol 100 mg QID than in those receiving CTC 200 mg BID. CONCLUSION: CTC 200 mg BID appears to be better tolerated than tramadol 100 mg QID, possibly because of reduced total exposure to tramadol. This may contribute to a more favorable benefit-risk profile for CTC versus individual components, making it a promising treatment for acute pain. TRIAL REGISTRATION: ClinicalTrials.gov identifiers: NCT03108482, NCT02982161 (EudraCT: 2016-000592-24), NCT03062644 (EudraCT: 2016-000593-38).

Efficacy of Co-Crystal of Tramadol-Celecoxib (CTC) in Patients with Acute Moderate-to-Severe Pain: A Pooled Analysis of Data from Two Phase 3 Randomized Clinical Trials.

BACKGROUND AND OBJECTIVES: New acute pain medications are needed that provide effective analgesia while minimizing side effects and opioid exposure. Clinical trials of co-crystal of tramadol-celecoxib (CTC) have demonstrated an improved benefit/risk profile versus tramadol or celecoxib alone. We pooled data from two phase 3 clinical trials to evaluate the efficacy of CTC 200 mg twice daily (BID) in acute moderate-to-severe pain. METHODS: Efficacy data were pooled from STARDOM1 [acute pain following oral surgery (NCT02982161)] and ESTEVE-SUSA-301 [acute pain following bunionectomy (NCT03108482)]. The primary efficacy outcome was sum of pain intensity difference from 0 to 48 h (SPID0-48). RESULTS: A total of 344 patients received CTC 200 mg BID, 342 received tramadol 50 or 100 mg four times a day, 181 received celecoxib 100 mg BID, and 172 received placebo. The least-squares mean difference in SPID0-48 was -21.8 (p = 0.002) for CTC versus tramadol and -72.8 (p < 0.001) for CTC versus placebo. A similar pattern of SPID0-48 was observed with CTC versus comparator whether patients had moderate or severe pain at baseline. Reduction in pain intensity was faster and reached mild intensity earlier with CTC versus comparators. Patients were significantly (p ≤ 0.005) less likely to receive rescue medication within 4 or 48 h with CTC compared with tramadol or placebo. CONCLUSIONS: This pooled analysis reinforces the efficacy profile of CTC versus tramadol and, given that CTC permits lower daily tramadol dosing and thereby reduces unnecessary opioid use, this highlights its improved benefit/risk profile and its potential for the management of moderate-to-severe pain.

Reprint of: Current strategies for the use of affinity tags and tag removal for the purification of recombinant proteins.

Affinity tags are highly efficient tools for protein purification. They allow the purification of virtually any protein without any prior knowledge of its biochemical properties. The use of affinity tags has therefore become widespread in several areas of research e.g., high throughput expression studies aimed at finding a biological function to large numbers of yet uncharacterized proteins. In some cases, the presence of the affinity tag in the recombinant protein is unwanted or may represent a disadvantage for the projected application of the protein, like for clinical use. Therefore, an increasing number of approaches are available at present that are designed for the removal of the affinity tag from the recombinant protein. Most of these methods employ recombinant endoproteases that recognize a specific sequence. These process enzymes can subsequently be removed from the process by affinity purification, since they also include a tag. Here, a survey of the most common affinity tags and the current methods for tag removal is presented, with special emphasis on the removal of N-terminal histidine tags using TAGZyme, a system based on exopeptidase cleavage. In the quest to reduce the significant costs associated with protein purification at large scale, relevant aspects involved in the development of downstream processes for pharmaceutical protein production that incorporate a tag removal step are also discussed. A comparison of the yield of standard vs. affinity purification together with an example of tag removal using TAGZyme is also included.

Reprint of: Current strategies for the use of affinity tags and tag removal for the purification of recombinant proteins.

Affinity tags are highly efficient tools for protein purification. They allow the purification of virtually any protein without any prior knowledge of its biochemical properties. The use of affinity tags has therefore become widespread in several areas of research e.g., high throughput expression studies aimed at finding a biological function to large numbers of yet uncharacterized proteins. In some cases, the presence of the affinity tag in the recombinant protein is unwanted or may represent a disadvantage for the projected application of the protein, like for clinical use. Therefore, an increasing number of approaches are available at present that are designed for the removal of the affinity tag from the recombinant protein. Most of these methods employ recombinant endoproteases that recognize a specific sequence. These process enzymes can subsequently be removed from the process by affinity purification, since they also include a tag. Here, a survey of the most common affinity tags and the current methods for tag removal is presented, with special emphasis on the removal of N-terminal histidine tags using TAGZyme, a system based on exopeptidase cleavage. In the quest to reduce the significant costs associated with protein purification at large scale, relevant aspects involved in the development of downstream processes for pharmaceutical protein production that incorporate a tag removal step are also discussed. A comparison of the yield of standard vs. affinity purification together with an example of tag removal using TAGZyme is also included.

Alcohol dehydrogenase 1 participates in the Crabtree effect and connects fermentative and oxidative metabolism in the Zygomycete Mucor circinelloides.

Mucor circinelloides is a dimorphic Zygomycete fungus that produces ethanol under aerobic conditions in the presence of glucose, which indicates that it is a Crabtree-positive fungus. To determine the physiological role of the alcohol dehydrogenase (ADH) activity elicited under these conditions, we obtained and characterized an allyl alcohol-resistant mutant that was defective in ADH activity, and examined the effect of adh mutation on physiological parameters related to carbon and energy metabolism. Compared to the Adh+ strain R7B, the ADH-defective (Adh-) strain M5 was unable to grow under anaerobic conditions, exhibited a considerable reduction in ethanol production in aerobic cultures when incubated with glucose, had markedly reduced growth capacity in the presence of oxygen when ethanol was the sole carbon source, and exhibited very low levels of NAD+-dependent alcohol de-hydrogenase activity in the cytosolic fraction. Further characterization of the M5 strain showed that it contains a 10-bp deletion that interrupts the coding region of the adhl gene. Complementation with the wild-type allele adh1+ by transformation of M5 remedied all the defects caused by the adh1 mutation. These findings indicate that in M. circinelloides, the product of the adh1 gene mediates the Crabtree effect, and can act as either a fermentative or an oxidative enzyme, depending on the nutritional conditions, thereby participating in the association between fermentative and oxidative metabolism. It was found that the spores of M. circinelloides possess low mRNA levels of the ethanol assimilation genes (adl2 and acs2), which could explain their inability to grow in the alcohol.

The use of TAGZyme for the efficient removal of N-terminal His-tags.

The use of affinity tags and especially histidine tags (His-tags) has become widespread in molecular biology for the efficient purification of recombinant proteins. In some cases, the presence of the affinity tag in the recombinant protein is unwanted or may represent a disadvantage for the projected use of the protein, like in clinical, functional or structural studies. For N-terminal tags, the TAGZyme system represents an ideal approach for fast and accurate tag removal. TAGZyme is based on engineered aminopeptidases. Using human tumor necrosis factor alpha as a model protein, we describe here the steps involved in the removal of a His-tag using TAGZyme. The tag used (UZ-HT15) has been optimized for expression in Escherichia coli and for TAGZyme efficiency. The UZ-HT15 tag and the method can be applied to virtually any protein. A description of the cloning strategy for the design of the genetic construction, two alternative approaches and a simple test to assess the performance of the tag removal process are also included.

The crystal structure of human dipeptidyl peptidase I (cathepsin C) in complex with the inhibitor Gly-Phe-CHN2.

hDDPI (human dipeptidyl peptidase I) is a lysosomal cysteine protease involved in zymogen activation of granule-associated proteases, including granzymes A and B from cytotoxic T-lymphocytes and natural killer cells, cathepsin G and neutrophil elastase, and mast cell tryptase and chymase. In the present paper, we provide the first crystal structure of an hDPPI-inhibitor complex. The inhibitor Gly-Phe-CHN2 (Gly-Phe-diazomethane) was co-crystallized with hDPPI and the structure was determined at 2.0 A (1 A=0.1 nm) resolution. The structure of the native enzyme was also determined to 2.05 A resolution to resolve apparent discrepancies between the complex structure and the previously published structure of the native enzyme. The new structure of the native enzyme is, within the experimental error, identical with the structure of the enzyme-inhibitor complex presented here. The inhibitor interacts with three subunits of hDPPI, and is covalently bound to Cys234 at the active site. The interaction between the totally conserved Asp1 of hDPPI and the ammonium group of the inhibitor forms an essential interaction that mimics enzyme-substrate interactions. The structure of the inhibitor complex provides an explanation of the substrate specificity of hDPPI, and gives a background for the design of new inhibitors.

Secondary metabolite production and the safety of industrially important members of the Bacillus subtilis group.

Members of the 'Bacillus subtilis group' include some of the most commercially important bacteria, used for the production of a wide range of industrial enzymes and fine biochemicals. Increasingly, group members have been developed for use as animal feed enhancers and antifungal biocontrol agents. The group has long been recognised to produce a range of secondary metabolites and, despite their long history of safe usage, this has resulted in an increased focus on their safety. Traditional methods used to detect the production of secondary metabolites and other potentially harmful compounds have relied on phenotypic tests. Such approaches are time consuming and, in some cases, lack specificity. Nowadays, accessibility to genome data and associated bioinformatical tools provides a powerful means for identifying gene clusters associated with the synthesis of secondary metabolites. This review focuses primarily on well-characterised strains of B. subtilis and B. licheniformis and their synthesis of non-ribosomally synthesised peptides and polyketides. Where known, the activities and toxicities of their secondary metabolites are discussed, together with the limitations of assays currently used to assess their toxicity. Finally, the regulatory framework under which such strains are authorised for use in the production of food and feed enzymes is also reviewed.

A built-in co-carcinogenic effect due to viruses involved in latent or persistent infections.

A new hypothesis for some cancers, which combines the chromosomal instability theories with a co-carcinogenic effect of viruses causing latent or persistent infection, is presented. The hypothesis incorporates the multi-step model of cancer and that pre-cancerous cells reach a state of chromosomal instability. Because of chromosomal instability, the genome of these cell lines will lead to changes from generation to generation and will face a remarkable selection pressure both from lost traits, apoptosis, and from the immune system. Viruses causing latent or persistent infections have evolved many different genes capable to evade the immune system. If these viruses are harboured in the genome of pre-cancerous cells they could provide them with "superpowers" and with genes that may assist the cells to elude the immune system. The theory explains why cancer predominantly is a disease of old age. Upon aging, the immune system becomes reduced including the ability to control and suppress the viruses that cause latent or persistent infections. The risk of cancer could thereby increase as the immune functions decrease. The theory provides new insights to the genesis of cancers.

Cloning strategy, production and purification of proteins with exopeptidase-cleavable His-tags.

Here, we present a cloning strategy for the production of recombinant proteins tagged with a polyhistidine sequence that can be cleaved by the exopeptidase, DAPase. The method can be used with most commonly available vectors and results in the expression of a His-tag protein that can be purified in its native form regardless of its natural sequence. This approach takes advantage of the TAGZyme system for the removal of amino-terminal affinity tags. Tag removal is accomplished either with DAPase (a recombinant dipeptidyl peptidase) alone or in combination with two accessory enzymes, Qcyclase and pGAPase. The system has been used for the production of intracellular proteins in Escherichia coli and can be applied to other expression hosts for the production of secreted proteins or proteins that require post-translational modification. The production of human interleukin 1beta in E. coli is used as an example to illustrate this method. The complete protocol from initial PCR to the production of a detagged protein with its authentic N terminus can be performed within 5 days.

Current strategies for the use of affinity tags and tag removal for the purification of recombinant proteins.

Affinity tags are highly efficient tools for protein purification. They allow the purification of virtually any protein without any prior knowledge of its biochemical properties. The use of affinity tags has therefore become widespread in several areas of research e.g., high throughput expression studies aimed at finding a biological function to large numbers of yet uncharacterized proteins. In some cases, the presence of the affinity tag in the recombinant protein is unwanted or may represent a disadvantage for the projected application of the protein, like for clinical use. Therefore, an increasing number of approaches are available at present that are designed for the removal of the affinity tag from the recombinant protein. Most of these methods employ recombinant endoproteases that recognize a specific sequence. These process enzymes can subsequently be removed from the process by affinity purification, since they also include a tag. Here, a survey of the most common affinity tags and the current methods for tag removal is presented, with special emphasis on the removal of N-terminal histidine tags using TAGZyme, a system based on exopeptidase cleavage. In the quest to reduce the significant costs associated with protein purification at large scale, relevant aspects involved in the development of downstream processes for pharmaceutical protein production that incorporate a tag removal step are also discussed. A comparison of the yield of standard vs. affinity purification together with an example of tag removal using TAGZyme is also included.

Papaya glutamine cyclotransferase shows a singular five-fold beta-propeller architecture that suggests a novel reaction mechanism.

Cyclisation of N-terminal glutamine and/or glutamate to yield pyroglutamate is an essential posttranslational event affecting a plethora of bioactive peptides and proteins. It is directly linked with pathologies ranging from neurodegenerative diseases to inflammation and several types of cancers. The reaction is catalysed by ubiquitous glutaminyl cyclotransferases (QCs), which present two distinct prototypes. Mammalian QCs are zinc-dependent enzymes with an alpha/beta-hydrolase fold. Here we present the 1.6-A-resolution structure of the other prototype, the plant analogue from Carica papaya (PQC). The hatbox-shaped molecule consists of an unusual five-fold beta-propeller traversed by a central channel, a topology that has hitherto been described only for some sugar-binding proteins and an extracellular nucleotidase. The high resistance of the enzyme to denaturation and proteolytic degradation is explained by its architecture, which is uniquely stabilised by a series of tethering elements that confer rigidity. Strikingly, the N-terminus of PQC specifically interacts with residues around the entrance to the central channel of a symmetry-related molecule, suggesting that this location is the putative active site. Cyclisation would follow a novel general-acid/base working mechanism, pivoting around a strictly conserved glutamate. This study provides a lead structure not only for plant QC orthologues, but also for bacteria, including potential human pathogens causing diphtheria, plague and malaria.

Molecular characterization of a G protein alpha-subunit-encoding gene from Mucor circinelloides.

Genes encoding the Galpha subunit were cloned from Mucor circinelloides, a zygomycete dimorphic fungus. There are at least four genes that encode for Galpha subunits, gpa1, gpa2, gpa3, and gpa4. The genes gpa1 and gpa3 were isolated and characterized, and their predicted products showed 36%-67% identity with Galpha subunits from diverse fungi. Northern blot analysis of gpa3 showed that it is present in spores and constitutively expressed during mycelium development and during yeast-mycelium and mycelium-yeast transitions. However, during yeast cell growth, decreased levels of mRNA were observed. Sequence analysis of gpa3 cDNA revealed that Gpa3 encodes a polypeptide of 356 amino acids with a calculated molecular mass of 40.8 kDa. The deduced sequence of Gpa3 protein contains all the consensus regions of Galpha subunits of the Galpha(i/o/t) subfamily except the cysteine near the C terminus for potential ADP-ribosylation by pertussis toxin. This cDNA was expressed in Escherichia coli and purified by affinity chromatography. Based on its electrophoretic mobility in SDS-PAGE, the molecular mass of the His6-tagged Gpa3 was 45 kDa. The recombinant protein was recognized by a polyclonal antibody against a fragment of a human Galpha(i/o/t). Furthermore, the recombinant Gpa3 was ADP-ribosylated by activated cholera toxin and [32P]NAD but not by pertussis toxin. These results indicate that in M. circinelloides the Galpha subunit Gpa3 is expressed constitutively during differentiation.

Cloning of glyceraldehyde-3-phosphate dehydrogenase-encoding genes in Mucor circinelloides (Syn. racemosus) and use of the gpd1 promoter for recombinant protein production.

Three genes (gpd1, gpd2, and gpd3) encoding glyceraldehyde-3-phosphate dehydrogenase were isolated from the dimorphic zygomycete Mucor circinelloides by PCR using degenerated primers. Transcription of gpd1 could be detected during vegetative growth under both aerobic and anaerobic conditions, whereas neither gpd2 nor gpd3 transcription was detected, indicating that gpd1 is the major transcribed gpd gene. The transcription of gpd1 was regulated by carbon source. The gpd1 promoter was successfully used for recombinant expression of genes of both homologous (crgA encoding a regulator of carotene biosynthesis) and heterologous (gox1 from Aspergillus niger encoding glucose oxidase; GOX) nature. Growth of a gox1 transformant strain resulted in the secretion of enzymatically active GOX. The potential advantages of using a dimorphic fungus for heterologous protein production are discussed.

Characterisation of the Mucor circinelloides regulated promoter gpd1P.

The promoter of the Mucor circinelloides gpd1 gene encoding glyceraldehyde-3-phosphate dehydrogenase (gpd1P) was recently cloned and used for the production of recombinant proteins, such as the Aspergillus niger glucose oxidase 1 (GOX). This represents the first example of the application of a strong and regulated promoter from this fungus for recombinant protein production. The original 741-bp gpd1P promoter fragment conferred hexose-dependent expression of GOX in M. circinelloides. To understand the regulatory mechanisms involved in gpd1P-driven expression and to develop improved promoter fragments, deletion derivatives of gpd1P were constructed. These derivatives were fused to the A. niger gox1 gene and used to construct strains containing a single copy of the expression cassette. GOX activity was detected in strains containing the full-length gpd1P and also in strains containing a 713-bp or a 361-bp derivative. Expression levels for the 361-bp derivative were high and comparable, regardless of the carbon source used. This promoter represents a useful derivative for constitutive heterologous gene expression in M. circinelloides.

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.

Optimization of signal peptide SP310 for heterologous protein production in Lactococcus lactis.

The authors have previously reported the identification of novel signal peptides (SPs) from Lactococcus lactis using transposon insertion. Of these, SP310 caused the highest level of secretion. However, the levels were lower than those obtained using the signal peptide from Usp45 (SPUSP), the major secreted lactococcal protein. In this study, site-directed mutagenesis of signal peptide SP310 was used to investigate the effect of amino acid alterations on lactococcal secretion and to improve secretion efficiency. Several mutated SPs caused higher secretion. This increase in secretion was due to modifications in the cleavage region. In fermenter experiments, the signal peptide SP310mut2 resulted in an extracellular Staphylococcus aureus nuclease (Nuc) yield which was 45 % higher than that with the natural SP310. Surprisingly, increasing the hydrophobicity of the hydrophobic core or increasing the number of positively charged amino acids in the N-terminal region of SP310 decreased secretion. High extracellular yields of Nuc resulted from more efficient secretion, as strains with less efficient SPs accumulated more intracellular SP-Nuc precursor.

Protein kinase A is involved in the control of morphology and branching during aerobic growth of Mucor circinelloides.

The cAMP signal transduction pathway controls many processes in fungi. The Mucor circinelloides pkaR and pkaC genes, encoding the regulatory (PKAR) and catalytic (PKAC) subunit of the cAMP-dependent protein kinase A (PKA), have been cloned recently. Expression analysis during the dimorphic shift and colony morphology suggested a role for PKAR in the control of morphology and branching. Here strain KFA121, which overexpresses the M. circinelloides pkaR gene, was used to quantify growth and branching under different aerobic growth conditions in a flow-through cell by computerized image analysis. An inverse relationship between the pkaR expression level in KFA121 and the hyphal growth unit length was observed in KFA121, suggesting a central role for PKAR in branching. A biochemical analysis of PKAR using antibodies and enzyme assay demonstrated that the level of PKAR is higher in KFA121 under inducing conditions, i.e. in the presence of high glucose, than in the vector control strain KFA89. Measurement of cAMP binding demonstrated a significant increase (two- to threefold) in PKAR level for KFA121 at the time of germ-tube emission in medium containing 10 g glucose l(-1). The level of PKA activity was determined using kemptide in the same crude cell extracts used to determine cAMP binding. Strain KFA121 showed a twofold increase in PKA activity. An excess of free PKAR subunit over PKA holoenzyme was determined using sucrose gradient centrifugation of extracts from KFA89 and KFA121. The data indicate that cAMP-dependent PKA in M. circinelloides might be down-regulated during hyphal-tube emergence and that an increase in PKAR levels results in increased branching.