Lysine biosynthesis in Penicillium chrysogenum is regulated by feedback inhibition of α-aminoadipate reductase

A partially purified preparation of alpha-aminoadipate reductase (EC 1.2.1.31) from Penicillium chrysogenum is competitively inhibited by lysine (Ki of 0.26 mM). Exogenous addition of 10 mM L-lysine to resting mycelia of P. chrysogenum increased the intracellular lysine pool concentration 2-fold, but decreased the incorporation of (6-14C)-alpha-aminoadipate into protein-bound lysine to a fifth. The distribution of radioactivity in the pathway metabolites alpha-aminoadipate, saccharopine and lysine was consistent with the assumption of a lysine sensitive enzyme step in vivo between alpha-aminoadipate and saccharopine. Hence lysine inhibition of alpha-aminoadipate reductase may be of physiologic importance.     

Regulation of alpha-aminoadipate reductase from Penicillium chrysogenum in relation to the flux from alpha-aminoadipate into penicillin biosynthesis.

The activity and regulation of alpha-aminoadipate reductase in three Penicillium chrysogenum strains (Q176, D6/1014/A, and P2), producing different amounts of penicillin, were studied. The enzyme exhibited decreasing affinity for alpha-aminoadipate with increasing capacity of the respective strain to produce penicillin. The enzyme from all three strains was inhibited by L-lysine, and the enzyme from the lowest producer, Q176, was least sensitive. Between pH 7.5 and 6.5, inhibition of alpha-aminoadipate reductase by L-lysine was pH dependent, being more pronounced at lower pH. The highest producer strain, P2, displayed the lowest alpha-aminoadipate reductase activity at pH 7.0. In Q176, the addition of 0.5-1 mM of exogenous lysine stimulated penicillin formation, whereas the same concentration was ineffective or inhibitory with strains D6/1014/A and P2. The addition of higher (up to 5 mM) lysine concentrations inhibited penicillin production in all three strains. In mutants of P. chrysogenum D6/1014/A, selected for resistance to 20 mM alpha-aminoadipate, highest penicillin production was observed in those strains whose alpha-aminoadipate reductase was most strongly inhibited by L-lysine. The results support the conclusion that the in vivo activity of alpha-aminoadipate reductase from superior penicillin producer strains of P. chrysogenum is more strongly inhibited by lysine, and that this is related to their ability to accumulate increased amounts of alpha-aminoadipate, and hence penicillin.     

alpha-Aminoadipate pool concentration and penicillin biosynthesis in strains of Penicillium chrysogenum

Intracellular amino acid pools in four Penicillium chrysogenum strains, which differed in their ability to produce penicillin, were determined under conditions supporting growth without penicillin production and under conditions supporting penicillin production. A significant correlation between the rate of penicillin production and the intracellular concentration of alpha-aminoadipate was observed, which was not shown with any other amino acid in the pool. In replacement cultivation, penicillin production was stimulated by alpha-aminoadipate, but not by valine or cysteine. Exogenously added alpha-aminoadipate (2 or 3 mM) maximally stimulated penicillin synthesis in two strains of different productivity. Under these conditions intracellular concentrations of alpha-aminoadipate were comparable in the two strains in spite of the higher rate of penicillin production in the more productive strain. Results suggest that the lower penicillin titre of strain Q 176 is due to at least two factors: (i) the intracellular concentration of alpha-aminoadipate is insufficient to allow saturation of any enzyme which is rate limiting in the conversion of alpha-aminoadipate to penicillin and (ii) the level of an enzyme, which is rate limiting in the conversion of alpha-aminoadipate to penicillin, is lower in Q 176 (relative to strain D6/1014/A). Results suggest that the intracellular concentration of alpha-aminoadipate in strain D6/1014/A is sufficiently high to allow saturation of the rate-limiting penicillin biosynthetic enzyme in that strain. The basis of further correlation of intracellular alpha-aminoadipate concentration and penicillin titre among strains D6/1014/A, P2, and 389/3, the three highest penicillin producers studied here, remains to be established.(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabolic flux distributions in Penicillium chrysogenum during fed-batch cultivations

Based on a review of the Penicillium chrysogenum biochemistry a stoichiometric model has been set up. The model considers 61 internal fluxes and there are 49 intracellular metabolites which are assumed to be in pseudo-steady state. In addition to the intracellular fluxes the model considers the uptake of 21 amino acids. From the stoichiometric model the maximum theoretical yield of penicillin V is calculated to 0.43 mol/mol glucose. If biosynthesis of cysteine is by direct sulfhydrylation rather than by transsulfuration, the maximum theoretical yield is about 20% higher, i.e., 0.50 mol/mol glucose. The theoretical yield decreases substantially if alpha-aminoadipate is converted to 6-oxo-piperidine-2-carboxylic acid (OPC). If only 40% of the alpha-aminoadipate is recycled, the maximum theoretical yield is 0.31 mol/mol glucose. The uptake rates of glucose, lactate, gamma-aminobutyrate, and 21 amino acids were measured during fed-batch cultivations. The rates of formation of penicillin V, delta-(L-alpha)-aminoadipyl-L-cysteinyl-D-valine (ACV), OPC, and the pool of isopenicillin N, 6-APA, and 8-HPA were also measured. Finally the synthesis rates of the biomass constituents RNA/DNA, protein, lipid, carbohydrate, and amino carbohydrate were measured. From these measured rates and the stoichiometric model the metabolic fluxes through the different intracellular pathways are calculated. The calculations show that penicillin formation is accompanied by a large flux through the pentose phosphate (PP) pathway due to a large requirement for nicotinamide-adenine dinucleotide phosphate (NADPH) used in the biosynthesis of cysteine. If cysteine is added to the medium, the flux through the PP pathway decreases. From the stoichiometric model Y(xATP) is calculated to 87 mmol adenosine triphosphate (ATP)/g dry weight (DW), and from the flux calculations m(ATP) is found to 3 mmol ATP/g DW/h. (c) 1995 John Wiley & Sons, Inc.     

Quantitative analysis of Penicillium chrysogenum Wis54-1255 transformants overexpressing the penicillin biosynthetic genes

The low penicillin-producing, single gene copy strain Wis54-1255 was used to study the effect of overexpressing the penicillin biosynthetic genes in Penicillium chrysogenum. Transformants of Wis54-1255 were obtained with the amdS expression-cassette using the four combinations: pcbAB, pcbC, pcbC-penDE, and pcbAB-pcbC-penDE of the three penicillin biosynthetic genes. Transformants showing an increased penicillin production were investigated during steady-state continuous cultivations with glucose as the growth-limiting substrate. The transformants were characterized with respect to specific penicillin productivity, the activity of the two pathway enzymes delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase (ACVS) and isopenicillin N synthetase (IPNS) and the intracellular concentration of the metabolites: delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (ACV), bis-delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (bisACV), isopenicillin N (IPN), glutathione (GSH), and glutathione disulphide (GSSG). Transformants with the whole gene cluster amplified showed the largest increase in specific penicillin productivity (r(p))-124% and 176%, respectively, whereas transformation with the pcbC-penDE gene fragment resulted in a decrease in r(p) of 9% relative to Wis54-1255. A marked increase in r(p) is clearly correlated with a balanced amplification of both the ACVS and IPNS activity or a large amplification of either enzyme activity. The increased capacity of a single enzyme occurs surprisingly only in the transformants where all the three biosynthetic genes are overexpressed but is not found within the group of pcbAB or pcbC transformants. The indication of the pcbAB and pcbC genes being closely regulated in fungi might explain why high-yielding strains of P. chrysogenum have been found to contain amplifications of a large region including the whole penicillin gene cluster and not single gene amplifications. Measurements of the total ACV concentration showed a large span of variability, which reflected the individual status of enzyme overexpression and activity found in each strain. The ratio ACV:bisACV remained constant, also at high ACV concentrations, indicating no limitation in the capacity of the thioredoxin-thioredoxin reductase (TR) system, which is assumed to keep the pathway intermediate LLD-ACV in its reduced state. The total GSH pool was at a constant level of approx. 5.7 mM in all cultivations.     

Inhibition of α-aminoadipate-semialdehyde dehydrogenase from Trichosporon adeninovorans bvy lysine and lysine analogues

The alpha-aminoadipate-semialdehyde dehydrogenase (EC 1.2.1.31) of Trichosporon adeninovorans, an enzyme of lysine biosynthesis, was partially purified, some properties of the enzyme were studied and a novel regulatory pattern was found. The Km values of the enzyme were estimated to be 0.78 mM for alpha-aminoadipate, 1.0 mM for ATP, 0.23 mM for NADPH and 0.77 mM for MgCl2. It is demonstrated that the enzyme can be regulated by lysine and lysine analogues. L-Lysine (Ki of 0.09 mM), S-(beta-aminoethyl)-L-cysteine (Ki of 0.007 mM) and delta-hydroxylysine (Ki of 1.65 mM) inhibited the enzyme activity. The inhibition was competitive with respect to alpha-aminoadipate and non-competitive with respect to both ATP and NADPH.     

Coinduction of α-L-arabinofuranosidase and α-D-galactosidase formation in Trichoderma reesei RUT C-30

Abstract Formation of α-L-arabinosidase can be induced in Trichoderma reesei by growing the fungus on L-arabinose or dulcitol, and by adding L-arabinose, L-arabitol, D-galactose, or dulcitol ot non-growing mycelia. The same conditions also stimulated the formation of α-D-galactosidase, but not that of various other enzymes involved in hemicellulose degradation. The optimal inducer concentration with all compounds was 4 mM for both enzymes. Using L-arabinose and D-galactose, the induction efficiency was highest at pH 6.5, whereas induction by arabitol and dulcitol was more efficient at low pH (2.5). The addition of 50 mM glucose did not repress α-L-arabinosidase or α-D-galactosidase formation. These findings suggest coregulation of two hemicellulose side-chain cleaving enzymes in T. reesei .     

Clusters of genes for the biosynthesis of antibiotics: Regulatory genes and overproduction of pharmaceuticals

Summary In the last decade numerous genes involved in the biosynthesis of antibiotics, pigments, herbicides and other secondary metabolites have been cloned. The genes involved in the biosynthesis of penicillin, cephalosporin and cephamycins are organized in clusters as occurs also with the biosynthetic genes of other antibiotics and secondary metabolites (see review by Martín and Liras [65]). We have cloned genes involved in the biosynthesis of -lactam antibiotics from five different -lactam producing organisms both eucaryotic (Penicillium chrysogenum, Cephalosporium acremonium (syn.Acremonium chrysogenum) Aspergillus nidulans) and procaryotic (Nocardia lactamdurans, Streptomyces clavuligerus). InP. chrysogenum andA. nidulans the organization of thepcbAB,pcbC andpenDE genes for ACV synthetase, IPN synthase and IPN acyltransferase showed a similar arrangement. InA. chrysogenum two different clusters of genes have been cloned. The cluster of early genes encodes ACV synthetase and IPN synthase, whereas the cluster of late genes encodes deacetoxycephalosporin C synthetase/hydroxylase and deacetylcephalosporin C acetyltransferase. InN. lactamdurans andS. clavuligerus a cluster of early cephamycin genes has been fully characterized. It includes thelat (for lysine-6-aminotransferase),pcbAB (for ACV synthase) andpcbC (for IPN synthase) genes. Pathway-specific regulatory genes which act in a positive (or negative) form are associated with clusters of genes involved in antibiotic biosynthesis. In addition, widely acting positive regulatory elements exert a pleiotropic control on secondary metabolism and differentiation of antibiotic producing microorganisms.The application of recombinant DNA techniques will contribute significantly to the improvement of fermentation organisms.     

The thioredoxin system of Penicillium chrysogenum and its possible role in penicillin biosynthesis.

Penicillium chrysogenum is an important producer of penicillin antibiotics. A key step in their biosynthesis is the oxidative cyclization of delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (ACV) to isopenicillin N by the enzyme isopenicillin N synthase (IPNS). bis-ACV, the oxidized disulfide form of ACV is, however, not a substrate for IPNS. We report here the characterization of a broad-range disulfide reductase from P. chrysogenum that efficiently reduces bis-ACV to the thiol monomer. When coupled in vitro with IPNS, it converts bis-ACV to isopenicillin N and may therefore play a role in penicillin biosynthesis. The disulfide reductase consists of two protein components, a 72-kDa NADPH-dependent reductase, containing two identical subunits, and a 12-kDa general disulfide reductant. The latter reduces disulfide bonds in low-molecular-weight compounds and in proteins. The genes coding for the reductase system were cloned and sequenced. Both possess introns. A comparative analysis of their predicted amino acid sequences showed that the 12-kDa protein shares 26 to 60% sequence identity with thioredoxins and that the 36-kDa protein subunit shares 44 to 49% sequence identity with the two known bacterial thioredoxin reductases. In addition, the P. chrysogenum NADPH-dependent reductase is able to accept thioredoxin as a substrate. These results establish that the P. chrysogenum broad-range disulfide reductase is a member of the thioredoxin family of oxidoreductases. This is the first example of the cloning of a eucaryotic thioredoxin reductase gene.     

Characterization of the biosynthesis in vivo of α-aminoadipyl-cysteinyl-valine inPenicillium chrysogenum

Summary The parameters affecting the formation in vivo of -aminoadipyl-cysteinyl-valine (ACV), an intermediate in penicillin biosynthesis, have been established in low- and high-penicillin producing strains ofPenicillium chrysogenum. ACV was found both in cell extracts and in the culture broth filtrates. (¹⁴C)valine, -(¹⁴C)aminoadipic acid and (¹⁴C)cysteine were efficiently incorporated into ACV. Formation of ACV was stimulated by phenylacetic acid when added during the growth of the culture. ACV biosynthesis was enhanced when protein synthesis was blocked with cycloheximide or anisomicin. The ACV-synthesising activity of the culture increased between 24 and 48 h of the culture preceeding penicillin biosynthesis, and remained constant thereafter. A decay of ACV-forming activity was observed when de novo protein synthesis was inhibited with cycloheximide. The apparent half-life of the ACV-synthesising enzyme system was 2.5 h.     

Localization of the pathway of the penicillin biosynthesis in Penicillium chrysogenum.

The localization of the enzymes involved in penicillin biosynthesis in Penicillium chrysogenum hyphae has been studied by immunological detection methods in combination with electron microscopy and cell fractionation. The results suggest a complicated pathway involving different intracellular locations. The enzyme delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase was found to be associated with membranes or small organelles. The next enzyme isopenicillin N-synthetase appeared to be a cytosolic enzyme. The enzyme which is involved in the last step of penicillin biosynthesis, acyltransferase, was located in organelles with a diameter of 200-800 nm. These organelles, most probably, are microbodies. A positive correlation was found between the capacity for penicillin production and the number of organelles per cell when comparing different P. chrysogenum strains.     

Nitrate regulation of alpha-aminoadipate reductase formation and lysine inhibition of its activity in Penicillium chrysogenum and Acremonium chrysogenum

alpha-Aminoadipate reductase (alpha-AAR) is a key enzyme in the branched pathway for lysine and beta-lactam biosynthesis of filamentous fungi since it competes with alpha-aminoadipyl-cysteinyl-valine synthetase for their common substrate L-alpha-aminoadipic acid. The alpha-AAR activity in two penicillin-producing Penicillium chrysogenum strains and two cephalosporin-producing Acremonium chrysogenum strains has been studied. The alpha-AAR activity peaked during the growth-phase preceding the onset of antibiotic production, which coincides with a decrease in alpha-AAR activity, and was lower in high penicillin- or cephalosporin-producing strains. The alpha-AAR required NADPH for enzyme activity and could not use NADH as electron donor for reduction of the alpha-aminoadipate substrate. The alpha-AAR protein of P. chrysogenum was detected by Western blotting using anti-alpha-AAR antibodies. The mechanism of lysine feedback regulation in these two filamentous fungi involves inhibition of the alpha-AAR activity but not repression of its synthesis by lysine. This is different from the situation in yeasts where lysine feedback inhibits and represses alpha-AAR. Nitrate has a strong negative effect on alpha-AAR formation as shown by immunoblotting studies of alpha-AAR. The nitrate effect was reversed by lysine.     

Role of the N-terminal α-helix in biogenesis of α7 nicotinic receptors

We studied the role of the α-helix present at the N-terminus of nicotinic acetylcholine receptor (nAChR) subunits in the expression of functional channels. Deletion of this motif in α7 subunits abolished expression of nAChRs at the membrane of Xenopus oocytes. The same effect was observed upon substitution by homologous motifs of other ligand-gated receptors. When residues from Gln4 to Tyr15 were individually mutated to proline, receptor expression strongly decreased or was totally abolished. Equivalent substitutions to alanine were less harmful, suggesting that proline-induced break of the α-helix is responsible for the low expression. Steady-state levels of wild-type and mutant subunits were similar but the formation of pentameric receptors was impaired in the latter. In addition, those mutants that reached the membrane showed a slightly increased internalization rate. Expression of α7 nAChRs in neuroblastoma cells confirmed that mutant subunits, although stable, were unable to reach the cell membrane. Analogous mutations in heteromeric nAChRs (α3β4 and α4β2) and 5-HT_3A receptors also abolished their expression at the membrane. We conclude that the N-terminal α-helix of nAChRs is an important requirement for receptor assembly and, therefore, for membrane expression.     

Evidence for a compartmentation of penicillin biosynthesis in a high- and a low-producing strain of Penicillium chrysogenum

Pulse-chase experiments using [U14C]valine were done with P2 and Q176, high- and low-penicillin-producing strains of Penicillium chrysogenum. The metabolic flux of this amino acid into protein and penicillin was measured, and compartmentation of penicillin biosynthesis was assessed. Strain P2 took up 14C-valine more slowly than strain Q176, but their rates of incorporation into protein were comparable. Incorporation of 14C-valine into penicillin occurred immediately with the high-producer P2, but exhibited a lag with Q176. After 14C-valine had been removed from the medium, the specific radioactivity of penicillin continued to increase in Q176 but started to decrease immediately in P2. The specific radioactivities of 14C-valine in protein and in penicillin were significantly different in both strains: Q176 had a higher specific radioactivity of valine in penicillin than P2, whereas P2 had a higher specific radioactivity of valine in protein than Q176. Moreover, the specific radioactivity of 14C-valine in penicillin was 20-fold higher in strain Q176 than in P2. These results indicate that penicillin and protein biosynthesis use different pools of cellular valine, and that exchange of valine between the two compartments is slow in the low-producer, but rapid in the high-producer strain. Hence these results indicate a further control point of penicillin biosynthesis in P. chrysogenum.     

α-Ketoglutarate biosynthesis in wild and industrial strains of Lactococcus lactis

Aims:  This study was carried out to explore the ability of wild and industrial strains of Lactococcus lactis to produce α-ketoglutarate (α-KG), which is essential during the conversion of amino acids to flavour compounds.
Methods and Results:  Two pathways in α-KG biosynthesis were explored in strains of L. lactis isolated from dairy products, vegetables and commercial dairy starter cultures. Half of the strains efficiently converted glutamine to glutamate (Glu) and grew in Glu-free medium. Strains did not present isocitrate dehydrogenase and aconitase activities. However, half of the strains presented glutamate dehydrogenase (GDH) activity.
Conclusions:  The ability of L. lactis to synthesize either α-KG or Glu via GDH was confirmed. However, L. lactis strains were not able to biosynthesize α-KG by the citrate–isocitrate pathway. NADP-GDH activity was mainly found in strains isolated from vegetables, whereas NAD-GDH activity was mainly found in strains isolated from dairy products.
Significance and Importance of the Study:  The origin of isolation highly influenced NAD or NADP-GDH activities. These enzymatic activities may be correlated to the flavour production capacity of the different strains.     

α4 but Not α3 and α7 Nicotinic Acetylcholine Receptor Subunits Are Lost from the Temporal Cortex in Alzheimer's Disease

Neuronal nicotinic acetylcholine receptors labelled with tritiated agonists are reduced in the cerebral cortex in Alzheimer's disease (AD), but to date it has not been demonstrated which nicotinic receptor subunits contribute to this deficit. In the present study, autopsy tissue from the temporal cortex of 14 AD cases and 15 age-matched control subjects was compared using immunoblotting with antibodies against recombinant peptides specific for alpha3, alpha4, and alpha7 subunits, in conjunction with [3H]epibatidine binding. Antibodies to alpha3, alpha4, and alpha7 produced one major band on western blots at 59, 51, and 57 kDa, respectively. [3H]Epibatidine binding and alpha4-like immunoreactivity (using antibodies against the extracellular domain and cytoplasmic loop of the alpha4 subunit) were reduced in AD cases compared with control subjects (p < 0.02) and with a subgroup of control subjects (n = 9) who did not smoke prior to death (p < 0.05) for the former two parameters. [3H]Epibatidine binding and cytoplasmic alpha4-like immunoreactivity were significantly elevated in a subgroup of control subjects (n = 4) known to have smoked prior to death (p < 0.05). There were no significant changes in alpha3- or alpha7-like immunoreactivity associated with AD or tobacco use. The selective involvement of alpha4 has implications for understanding the role of nicotinic receptors in AD and potential therapeutic targets.     

Inhibition and repression of homocitrate synthase by lysine in Penicillium chrysogenum

Homocitrate synthase in the first enzyme of the lysine biosynthetic pathway. It is feedback regulated by L-lysine. Lysine decreases the biosynthesis of penicillin (determined by the incorporation of [14C]valine into penicillin) by inhibiting and repressing homocitrate synthase, thereby depriving the cell of alpha-aminoadipic acid, a precursor of penicillin. Lysine feedback inhibited in vivo the biosynthesis and excretion of homocitrate by a lysine auxotroph, L2, blocked in the lysine pathway after homocitrate. Neither penicillin nor 6-aminopenicillanic acid exerted any effect at the homocitrate synthase level. The molecular mechanism of lysine feedback regulation in Penicillium chrysogenum involved both inhibition of homocitrate synthase activity and repression of its synthesis. In vitro studies indicated that L-lysine feedback inhibits and represses homocitrate synthase both in low- and high-penicillin-producing strains. Inhibition of homocitrate synthase activity by lysine was observed in cells in which protein synthesis was arrested with cycloheximide. Maximum homocitrate synthase activity in cultures of P. chrysogenum AS-P-78 was found at 48 h, coinciding with the phase of high rate of penicillin biosynthesis.     

delta-(L-alpha-Aminoadipyl)-L-cysteinyl-D-valine synthetase,that mediates the first committed step in penicillin biosynthesis,is a cytosolic enzyme

Penicillin biosynthesis by Penicillium chrysogenum is a compartmentalized process. The first catalytic step is mediated by delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase (ACV synthetase), a high molecular mass enzyme that condenses the amino acids L-alpha-aminoadipate, L-cysteine, and L-valine into the tripeptide ACV. ACV synthetase has previously been localized to the vacuole where it is thought to utilize amino acids from the vacuolar pools. We localized ACV synthetase by subcellular fractionation and immuno-electron microscopy under conditions that prevented proteolysis and found it to co-localize with isopenicillin N synthetase in the cytosol, while acyltransferase localizes in microbodies. These data imply that the key enzymatic steps in penicillin biosynthesis are confined to only two compartments, i.e., the cytosol and microbody.