Methionine induction of ACV synthetase inCephalosporium acremonium

Nitrogen source as a modulator of the metabolic activity of Pedobacter lusitanus NL19: a transcriptomic approach

Secondary metabolites (SMs) are compounds with relevant biological activities. Their production under laboratory conditions, especially in broth, is still challenging. An example is the pedopeptins, which are nonribosomal peptides active against some bacteria listed by the WHO for which new antibiotics are urgently needed. Their biosynthesis is inhibited by high concentrations of peptone from casein (PC) in tryptic soy broth (TSB), and we applied a RNA-seq approach to identify Pedobacter lusitanus NL19 cellular pathways modulated by this condition. Results were validated by qPCR and revealed 261 differentially expressed genes (DEGs), 46.3% of them with a predicted biological function. Specifically, high concentration of PC significantly repressed the de novo biosynthesis of biotin (- 60X) and the production of nonribosomal peptide synthetases (NRPS) of pedopeptins (about - 14X), but no effect was observed on the expression of other NRPS. Transcription of a L-Dap synthesis operon that includes a protein with a σ⁷⁰-like domain was also reduced (about - 7X). High concentrations of PC led to a significant overexpression of MFS and RND efflux pumps and a ferrous iron uptake system, suggesting the redirection of cell machinery to export compounds such as amino acids, sugars and metal divalent cations, alongside with a slight increase of iron import. KEY POINTS: • Higher concentrations of phosphate sources highly repress many operons • High concentrations of peptone from casein (PC) cause biotin's operon repression • High concentrations of PC downregulate the production of peptides of unknown function.

Involvement of nitrogen-containing compounds in beta-lactam biosynthesis and its control

Biosynthesis of beta-lactam antibiotics by fungi and actinomycetes is markedly affected by compounds containing nitrogen. The different processes employed by the spectrum of microbes capable of making these valuable compounds are affected differently by particular compounds. Ammonium ions, except at very low concentrations, exert negative effects via nitrogen metabolite repression, sometimes involving the nitrogen regulatory gene nre. Certain amino acids are precursors or inducers, whereas others are involved in repression and, in certain cases, as inhibitors of biosynthetic enzymes and of enzymes supplying precursors. The most important amino acids from the viewpoint of regulation are lysine, methionine, glutamate and valine. Surprisingly, diamines such as diaminopropane, putrescine and cadaverine induce cephamycin production by actinomycetes. In addition to penicillins and cephalosporins made by fungi and cephamycins made by actinomycetes, other beta-lactams are made by actinomycetes and unicellular bacteria. These include clavams (e.g., clavulanic acid), carbapenems (e.g., thienamycin), nocardicins and monobactams. Here also, amino acids are precursors and inhibitors, but only little is known about regulation. In the case of the simplest carbapenem made by unicellular bacteria, i.e., 1-carba-2-em-3-carboxylic acid, quorum sensors containing homoserine lactone are inducers.

Unraveling the methionine-cephalosporin puzzle in Acremonium chrysogenum

Methionine has long been known as the major stimulant of the formation of cephalosporin C in Acremonium chrysogenum. Enzymatic and genetic studies of methionine have revealed that it induces four of the enzymes of cephalosporin-C biosynthesis at the level of transcription. It is also converted to cysteine, one of three precursors of cephalosporin C, by cystathionine-gamma-lyase. The main effect of methionine on cephalosporin production results from its regulatory role, which can be duplicated by the non-sulfur analog norleucine. Eliminating cystathionine-gamma-lyase prevents the enhancing precursor effect of methionine on cephalosporin-C production, and cystathionine-gamma-lyase overproduction in moderate doses increases cephalosporin-C formation.

Targeted inactivation of the mecB gene, encoding cystathionine-gamma-lyase, shows that the reverse transsulfuration pathway is required for high-level cephalosporin biosynthesis in Acremonium chrysogenum C10 but not for methionine induction of the cephalosporin genes

Targeted gene disruption efficiency in Acremonium chrysogenum was increased 10-fold by applying the double-marker enrichment technique to this filamentous fungus. Disruption of the mecB gene by the double-marker technique was achieved in 5% of the transformants screened. Mutants T6 and T24, obtained by gene replacement, showed an inactive mecB gene by Southern blot analysis and no cystathionine-gamma-lyase activity. These mutants exhibited lower cephalosporin production than that of the control strain, A. chrysogenum C10, in MDFA medium supplemented with methionine. However, there was no difference in cephalosporin production between parental strain A. chrysogenum C10 and the mutants T6 and T24 in Shen's defined fermentation medium (MDFA) without methionine. These results indicate that the supply of cysteine through the transsulfuration pathway is required for high-level cephalosporin biosynthesis but not for low-level production of this antibiotic in methionine-unsupplemented medium. Therefore, cysteine for cephalosporin biosynthesis in A. chrysogenum derives from the autotrophic (SH(2)) and the reverse transsulfuration pathways. Levels of methionine induction of the cephalosporin biosynthesis gene pcbC were identical in the parental strain and the mecB mutants, indicating that the induction effect is not mediated by cystathionine-gamma-lyase.

Cephalosporin C production by Cephalosporium acremonium: the methionine story

More than 40 years ago, it was reported that methionine markedly stimulated production of cephalosporin C by Cephalosporium acremonium. Over the years, many hypotheses were put forth to explain this phenomenon. The accumulating evidence strongly supported the concept that methionine stimulates by inducing enzymes of the biosynthetic pathway such as delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase, isopenicillin N synthase, and deacetoxycephalosporin C synthase. This mechanism has been strengthened by the finding that transcription of the genes encoding the above enzymes is markedly enhanced by growth with methionine. An effect of methionine in the fermentation unrelated to the titer stimulation is its contribution of the sulfur atom to the cephalosporin molecule. Methionine also stimulates mycelial fragmentation; the relationship between this effect on hyphal differentiation and the induction of the cephalosporin synthases remains to be elucidated.

Exogenous methionine increases levels of mRNAs transcribed from pcbAB, pcbC, and cefEF genes, encoding enzymes of the cephalosporin biosynthetic pathway, in Acremonium chrysogenum

Methionine stimulated cephalosporin production in cultures of three different strains of Acremonium chrysogenum when added either at inoculation time or at 72 h to cells grown previously in the absence of methionine. When methionine was added at 72 h, the stimulation of cephalosporin biosynthesis was observed only 12 h later and required de novo protein synthesis. Methionine increased the levels of enzymes (isopenicillin N synthase and deacetylcephalosporin C acetyltransferase) expressed from genes (pcbC and cefG, respectively) located in the two clusters of cephalosporin biosynthesis genes in the wild-type A. chrysogenum strain and also in the two improved strains, CW19 and C10. Methionine-supplemented cells showed higher levels of transcripts of the four known genes (pcbAB, pcbC, cefEF and, to a slight extent, cefG) of the cephalosporin biosynthetic pathway than cells grown in the absence of methionine. The levels of the cefG transcript were much lower than those of the pcbAB, pcbC, and cefEF transcripts. The induction by methionine of transcription of the four cephalosporin biosynthesis genes and the known effect of this amino acid on the differentiation of A. chrysogenum indicate that methionine exerts a pleiotropic effect that coordinately regulates cephalosporin biosynthesis and differentiation.

Expression of genes and processing of enzymes for the biosynthesis of penicillins and cephalosporins

The genes pcbAB, pcbC and penDE encoding the enzymes (alpha-aminoadipyl-cysteinyl-valine synthetase, isopenicillin N synthase and isopenicillin N acyltransferase, respectively) involved in the biosynthesis of penicillin have been cloned from Penicillin chrysogenum and Aspergillus nidulans. They are clustered in chromosome I (10.4 Mb) of P. chrysogenum, in chromosome II of Penicillium notatum (9.6 Mb) and in chromosome VI (3.0 Mb) of A. nidulans. Each gene is expressed as a single transcript from separate promoters. Enzyme regulation studies and gene expression analysis have provided useful information to understand the control of genes involved in penicillin biosynthesis. The enzyme isopenicillin N acyltransferase encoded by the penDE gene is synthesized as a 40 kDa protein that is (self)processed into two subunits of 29 and 11 kDa. Both subunits appear to be required for acyl-CoA 6-APA acyltransferase activity. The isopenicillin N acyltransferase was shown to be located in microbodies, whereas the isopenicillin N synthase has been reported to be present in vesicles of the Golgi body and in the cell wall. A mutant in the carboxyl-terminal region of the isopenicillin N acyltransferase lacking the three final amino acids of the enzymes was not properly located in the microbodies and failed to synthesize penicillin in vivo. In C. acremonium the genes involved in cephalosporin biosynthesis are separated in at least two clusters. Cluster I (pcbAB-pcbC) encodes the first two enzymes (alpha-aminoadipyl-cysteinyl) valine synthetase and isopenicillin N synthase) of the cephalosporin pathway which are very similar to those involved in penicillin biosynthesis. Cluster II (cefEF-cefG), encodes the last three enzymatic activities (deacetoxycephalosporin C synthetase/hydroxylase and deacetylcephalosporin C acetyltransferase) of the cephalosporin pathway. It is unknown, at this time, if the cefD gene encoding isopenicillin epimerase is linked to any of these two clusters. Methionine stimulates cephalosporin biosynthesis in cultures of three different strains of A. chrysogenum. Methionine increases the levels of enzymes (isopenicillin N synthase and deacetylcephalosporin C acetyltransferase) expressed from genes (pcbC and cefG respectively) which are separated in the two different clusters of cephalosporin biosynthesis genes. This result suggests that both clusters of genes have regulatory elements which are activated by methionine. Methionine-supplemented cells showed higher levels of transcripts of the pcbAB, pcbC, cefEF genes and to a lesser extent of cefG than cells grown in absence of methionine. The levels of the cefG transcript were very low as compared to those of pcbAB, pcbC and cefEF.(ABSTRACT TRUNCATED AT 400 WORDS)

Low-molecular-weight thiols in streptomycetes and their potential role as antioxidants

The intracellular low-molecular-weight thiols present in five gram-positive Streptomyces species and one Flavobacterium species were analyzed by high-performance liquid chromatography after fluorescence labeling with monobromobimane. Bacteria were chosen to include penicillin and cephalosporin beta-lactam producers and nonproducers. No significant amount of glutathione was found in any of the streptomycetes. Major intracellular thiols in all strains examined were cysteine, coenzyme A, sulfide, thiosulfate, and an unknown thiol designated U17. Those streptomycetes that make beta-lactam antibiotics also produce significant amounts of delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (ACV), a key intermediate in their biosynthesis. In Streptomyces clavuligerus, a potent producer of beta-lactams, the level of ACV was low during the early phase of growth and increased rapidly toward the end of exponential growth, paralleling that of antibiotic production. These and other observations indicate that ACV does not function as a protective thiol in streptomycetes. U17 may have this role since it was the major thiol in all streptomycetes and appeared to occur at levels about 10-fold higher than those of the other thiols measured, including ACV. Purification and amino acid analysis of U17 indicated that it contains cysteine and an unusual amine that is not one of the common amino acids. This thiol is identical to an unknown thiol found previously in Micrococcus roseus and Streptomyces griseus. A high level of ergothioneine was found in Streptomyces lactamdurans, and several unidentified thiols were detected in this and other streptomycetes.

Production kinetics and stability properties of delta(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase from Streptomyces clavuligerus

Delta-(L-alpha-Aminoadipyl)-L-cysteinyl-D-valine (ACV)-synthetase is a key enzyme that channels primary metabolites to a tripeptide common to cephalosporin and cephamycin biosynthesis in Streptomyces clavuligerus. Time-course studies indicated that the S. clavuligerus ACV-synthetase was stable during the cephamycin C fermentation: the enzyme was produced early in the growth phase and its activity remained high up to 96 h of growth. The detection of crude ACV-synthetase activity in older cultures was best achieved with an assay medium supplemented with 5 mM phosphoenolpyruvate, at lower ATP concentrations. During storage at 4 degrees C, a progressive decrease in the stability of crude ACV-synthetase was observed with increasing culture age. Although a proteinolytic activity with a pH optimum at 8.2 was detected in crude cell-free extracts, no significant variation was observed in its activity with increasing culture age to account for the instability of ACV-synthetase in vitro. Addition of proteinase inhibitors did not improve the stability of the enzyme. However, a stabilization cocktail containing dithiothreitol, MgCl2, the three substrate amino acids, and glycerol increased the stability of the enzyme isolated from cultures grown for 30-40 h, which was shortly after the appearance of antibiotics in the culture fluid. This stabilized enzyme retained half of its initial activity after 6 days at 4 degrees C.

Regulation of ACV synthetase in penicillin- and cephalosporin-producing microorganisms

ACV synthetase is the first enzyme in the biosynthetic pathway for all natural penicillins and cephalosporins. Its activity catalyzes the possible rate-limiting step and is subject to various regulatory controls. In both the fungus Cephalosporium acremonium and the actinomycete Streptomyces clavuligerus, formation of the enzyme is repressed by ammonium and phosphate ions, but not by easily-utilized carbon sources; it is induced by methionine in C. acremonium. The action of the crude enzyme is indirectly inhibited in vitro by sugars such as glucose and by the carbon source metabolite glyceraldehyde-3-phosphate (G3P). Sugars are not inhibitory to the purified enzyme activity but G3P is inhibitory. The sugar inhibition is reversed by ATP and the G3P inhibition by L-cysteine (L-cys). Addition of L-cys to fermentation media increases beta-lactam production by both microorganisms. Phosphate and ferrous ions inhibit enzyme activity. Dissolved oxygen levels do not affect enzyme formation. Regulation of ACVS formation most likely occurs at the transcriptional level.

Biosynthesis and control of β-lactam antibiotics: The early steps in the “classical” tripeptide pathway

The interaction between growth and secondary metabolism develops from physiological responses of the producer organism to its environment. Nutrients are channelled into primary growth processes or into secondary processes such as antibiotic biosynthesis by a variety of metabolic controls, the nature of which has been extensively studied in organisms producing beta-lactam antibiotics via the tripeptide, delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine. In the following article we review the early stages of beta-lactam biosynthesis in fungi and actinomycetes, keeping in mind the regulation of primary pathways that provide the amino acid precursors of this group of antibiotics, as well as the regulation of the secondary pathway itself. Of special importance to organisms engaging in secondary metabolism are the control mechanisms that suppress the nonessential process during rapid growth but allow secondary metabolic genes to be expressed and resources to be diverted when environmental factors generate the appropriate biochemical signals.

Measurement of delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase activity in Streptomyces clavuligerus by high-performance liquid chromatography after precolumn derivatization with o-phthaldialdehyde

A procedure for rapid, sensitive measurement of delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (ACV) in complex cell extracts has been developed by adapting a widely used method for high-performance liquid chromatography analysis of amino acids. Samples were oxidized with performic acid and derivatized with o-phthaldialdehyde-mercaptoethanol to give the fluorescent isoindole derivative of the peptide sulfonate. The procedure was used to assay Streptomyces clavuligerus cell extracts and partially purified fractions for ACV synthetase activity and to determine some characteristics of the enzyme reaction. The presence of a second enzyme with cysteine as a substrate was also indicated.