Bioprocess monitoring of dissolved oxygen using a computerized pulsing membrane electrode

A membrane oxygen electrode usually suffers from long-term signal deterioration due to environmental factors such as changes in hydrodynamic conditions and alteration of membrane oxygen diffusivity due to fouling. These problems can theoretically be overcome by the use of the same oxygen electrode in a pulsing mode. The effects of stirring rate, viscosity of the culture media, and addition of antifoam agents on the direct reading of this pulsing oxygen electrode were investigated, and the results were compared to the traditional pseudo-steady-state operation of the electrode. With a pulsing period controlled at 1 s and a rest period of longer than 5 min, the transient signal obtained can be quite stable, and it showed minimal interference from various environmental changes. Dissolved oxygen tension of a cephalosporin C fermentation was monitored using both this pulsing and the conventional pseudo-steady-state methods, and their readings were compared to those measured from an off-line newly calibrated oxygen electrode. The reading from the pulsed electrode showed more favorable agreement with that of the off-line measurement. The error encountered in using the conventional method of dissolved oxygen measurement could vary as much as 40% in comparison with the actual dissolved oxygen tension during a fermentation process.

Cloning, characterization, and use in strain improvement of the Cephalosporium acremonium gene cefG encoding acetyl transferase

A long open reading frame (ORF) closely linked to the Cephalosporium acremonium gene cefEF was identified by DNA sequencing. The cefEF gene encodes the enzyme involved in cephalosporin C (CPC) biosynthesis known as expandase/hydroxylase. Complementation of a C. acremonium cefG mutant, as well as expression of the gene in Aspergillus niger, showed this ORF to be the cefG gene, encoding cephalosporin C acetyltransferase, which catalyzes the last step in CPC biosynthesis. Analysis of transformants containing additional copies of this gene showed that a direct relationship exists between cefG copy number, cefG message levels, and CPC titers. This gene encodes an enzyme for what may be a rate-limiting step in CPC production.

Identification of rate-limiting steps in cephalosporin C biosynthesis in Cephalosporium acremonium: a theoretical analysis

A kinetic model describing the biosynthesis of cephalosporin C in Cephalosporium acremonium has been developed to identify the rate-limiting step(s). Using this model and in-vitro kinetic data of the biosynthetic enzymes, the production kinetics of cephalosporin C were examined theoretically. The predicted time profile of the specific production rate during batch culture is in good agreement with that of experimental results published previously. Sensitivity analysis indicates that delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (ACV) synthetase is the rate-limiting enzyme. Our analysis also predicts that increasing ACV synthetase enhances the production rate initially until expandase/hydroxylase becomes rate-limiting. Furthermore, increasing expandase/hydroxylase reduces the accumulation of penicillin N, and thus, enhances the production of cephalosporin C. Based on our analysis, amplifying both ACV synthetase and expandase/hydroxylase concurrently should enhance the production rate to a great extent.

Penicillin and cephalosporin biosynthetic genes: structure, organization, regulation, and evolution

Penicillins and cephalosporins are produced by a wide variety of microorganisms, including some filamentous fungi, many gram-positive streptomycetes, and a few gram-negative unicellular bacteria. All produce these beta-lactam antibiotics by essentially the same biosynthetic pathway. Recently, most of the penicillin and cephalosporin biosynthetic genes have been cloned, sequenced, and expressed. The biosynthetic genes code for enzymes that possess multifunctional peptide synthetase, cyclase, epimerase, expandase, hydroxylase, lysine aminotransferase, and acetyltransferase activities and are organized in chromosomal gene clusters and coordinately expressed. DNA hybridization screens of streptomycetes demonstrate that beta-lactam biosynthetic genes may be more widespread in nature than is indicated by conventional antibiotic screens. They offer the possibility of expanding the search for organisms with potential to make new beta-lactam antibiotics. Attempts to improve current yields of beta-lactams in production strains by introducing into them additional copies of biosynthetic genes have been partially successful. Comparative sequence analysis of bacterial and fungal beta-lactam biosynthetic genes show they share very high sequence identity. A model that explains the similarity of biosynthetic genes from an evolutionary standpoint assumes horizontal gene-transfer between the two groups of organisms. Indirect evidence suggests the transfer occurred from the bacteria to the fungi.

Influence of dissolved oxygen concentration on the biosynthesis of cephalosporin C

Cephalosporin C was produced by a highly productive strain of Cephalosporium acremonium under industrial production conditions by fed-batch cultivation in a 40-l stirred-tank reactor using a complex medium containing 50 g l-1 peanut flour. The influence of dissolved oxygen concentration (pO2, DOC), which was maintained at different constant levels between 5 and 40% of its saturation value, during the production phase by means of a parameter-adaptive pO2-controller, on the cephalosporin C biosynthesis, was investigated. The concentrations of cephalosporin C (CPC) and its precursors penicillin N (PEN N), deacetoxycephalosporin C (DAOC), and deacetylcephalosporin C (DAC) were monitored by on-line HPLC. The concentrations of amino acids, valine (VAL), cysteine (CYS), alpha-amino-adipic acid (alpha-AAA), the dipeptide alpha-amino-adipyl-cysteine (AC), and the tripeptide alpha-amino-adipyl-cysteinyl-valine (ACV) were determined by off-line HPLC. By reducing the pO2 in the production phase from 40 to 5% of its saturation value, the CPC concentration diminished from 7.2 to 1.1 g l-1 and the PEN N concentration increased from 2.57 to 7.65 g l-1. The DAC concentration also dropped from 3.13 to 0.42 g l-1; however, the DAOC concentration was less influenced. The concentrations of AC and ACV were also less affected. The small DOC did not lead to an accumulation of the intermediate AC and ACV during the production phase. With increasing DOC in the range of 5-20%, the maximal specific production rate, the cell mass concentration-based and the substrate-based yield coefficients for CPC increased almost linearly, and fell back for PEN N.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of deacetoxycephalosporin C from fermentation broths of Penicillium chrysogenum transformants: construction of a new fungal biosynthetic pathway

Deacetoxycephalosporin C (DAOC), a precursor of cephalosporins excreted by Cephalosporium and Streptomyces species, has been produced in Penicillium chrysogenum transformed with DNA containing a hybrid penicillin N expandase gene (cefEh) and a hybrid isopenicillin N epimerase gene (cefDh). DAOC from a P. chrysogenum transformant was identified by ultraviolet light (UV), high performance liquid chromatography (HPLC), nuclear magnetic resonance (NMR) and mass spectrum analyses. P. chrysogenum transformed with DNA containing cefEh without cefDh did not produce DAOC. Untransformed P. chrysogenum produced penicillin V (phenoxymethylpenicillin) but not DAOC. Transformants also produced penicillin V but, in general, less than untransformed P. chrysogenum. The cefEh and cefDh genes were constructed by replacing the open reading frame (ORF) of cloned P. chrysogenum pcbC and penDE genes with the ORF of the Streptomyces clavuligerus expandase gene, cefE, and the ORF of the Streptomyces lipmanii epimerase gene, cefD, respectively. Analyses of representative transformants suggested that production of DAOC occurred via cefEh and cefDh genes stably integrated in the P. chrysogenum genome. DNA from untransformed P. chrysogenum did not hybridize to cefE or cefD gene probes.

Determination of the rate-limiting step(s) in the biosynthetic pathways leading to penicillin and cephalosporin

This paper is a review of strategies that have been used, or that could be used, to determine the rate-limiting step(s) in the biosynthetic pathways leading to penicillin or cephalosporin. Information is summarized from published material that involves studies with low-producing strains of Penicillium chrysogenum and Cephalosporium acremonium. We also summarize information derived from some high-producing production strains. Identification of the rate-limiting step(s) was of great interest to us as the first step in a rational program to further improve antibiotic titers of these highly developed strains. A number of approaches that could be used to elucidate the rate-limiting step(s) are described herein.

Influence of medium composition on the cephalosporin C production with a highly productive strain Cephalosporium acremonium

Cephalosporin production by a highly productive Cephalosporium acremonium strain was carried out and optimized by fed-batch operation in a 40 l stirred tank reactor using a complex medium containing 30-120 g l-1 peanut flour. The concentrations of cephalosporin C (CPC) and its precursors: penicillin N (PEN N), deacetoxy cephalosporin C (DAOC), and deacetyl cephalosporin C (DAC) were monitored with an on-line HPLC. The concentrations of amino acids valine (VAL), cysteine (CYS), alpha-amino adipic acid (alpha-AAA), the dipeptide alpha-amino-adipyl-cysteine (AC), and the tripeptide alpha-amino-adipyl-cysteinyl-valine (ACV), were determined off-line by HPLC. The RNA content and dry weight of the sediment as well as the oxygen transfer rate (OTR) and the CO2 production rate (CPR) were used to calculate the cell mass concentration (X). The influences of peanut flour (PF) and the on-line monitored and controlled medium components: glucose (GLU), phosphate, methionine (MET) as well as the dissolved oxygen (DOC) on the cell growth, the product formation, and the pathway of cephalosporin C biosynthesis were investigated and evaluated. When the glucose fed-batch cycle was optimized and oxygen transfer limitation was avoided (DOC greater than 20% of the saturation value), high process performance (103.5 g l-1 X, 11.84 g l-1 CPC, a maximum CPC productivity of 118 mg l-1 h-1, and the whole concentration of the beta-lactam antibiotics CPC, DAC, DAOC, PEN N 17.34 g l-1) was achieved by using 100 g l-1 PF in the medium with the optimum concentration of phosphate (260-270 mg l-1) and a low glucose concentration (less than 0.5 g l-1). The cultivations with different medium concentrations demonstrated that the product formation was directly proportional to the cell mass concentration. On the average, the cell mass-based yield coefficient of CPC: YCPC/X amounted to 0.115 g CPC per g cell mass.

The cefG gene of Cephalosporium acremonium is linked to the cefEF gene and encodes a deacetylcephalosporin C acetyltransferase closely related to homoserine O-acetyltransferase

The gene (cefG) encoding the acetyl coenzyme A:deacetylcephalosporin C acetyltransferase of Cephalosporium acremonium (synonym Acremonium chrysogenum) C10 has been cloned. It contains two introns and encodes a protein of 444 amino acids with an M(r) of 49,269 that correlates well with the M(r) deduced by gel filtration. The cefG gene is linked to the cefEF gene (encoding the bifunctional deacetoxycephalosporin C synthase/hydroxylase), but it is expressed in an orientation opposite that of the cefEF gene. Two transcripts of 1.2 and 1.4 kb were found in C. acremonium that correspond to the cefEF and cefG genes, respectively; the degree of expression of the cefG gene was clearly lower than that of the cefEF gene in 48-h cultures. The cloned cefG complemented the deficiency of deacetylcephalosporin acetyltransferase in the nonproducer mutant C. acremonium ATCC 20371 and restored cephalosporin biosynthesis in this strain. Heterologous expression of the cefG genes took place in Penicillium chrysogenum. The deacetylcephalosporin acetyltransferase showed a much higher degree of homology with the O-acetylhomoserine acetyltransferases of Saccharomyces cerevisiae and Ascobolus immersus than with other O-acetyltransferases. The cefEF-cefG cluster of genes encodes the enzymes that carry out the three late steps of the cephalosporin biosynthetic pathway and is not linked to the pcbAB-pcbC gene cluster that encodes the first two steps of the pathway.

ACV synthetase

ACV synthetase (ACVS) is the first enzyme and plays a key role in the biosynthesis of all natural penicillins and cephalosporins. The enzyme is extremely unstable and little had been known about it until recently. This article summarizes the progress in research on this enzyme, including the establishment of a cell-free assay system, stabilization, purification, characterization, and gene cloning. A possible reaction sequence for ACVS catalysis is suggested.

Production of beta-lactam antibiotics and its regulation

The discovery of penicillin was announced over 60 years ago. It was the first beta-lactam antibiotic and the importance of this group is greater today than it has ever been. It is clear that even at 60 years of age, beta-lactams are going strong and no one contemplates their early retirement. Currently, sales of beta-lactam compounds form the largest share by far of the world's antibiotic market. The beta-lactam antibiotics include penicillins such as penicillin G, penicillin V, ampicillin, cloxacillin, and piperacillin; cephalosporins such as cephalothin, cephaloridine, cephalexin, and cefaclor; and cephamycins such as cefoxitin. In addition, beta-lactam antibiotics include the more recently developed nonclassical structures such as monobactams, including aztreonam; clavulanic acid, which is a component of the combination drug augmentin; and thienamycin, which is chemically transformed into imipenem, a component of the combination drug known as primaxin (or tienam). The classical beta-lactam antibiotics can be divided into hydrophobic and hydrophilic fermentation products. The hydrophobic members, e.g. benzylpenicillin (penicillin G) and phenoxymethylpenicillin (penicillin V), contain non-polar side chains, e.g. phenylacetate and phenoxyacetate, respectively, and are made only by filamentous fungi; the best known of these is Penicillium chrysogenum. The antibacterial spectrum of the hydrophobic penicillins is essentially Gram-positive. The hydrophilic types are penicillin N, cephalosporins and 7-alpha-methoxycephalosporins (cephamycins) which are made by fungi, actinomycetes and unicellular bacteria. They all contain the polar side chain, D-alpha-aminoadipate. We can draw a sequence of reactions which describes the biosynthesis of all penicillins and cephalosporins, however the total sequence exists in no one microorganism. All penicillin and cephalosporin biosynthetic pathways possess the first three steps in common and all cephalosporin pathways go through deacetylcephalosporin C. However, there are many subsequent biosynthetic reactions which vary in the different producing organisms. Production of beta-lactam antibiotics occurs best under conditions of nutrient imbalance and at low growth rates. Nutrient imbalance can be brought about by limitation of the carbon, nitrogen or phosphorus source. In addition to these factors, amino acids such as lysine and methionine exert marked effects on production of penicillins and/or cephalosporins by some microorganisms. Induction of some of the synthetases, especially the first enzyme, ACV synthetase, by methionine is the basis of the methionine stimulation of cephalosporin C synthesis in C. acremonium. Inhibition of homocitrate synthase is the mechanism involved in lysine inhibition of penicillin synthesis in Penicillium chrysogenum.(ABSTRACT TRUNCATED AT 400 WORDS)

Stringent response and initiation of secondary metabolism in Streptomyces clavuligerus

Cephalosporin biosynthetic activity and extracellular protease production increased during growth of Streptomyces clavuligerus in defined medium, while the level of guanosine 5'-diphosphate 3'-diphosphate (ppGpp) remained very low and stable. Cephalosporin biosynthesis (measured in resting cell systems) was initiated during early exponential growth in complex media, without appreciable change in the small ppGpp pool. Nutritional shift-down induced by withdrawal of Casamino acids caused a transient increase in ppGpp and a reduction of RNA accumulation. The increase in ppGpp was small in very young cultures, but increased as the culture aged. Twenty-seven spontaneous thiostrepton-resistant mutants were isolated and partially characterized. Most of them had a reduced ppGpp-forming ability and gave normal titres of cephalosporin. However, in complex medium, some mutants did not produce cephalosporins or extracellular protease, whereas others overproduced cephalosporins. The results indicate that, in S. clavuligerus, there is no obligatory relationship between the initiation of secondary metabolism and the stringent response.

Genes directly involved in the biosynthesis of beta-lactam antibiotics

Several genes encoding enzymatic activities involved in penicillin and cephalosporin biosynthesis have been identified. The first two steps in the biosynthesis of both antibiotics are common in penicillin, cephalosporin and cephamycin producers: condensation of the three precursor amino acids to form the tripeptide delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine, and oxidative cyclization of the tripeptide to form isopenicillin N. The genes involved in the two steps are pcbAB and pcbC respectively. The conversion of isopenicillin N to penicillin G is carried out by the enzyme isopenicillin N:6-APA acyltransferase encoded by the gene penDE. The biosynthesis of cephalosporin diverges from that of penicillin G at the isopenicillin N level. The isopenicillin N is first isomerized to penicillin N by an epimerase that is encoded by the gene cefD. The penicillin N is converted in deacetoxycephalosporin C by an expansion of the five-membered thiazolidine ring to the six-membered dihydrothiazine ring. The deacetoxycephalosporin C is finally converted into cephalosporin C by a hydroxylation and O-acetylation. The enzymes which catalyze these last three steps are encoded by the genes cefE, cefF and cefG. The penicillin, cephalosporin and cephamycin biosynthetic genes are organized in clusters (and subclusters) of genes.

Studies on cephalosporin antibiotics. III. Synthesis, antibacterial activity and oral absorption of new 3-(substituted-alkylthio)-7 beta-[(Z)-2-(2-aminothiazol-4-yl)-2- (carboxymethoxyimino)acetamido]cephalosporins

The synthesis, antibacterial activity and oral absorption in rats of new 7 beta-[(Z)-2-(2-aminothiazol-4-yl)-2-(carboxymethoxyimino)ace tamido] cephalosporins (1) having various substituted-alkylthio groups at the C-3 position of the cephem nucleus are described. Of these, the cephalosporins with a cyanomethylthio group (1d) and fluoroethylthio group (1p) at the C-3 position showed a potent in vitro antibacterial activity against Gram-positive and Gram-negative bacteria as well as good oral absorption in rats. When administered orally to mice infected with Klebsiella pneumoniae, 1d had stronger protective effect than 1p. The structure-activity relationships of 1 are also presented.

Characterization of the Cephalosporium acremonium pcbAB gene encoding alpha-aminoadipyl-cysteinyl-valine synthetase, a large multidomain peptide synthetase: linkage to the pcbC gene as a cluster of early cephalosporin biosynthetic genes and evidence of multiple functional domains

A 24-kb region of Cephalosporium acremonium C10 DNA was cloned by hybridization with the pcbAB and pcbC genes of Penicillium chrysogenum. A 3.2-kb BamHI fragment of this region complemented the mutation in the structural pcbC gene of the C. acremonium N2 mutant, resulting in cephalosporin production. A functional alpha-aminoadipyl-cysteinyl-valine (ACV) synthetase was encoded by a 15.6-kb EcoRI-BamHI DNA fragment, as shown by complementation of an ACV synthetase-deficient mutant of P. chrysogenum. Two transcripts of 1.15 and 11.4 kb were found by Northern (RNA blot) hybridization with probes internal to the pcbC and pcbAB genes, respectively. An open reading frame of 11,136 bp was located upstream of the pcbC gene that matched the 11.4-kb transcript initiation and termination regions. It encoded a protein of 3,712 amino acids with a deduced Mr of 414,791. The nucleotide sequence of the gene showed 62.9% similarity to the pcbAB gene encoding the ACV synthetase of P. chrysogenum; 54.9% of the amino acids were identical in both ACV synthetases. Three highly repetitive regions occur in the deduced amino acid sequence of C. acremonium ACV synthetase. Each is similar to the three repetitive domains in the deduced sequence of P. chrysogenum ACV synthetase and also to the amino acid sequence of gramicidin synthetase I and tyrocidine synthetase I of Bacillus brevis. These regions probably correspond to amino acid activating domains in the ACV synthetase protein. In addition, a thioesterase domain was present in the ACV synthetases of both fungi. A similarity has been found between the domains existing in multienzyme nonribosomal peptide synthetases and polyketide and fatty acid synthetases. The pcbAB gene is linked to the pcbC gene, forming a cluster of early cephalosporin-biosynthetic genes.

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.

Construction of a 7-Aminocephalosporanic Acid (7ACA) Biosynthetic Operon and Direct Production of 7ACA in Acremonium Chrysogenum

We have used cDNA encoding D-amino acid oxidase, and genomic DNA encoding cephalosporin acylase from Fusarium solani and Pseudomonas diminuta, respectively, to construct a novel hybrid 7-aminocephalosporanic acid (7ACA) biosynthetic operon under the control of regulatory elements from the alkaline protease gene of Acremonium chrysogenum. Transformants of A. chrysogenum BC2116, a high cephalosporin-producing strain, containing this operon, synthesized and secreted low levels of 7ACA. Although the amounts are not yet commercially significant, this represents the first microbial production of 7ACA and demonstrates the feasibility of introducing new biosynthetic capabilities into industrial microorganisms by combining fungal and bacterial genes.

Characterization and complementation of a cephalosporin-deficient mutant of Streptomyces clavuligerus NRRL 3585

We have characterized a mutant of Streptomyces clavuligerus NRRL 3585 which is almost completely blocked in cephalosporin biosynthesis and exhibits depressed activities of both the delta(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (ACV) synthetase and cyclase enzymes of the cephalosporin pathway. A wild-type DNA region was cloned which partially restores antibiotic production, ACV synthetase and cyclase activities to this mutant. The recombinant plasmid exhibits a variable copy number in different transformants. Hybridization experiments indicate that sequences homologous to the cloned region are present in various beta-lactam-producing Streptomyces spp. but absent in species which are not known to produce this class of antibiotics. Furthermore, the chromosomal copy of the cloned region lies in close proximity to a gene coding for the isopenicillin N synthase gene of the cephalosphorin pathway.

Defining an optimal carbon source/methionine feed strategy for growth and cephalosporin C formation by Cephalosporium acremonium

The effect of the method of methionine addition, growth-limiting carbon source (glucose vs sucrose), and culture growth rate on cephalosporin C production was investigated in a Cephalosporium acremonium defined medium fed batch fermentation. Batch addition of methionine, at a concentration of 3 g/L, prior to the start of a fed sucrose fermentation was found to interfere with the ability of the culture to utilize this sugar, thus limiting growth and decreasing cephalosporin C production. Batch methionine addition had no effect on glucose-limited cultures. Concurrent exponential feeding of methionine with sucrose improved both culture growth and productivity. Under the control of identical carbon source limiting feed profiles, sucrose was observed to support greater cephalosporin C production than glucose. Optimal cephalosporin C production in a C. acremonium defined medium fed batch fermentation was obtained through controlling culture growth during the rapid growth phase at a relatively low level with respect to mumax (mu approximately 0.036 h-1) until achieving a desired cell mass with a concurrent sucrose and methionine feed, followed by maintaining relatively vigorous growth (mu approximately 0.01 h-1) with sucrose for the duration of the fermentation.

[Cloning in Escherichia coli of a mitochondrial DNA fragment from Acremonium chrysogenum containing a region responsible for DNA replication]

A bireplicone plasmid pSU901,4.6 kb in length, was constructed on the basis of plasmid pUC19 and the pstIB fragment, 1.9 kb in length, from mitochondrial DNA of A. chrysogenum. Based on the hybrid plasmid pSU901 and kanamycin resistance determinant, an autonomically replicating vector for A. chrysogenum, a culture producing cephalosporin C, is being constructed.

[Preservation of microorganisms by the method of L-drying]

A procedure for L-drying of microorganisms or their drying under vacuum from liquid state is described in detail. The procedure is used in the Culture Collection of the All-Union Research Institute of Antibiotics. Preservation of Acremonium chrysogenum VNIIA 313A, the cephalosporin-producing culture, with the described procedure allowed one not only to maintain its viability for prolonged periods, but also to completely reproduce its initial antibiotic activity. L-Drying of some poorly kept cultures belonging to Acremonium was a success.

Purification from Cephalosporium acremonium of the initial enzyme unique to the biosynthesis of penicillins and cephalosporins

The stability of the unstable enzyme, delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (ACV) synthetase from Cephalosporium acremonium C-10, was increased 10-fold which facilitated its purification. The active enzyme was purified over 100 fold to electrophoretic homogeneity by protamine sulfate treatment, ammonium sulfate fractionation, gel filtration and hydrophobic interaction chromatography. It appears to have a minimal size of 360 kDa based on SDS-polyacrylamide gel electrophoresis.