Protoplast transformation in coryneform bacteria and introduction of an alpha-amylase gene from Bacillus amyloliquefaciens into Brevibacterium lactofermentum

The soil bacterium, Corynebacterium glutamicum, from biosynthesis of value-added products to bioremediation: A master of many trades

Ever since its discovery in 1957, Corynebacterium glutamicum has become a well-established industrial strain and is known for its massive capability of producing various amino acids (like L-lysine and L-glutamate) and other value-added chemicals. With the rising demand for these bio-based products, the revelation of the whole genome sequences of the wild type strains, and the astounding advancements made in the fields of metabolic engineering and systems biology, our perspective of C. glutamicum has been revolutionized and has expanded our understanding of its strain development. With these advancements, a new era for C. glutamicum supremacy in the field of industrial biotechnology began. This led to remarkable progress in the enhancement of tailor-made over-producing strains and further development of the substrate spectrum of the bacterium, to easily accessible, economical, and renewable resources. C. glutamicum has also been metabolically engineered and used in the degradation/assimilation of highly toxic and ubiquitous environmental contaminant, arsenic, present in water or soil. Here, we review the history, current knowledge, progress, achievements, and future trends relating to the versatile metabolic factory, C. glutamicum. This review paper is devoted to C. glutamicum which is one of the leading industrial microbes, and one of the most promising and versatile candidates to be developed. It can be used not only as a platform microorganism to produce different value-added chemicals and recombinant proteins, but also as a tool for bioremediation, allowing to enhance specific properties, for example in situ bioremediation.

Recombinant Protein Expression System in Corynebacterium glutamicum and Its Application

Corynebacterium glutamicum, a soil-derived gram-positive actinobacterium, has been widely used for the production of biochemical molecules such as amino acids (i.e., L-glutamate and L-lysine), nucleic acids, alcohols, and organic acids. The metabolism of the bacterium has been engineered to increase the production of the target biochemical molecule, which requires a cytosolic enzyme expression. As recent demand for new proteinaceous biologics (such as antibodies, growth factors, and hormones) increase, C. glutamicum is attracting industrial interest as a recombinant protein expression host for therapeutic protein production due to the advantages such as low protease activity without endotoxin activity. In this review, we have summarized the recent studies on the heterologous expression of the recombinant protein in C. glutamicum for metabolic engineering, expansion of substrate availability, and recombinant protein secretion. We have also outlined the advances in genetic components such as promoters, surface anchoring systems, and secretory signal sequences in C. glutamicum for effective recombinant protein expression.

Bottleneck in secretion of α-amylase in Bacillus subtilis

Amylase plays an important role in biotechnology industries, and Gram-positive bacterium Bacillus subtilis is a major host to produce heterogeneous α-amylases. However, the secretion stress limits the high yield of α-amylase in B. subtilis although huge efforts have been made to address this secretion bottleneck. In this question-oriented review, every effort is made to answer the following questions, which look simple but are long-standing, through reviewing of literature: (1) Does α-amylase need a specific and dedicated chaperone? (2) What signal sequence does CsaA recognize? (3) Does CsaA require ATP for its operation? (4) Does an unfolded α-amylase is less soluble than a folded one? (5) Does α-amylase aggregate before transporting through Sec secretion system? (6) Is α-amylase sufficient stable to prevent itself from misfolding? (7) Does α-amylase need more disulfide bonds to be stabilized? (8) Which secretion system does PrsA pass through? (9) Is PrsA ATP-dependent? (10) Is PrsA reused after folding of α-amylase? (11) What is the fate of PrsA? (12) Is trigger factor (TF) ATP-dependent? The literature review suggests that not only the most of those questions are still open to answers but also it is necessary to calculate ATP budget in order to better understand how B. subtilis uses its energy for production and secretion.

Novel expression system for Corynebacterium acetoacidophilum and Escherichia coli based on the T7 RNA polymerase-dependent promoter

The industrially important species of corynebacteria viz. Corynebacterium acetoacidophilum appear to be alternative hosts for recombinant protein production; despite many efforts, a strong promoter-based system in corynebacteria has not been established so far. Described here is a T7 promoter-based expression system which was functional in both gram-positive C. acetoacidophilum and gram-negative Escherichia coli in an external inducer independent manner. This is the very first report of a T7 expression system for Corynebacterium sp. Also, it is a useful addition in the existing T7 expression systems of E. coli.

High yield secretion of heterologous proteins in Corynebacterium glutamicum using its own Tat-type signal sequence

Efficient protein secretion, the basis of large-scale production of many compounds central to the biotechnology industry, is achieved by signal peptide and propeptide optimization in addition to optimizing host factors affecting heterologous protein production. Here, we fused green fluorescent protein (GFP) to the recently identified Tat-type secretory signal peptide of CgR0949 to demonstrate a high-yield protein secretion system of Corynebacterium glutamicum. The resultant secretion vector facilitated effective secretion of active-form GFP (20 mg l(-1)) into C. glutamicum culture medium. The expression of GFP was enhanced 2.9-fold using the Shine-Dalgarno sequence of triosephosphate isomerase in the secretion vector. Moreover, GFP drastically accumulated in the culture supernatant upon addition of calcium chloride even though Ca(2+) addition did neither enhanced the transcription of gfp nor resulted in the accumulation of cytosolic GFP. Active-form GFP concentration reached 1.8 g l(-1) after 48-h incubation in a jar fermentor. Likewise, α-amylase accumulation in C. glutamicum cultures was also enhanced by Ca(2+) addition, suggesting that Ca(2+) may affect general protein secretion in C. glutamicum.

Scanning the Corynebacterium glutamicum R genome for high-efficiency secretion signal sequences

Systematic screening of secretion proteins using an approach based on the completely sequenced genome of Corynebacterium glutamicum R revealed 405 candidate signal peptides, 108 of which were able to heterologously secrete an active-form alpha-amylase derived from Geobacillus stearothermophilus. These comprised 90 general secretory (Sec)-type, 10 twin-arginine translocator (Tat)-type and eight Sec-type with presumptive lipobox peptides. Only Sec- and Tat-type signals directed high-efficiency secretion. In two assays, 11 of these signals resulted in 50- to 150-fold increased amounts of secreted alpha-amylase compared with the well-known corynebacterial secretory protein PS2. While the presence of an AXA motif at the cleavage sites was readily apparent, it was the presence of a glutamine residue adjacent to the cleavage site that may affect secretion efficiency.

Temperature-sensitive cloning vector for Corynebacterium glutamicum

We constructed a temperature-sensitive form of the Corynebacterium glutamicum ATCC13869 cryptic plasmid, pBL1. The C. glutamicum/Escherichia coli shuttle vector pSFK6, which is composed of pBL1 and the E. coli cloning vector pK1, was mutagenized in vitro by treatment with hydroxylamine, and introduced into C. glutamicum cells. A mutant plasmid, which was stably maintained at 25 degrees C but not at 34 degrees C, was isolated from the cells. Sequencing the plasmid, which was named p48K, revealed four substitutions in the Rep protein coding region. Moreover, site-directed single-nucleotide substitutions showed that a G to A transition at position 2,920, which resulted in a Pro-47 to Ser substitution in the Rep protein, was responsible for its temperature-sensitive replication. Pro-47 is conserved among the Rep proteins of the pIJ101/pJV1 family of plasmids. This temperature-sensitive cloning vector will be useful for disrupting genes in this industrially important bacterium.

Plasmids in Corynebacterium glutamicum and their molecular classification by comparative genomics

Endogenous plasmids and selectable resistance markers are a fundamental prerequisite for the development of efficient recombinant DNA techniques in industrial microorganisms. In this article, we therefore summarize the current knowledge about endogenous plasmids in amino acid-producing Corynebacterium glutamicum isolates. Screening studies identified a total of 24 different plasmids ranging in size from 2.4 to 95 kb. Although most of the C. glutamicum plasmids were cryptic, four plasmids carried resistance determinants against the antibiotics chloramphenicol, tetracycline, streptomycin-spectinomycin, and sulfonamides. Considerable information is now available on the molecular genetic organization of 12 completely sequenced plasmid genomes from C. glutamicum. The deduced mechanism of plasmid DNA replication and the degree of amino acid sequence similarity among replication initiator proteins was the basis for performing a classification of the plasmids into four distinct C. glutamicum plasmid families.

Secretion of active-form Streptoverticillium mobaraense transglutaminase by Corynebacterium glutamicum: processing of the pro-transglutaminase by a cosecreted subtilisin-Like protease from Streptomyces albogriseolus

The transglutaminase secreted by Streptoverticillium mobaraense is a useful enzyme in the food industry. A fragment of transglutaminase was secreted by Corynebacterium glutamicum when it was coupled on a plasmid to the promoter and signal peptide of a cell surface protein from C. glutamicum. We analyzed the signal peptide and the pro-domain of the transglutaminase gene and found that the signal peptide consists of 31 amino acid residues and the pro-domain consists of 45 residues. When the pro-domain of the transglutaminase was used, the pro-transglutaminase was secreted efficiently by C. glutamicum but had no enzymatic activity. However, when the plasmid carrying the S. mobaraense transglutaminase also encoded SAM-P45, a subtilisin-like serine protease derived from Streptomyces albogriseolus, the peptide bond to the C side of 41-Ser of the pro-transglutaminase was hydrolyzed, and the pro-transglutaminase was converted to an active form. Our findings suggest that C. glutamicum has potential as a host for industrial-scale protein production.

Construction of a xylanase-producing strain of Brevibacterium lactofermentum by stable integration of an engineered xysA gene from Streptomyces halstedii JM8

A xylanolytic strain of Brevibacterium lactofermentum containing the Streptomyces halstedii His-tagged xysA gene was generated. The new strain contains DNA derived from S. halstedii, expresses xylanolytic activity, and was obtained by an integrative process mediated by a conjugative plasmid targeted to a dispensable chromosomal region located downstream from the essential cell division gene ftsZ. The His-tagged Xys1 enzyme was constitutively expressed under the control of the kan promoter from Tn5 and was easily purified by use of Ni-nitrilotriacetic acid-agarose. The new strain is stable for more than 200 generations, lacks any known antibiotic resistance gene, and does not need any selective pressure to maintain the integrated gene. This strategy can be used to integrate any gene into the B. lactofermentum chromosome and to maintain it stably without the use of antibiotics for selection.

A novel system with positive selection for the chromosomal integration of replicative plasmid DNA in Corynebacterium glutamicum

A simple system has been developed for generating Corynebacterium glutamicum strains containing stable replicative plasmids integrated into the chromosome via homologous recombination. The system is based upon extremely strong incompatibility between two plasmids, which cannot be co-maintained even under antibiotic selective pressure. Integration of the resident plasmid that contained the trpD gene of C. glutamicum was achieved by introduction of a second plasmid and subsequent selection for the maintenance of both plasmids. Plasmid integrates positive for both plasmid markers were obtained at a frequency about 10(-3) of the normal transformation frequency with selection for the maintenance of only the second plasmid. Southern analysis revealed that the integration had occurred through a single-crossover homologous recombination between the trpD regions of the host genome and the plasmid. On the basis of the Campbell-type integration, chromosome walking was attempted by using Escherichia coli replication origins that were also present in the integrated plasmid. The chromosomal DNA was digested, ligated, and used to transform E. coli, which enabled recovery of the expected adjacent genomic DNA regions. The plasmid integrate was stably maintained for 30 generations under non-selective culture conditions, suggesting that the integrated sequences carrying a replicon active in the host were maintained as a stable chromosomal insert in C. glutamicum.

Mycotic acid composition of Corynebacterium glutamicum and its cell surface mutants: effects of growth with glycine and isonicotinic acid hydrazide

Auxotrophic mutants of Corynebacterium glutamicum strain ATCC 13059 (parent of AS019, a rifampicin-resistant variant), which were morphologically distinct from the parent and formed protoplasts more readily, had been isolated previously. Mutants MLB130-133 and MLB194 were more sensitive to growth inhibition by isonicotinic acid hydrazide (INH) and glycine, which caused branching and budding. Fatty acid and mycolic acid (MA) profiles were determined after growth in LBG (Luria broth plus glucose), LBG-glycine (LBG- and LBG-INH (LBG-I). The fatty acid profiles of all strains were similar, except that mutant MLB133 showed some increase in stearic acid (C18:0), normally a minor component, late in the growth cycle and oleic acid proportionately decreased. All strains had five major types of MAs (C32:0, C34:0, C34:1, C36:1, C36:2) but the relative proportion of each varied with the strain, age of culture and medium composition. Mutants MLB133 and MLB194 showed slightly higher levels of non-covalentiy bound MAs than the parent and normally showed a higher proportion of longer-chained, unsaturated MAs. The proportion of extracellular MAs increased with culture age for these mutants. Typically, by late stationary phase, mycolic acids in culture fluids increased to 6.5% of the total MAs for MLB194 and 7.9% for MLB133 compared with 3.5% for the parent strain grown in LBG. The main effect of glycine (2%, w/v) addition was to increase the proportion of mycolic acids found in extracellular fluids (16.1 % for AS019 and 31% for MLB133). The most significant effects of INH were seen when strains were cultured in LBG with 8 mg INH ml−1. When harvested at late stationary phase, strains MLB133 and MLB194 had 18.8% and 21.2% extracellular mycolic acids respectively, with a significant increase in the relative proportion of unsaturated mycolic acids. This effect was not as marked for AS019, which also showed a similar decrease in C32:0 relative to increases in the proportion of C34:1 and C36:2 plus a corresponding increase in the overall proportion of unsaturated mycolic acids and increased extracellular mycolates (8.5%). These results suggest that the mutations in strains MLB133 and MLB194 are associated with synthesis of specific mycolic acids (e.g. C32:0) and attachment of mycolic acids to the cell surface, both of which are likely target sites for glycine and INH action for cell-surface modifications. In addition to previously reported targeting of the peptidoglycan cross-linking, these results show that glycine affects mycolic acid attachment to the cell surface of C. glutamicum.

Construction and characterization of new corynebacterial plasmids carrying the alpha-amylase gene

Novel corynebacterial plasmids carrying alpha-amylase gene from Bacillus have been constructed. The level of alpha-amylase expression depends on the size of the vector. The highest expression levels were measured in brevibacteria harboring pA61 plasmid.

Expression and secretion of heterologous proteases by Corynebacterium glutamicum

Genes encoding the basic protease of Dichelobacter nodosus (bprV) and the subtilisin of Bacillus subtilis (aprE) were cloned and expressed in Corynebacterium glutamicum. In each case, enzymatically active protein was detected in the supernatants of liquid cultures. While the secretion of subtilisin was directed by its own signal peptide, the natural signal peptide of the bprV basic protease did not facilitate secretion. A hybrid aprE-bprV gene in which the promoter and signal peptide coding sequences of subtilisin replaced those of bprV could be expressed, and basic protease was secreted by C. glutamicum. Expression of these proteases in C. glutamicum provides an opportunity to compare protein secretion from this gram-positive host with that from other gram-positive and gram-negative bacteria.

Recent advances in the physiology and genetics of amino acid-producing bacteria

Corynebacterium glutamicum and its close relatives, C. flavum and C. lactofermentum, have been used for over 3 decades in the industrial production of amino acids by fermentation. Since 1984, several research groups have started programs to develop metabolic engineering principles for amino acid-producing Corynebacterium strains. Initially, the programs concentrated on the isolation of genes encoding (deregulated) biosynthetic enzymes and the development of general molecular biology tools such as cloning vectors and DNA transfer methods. With most of the genes and tools now available, recombinant DNA technology can be applied in strain improvement. To accomplish these improvements, it is critical and advantageous to understand the mechanisms of gene expression and regulation as well as the biochemistry and physiology of the species being engineered. This review explores the advances made in the understanding and application of amino acid-producing bacteria in the early 1990s.

Characterization of a region of plasmid pBL1 of Brevibacterium lactofermentum involved in replication via the rolling circle model

The minimal region for autonomous replication of pBL1, a 4.5-kb cryptic plasmid of Brevibacterium lactofermentum ATCC 13869 that has been used to construct a variety of corynebacterium vectors, was shown to be contained on a 1.8-kb HindII-SphI DNA fragment. This region contains two open reading frames (ORFs) (ORF1 and ORF5) which are essential for pBL1 replication in B. lactofermentum. Accumulation of single-strand intermediates in some of the constructions indicates that plasmid pBL1 replicates via the rolling circle replication model; its plus strand and minus strand were identified by hybridization with two synthetic oligonucleotide probes complementary to each pBL1 strand. ORF1 seems to encode the Rep protein and showed partial homology with sequences for Rep proteins from Streptomyces plasmids which replicate via rolling circle replication such as pIJ101, pSB24, and pJV1.

Expression, secretion, and processing of staphylococcal nuclease by Corynebacterium glutamicum

The gene for staphylococcal nuclease (SNase), an extracellular enzyme of Staphylococcus aureus, was introduced into Corynebacterium glutamicum. The heterologous gene was expressed in this host organism, and SNase was efficiently exported to the culture medium. Amino-terminal sequencing of SNase secreted by C. glutamicum revealed that the signal peptide was apparently cleaved off at precisely the same position as in the original host, S. aureus. As with S. aureus, a second smaller form of SNase (A form), whose appearance is presumably the result of a secondary processing step, was found in the culture medium of the recombinant C. glutamicum strain. The A form was one residue shorter than the mature nuclease A produced by S. aureus. Variation of the sodium chloride concentration in the growth medium had a marked influence on the location and the processing of SNase by C. glutamicum. In a complex growth medium containing 4% sodium chloride, SNase was exclusively located in the supernatant, but a significant amount of the enzyme remained cell associated if the strain was grown in a low-salt medium. Also, high salt concentrations seemed to inhibit processing of the high-molecular-weight form of SNase (B form) to the smaller A form. Similarities and differences in the export and modes of processing of SNase by three different, nonrelated gram-positive host organisms are discussed. Finally, a versatile Escherichia coli-C. glutamicum tac-lacIq expression shuttle vector was constructed. With this vector, it was possible to achieve isopropyl-beta-D-galactopyranoside (IPTG)-inducible overexpression and secretion of SNase in C. glutamicum, whereby the expression level was dependent on the concentration of the inducer.

Characterization of pGA1, a new plasmid from Corynebacterium glutamicum LP-6

A new plasmid, pGA1, has been isolated from Corynebacterium glutamicum LP-6, and its detailed restriction map has been prepared. The 4.9-kb plasmid has a G + C content of 57%. It replicates in C. glutamicum ATCC13032 and is compatible with the three other plasmids, pCC1, pBL1 and pHM1519, commonly used for vector construction for amino acid-producing corynebacteria. Fusions of pGA1 with different Escherichia coli replicons (transferred from E. coli to Corynebacterium via transformation of spheroplasts or by filter mating experiments with intact cells) are shown to be suitable as shuttle plasmids; some of them are highly stable in C. glutamicum, even when propagated without any selection pressure.

Cloning vectors and antibiotic-resistance markers for Brevibacterium sp. R312

Replication of several cryptic plasmids from coryneform strains was investigated in Brevibacterium sp. R312. Only the Corynebacterium glutamicum pSR1 replicon was found to be suitable for establishing a host-vector system. Two pSR1 derivatives, pRPCG200 and pHYCG1, were used as cloning vectors. They carry a neomycin-resistance-encoding and a tetracycline-resistance-encoding gene, respectively.

Microbial amylolytic enzymes

Starch-degrading, amylolytic enzymes are widely distributed among microbes. Several activities are required to hydrolyze starch to its glucose units. These enzymes include alpha-amylase, beta-amylase, glucoamylase, alpha-glucosidase, pullulan-degrading enzymes, exoacting enzymes yielding alpha-type endproducts, and cyclodextrin glycosyltransferase. Properties of these enzymes vary and are somewhat linked to the environmental circumstances of the producing organisms. Features of the enzymes, their action patterns, physicochemical properties, occurrence, genetics, and results obtained from cloning of the genes are described. Among all the amylolytic enzymes, the genetics of alpha-amylase in Bacillus subtilis are best known. Alpha-Amylase production in B. subtilis is regulated by several genetic elements, many of which have synergistic effects. Genes encoding enzymes from all the amylolytic enzyme groups dealt with here have been cloned, and the sequences have been found to contain some highly conserved regions thought to be essential for their action and/or structure. Glucoamylase appears usually in several forms, which seem to be the results of a variety of mechanisms, including heterogeneous glycosylation, limited proteolysis, multiple modes of mRNA splicing, and the presence of several structural genes.

Nucleotide sequence and fine structural analysis of the Corynebacterium glutamicum hom-thrB operon

The complete nucleotide sequence of the Corynebacterium glutamicum hom-thrB operon has been determined and the structural genes and promoter region mapped. A polypeptide of Mr 46,136 is encoded by hom and a polypeptide of Mr 32,618 is encoded by thrB. Both predicted protein sequences show amino acid sequence homology to their counterparts in Escherichia coli and Bacillus subtilis. The promoter region has been mapped by S1-nuclease and deletion analysis. Located between -88, RNA start site and -219 (smallest deletion clone with complete activity) are sequence elements similar to those found in E. coli and B. subtilis promoters. Although there are no obvious attenuator-like structures in the 5'-untranslated region, there is a dyad-symmetry element, which may act as an operator.

The expression of Cellulomonas fimi cellulase genes in Brevibacterium lactofermentum

The exoglucanase gene (cex) and the endoglucanase A gene (cenA) from Cellulomonas fimi were subcloned into the Escherichia coli/Brevibacterium lactofermentum shuttle vector pBK10. Both genes were expressed to five to ten times higher levels in B. lactofermentum than in E. coli, probably because these genes were expressed from C. fimi promoters. In B. lactofermentum virtually all of the enzyme activities were in the culture supernatant. This system will facilitate analysis of the expression of the C. fimi genes in and secretion of their products from a Gram-positive bacterium.

Identification of a promoter sequence in the plasmid pUL340 of Brevibacterium lactofermentum and construction of new cloning vectors for corynebacteria containing two selectable markers

A strong promoter P1 has been found in plasmid pUL340, a cloning vector used to transform corynebacteria. This promoter is also expressed efficiently in Escherichia coli. A gene (cat) for chloramphenicol acetyltransferase from Streptomyces acrimycini and a gene (hyg) for hygromycin phosphotransferase from Streptomyces hygroscopicus were subcloned in different positions of the Brevibacterium lactofermentum plasmid pUL340. Both resistance genes are expressed in B. lactofermentum from their own promoters or from the endogenous promoter in pUL340. These genes provide useful screening markers for selecting transformants of B. lactofermentum together with the kanamycin-resistance gene from the transposon Tn5.