Recombinant expression and characterization of Oryctolagus cuniculus chymosin in Komagataella phaffii (Pichia pastoris)

This study was conducted for investigating expression and enzymatic characteristics of recombinant Oryctolagus cuniculus chymosin (ROCC) expressed in Pichia pastoris. SDS-PAGE of partially purified supernatant displayed two distinct molecular bands approximately at the sizes of 40 kDa and 45 kDa corresponding to chymosin and partially glycosylated chymosin, respectively. Proteolysis assay demonstrated that rabbit chymosin was more specific compared to bovine and camel chymosins when it comes to hydrolyzing α, β, and κ-casein. Rabbit chymosin kept its stability in a wide pH range (3.0-6.0) at 37 °C for 8 h. Active chymosin exhibited maximum enzymatic activity at 40 °C and pH 4.0 with the addition of 75 mM CaCl₂. The ROCC clotting activity on donkey, cow, goat, lamb, camel milk was determined as 40, 10, 5.7, 3.07, and 2.66 IMCU/mL, respectively. These results revealed that ROCC might possess a potential for incorporation into cheese manufacture technology as a milk-clotting enzyme.

Enhanced production of bovine chymosin by autophagy deficiency in the filamentous fungus Aspergillus oryzae

Aspergillus oryzae has been utilized as a host for heterologous protein production because of its high protein secretory capacity and food-safety properties. However, A. oryzae often produces lower-than-expected yields of target heterologous proteins due to various underlying mechanisms, including degradation processes such as autophagy, which may be a significant bottleneck for protein production. In the present study, we examined the production of heterologous protein in several autophagy (Aoatg) gene disruptants of A. oryzae. We transformed A. oryzae gene disruptants of Aoatg1, Aoatg13, Aoatg4, Aoatg8, or Aoatg15, with a bovine chymosin (CHY) expression construct and found that the production levels of CHY increased up to three fold compared to the control strain. Notably, however, conidia formation by the Aoatg gene disruptants was significantly reduced. As large amounts of conidia are necessary for inoculating large-scale cultures, we also constructed Aoatg gene-conditional expression strains in which the promoter region of the Aoatg gene was replaced with the thiamine-controllable thiA promoter. Conidiation by the resultant transformants was clearly enhanced in the absence of thiamine, while autophagy remained repressed in the presence of thiamine. Moreover, these transformants displayed increased CHY productivity, which was comparable to that of the Aoatg gene disruptants. Consequently, we succeeded in the construction of A. oryzae strains capable of producing high levels of CHY due to defects in autophagy. Our finding suggests that the conditional regulation of autophagy is an effective method for increasing heterologous protein production in A. oryzae.

Disruption of PMR1 in Kluyveromyces lactis improves secretion of calf prochymosin

BACKGROUND

Chymosin is an important industrial enzyme widely used in cheese manufacturing. Kluyveromyces lactis is a promising host strain for expression of the chymosin gene. However, only low yields of chymosin (80 U mL(-1) in shake flask culture) have been obtained using K. lactis GG799. The aim of this study was to increase the amount of recombinant calf chymosin secreted by K. lactis GG799 by disrupting the PMR1 gene.

RESULTS

Kluyveromyces lactis GG799 harbouring the disrupted PMR1 gene showed reduced growth in ethylene glycol tetraacetic acid-containing and Ca(2+) -deficient medium, but Ca(2+) supplementation eliminated the growth problem. The calf chymosin gene was ligated into the K. lactis GG799 expression vector, generating the plasmid pKLAC1-N-prochymosin. The linearised plasmid was homologously integrated into the genome of K. lactis GG799. In shake flask culture, chymosin activity was 496 U mL(-1) in the K. lactis PMR1-deficient mutant, sixfold higher than that in wild-type K. lactis GG799.

CONCLUSION

Disrupting the PMR1 gene improved chymosin production in K. lactis GG799 sixfold. This knowledge could be applied to industrial chymosin production.

[Gene synthesis of the bovine prochymosin gene and high-level expression in Kluyvermyces lactis]

Chymosin is an important industrial enzyme widely used in cheese manufacture. To improve expression efficiency of recombinant bovine chymosin in Kluyveromyces lactis strain GG799, we designed and synthesized a DNA sequence encoding bovine prochymosin gene (GenBank Accession No. AA30448) by using optimized codons. The synthesized prochymosin gene was amplified by two-step PCR method, and then cloned into the expression vector pKLAC1, resulting in pKLAC1-Prochy. pKLAC1-Prochy was linearized and transformed into K. lactis GG799 by electrotransformation. Positive clones were screened by YEPD plates containing 1% casein. A recombinant strain chyl with highest activities and multi-copy integration which was detected by using specifical integration primers was chosen and fermented in flask. Prochymosin was expressed in K. lactis successfully. SDS-PAGE analysis revealed that the purified recombinant bovine prochymosin had a molecular mass of 41 kDa. After acid treatment, molecular weight of chymosin is about 36 kDa, the same as native bovine chymosin. Activity tests showed that the chymosin activity of the culture supernatant was 99.67 SU/mL after 96 h cultivation. The activities of chymosin were not prominent increased when galactose was used as carbon source instead of glucose, which proved that the fermentation of recombinant strain does not need galactose inducing. The recombinant K. lactis strain obtained in this study could be further used to produce recombinant chymosin for cheese making.

[Recombinant expression of bovine chymosin in Pichia pastoris]

To express bovine chymosin in yeast, we amplified the prochymosin gene from the plasmid pMD18T-Prochy by PCR, and then cloned the gene into the expression vector pPICZaA, resulting in pPICZaA-Prochy. Pichia pastoris GS115 was used as host cells. Integration of the prochymosin cDNA into the Pichia pastoris genome was confirmed by PCR and sequencing analysis. Chymosin was expressed in Pichia pastoris successfully, and a strong band at about 37 kD was shown by SDS-PAGE. Activity tests showed that the chymosin activity of the culture supernatant was 12.2 SU/mL. This is the first report of successful expression of chymosin in Pichia pastoris. The recombinant Pichia pastoris strain obtained in this study could be further used to produce recombinant chymosin for cheese making.

Increased production of chymosin by glycosylation

Filamentous fungi are well known in the industry as producers of large amounts of extracellular proteins. However, production levels of heterologous proteins are often disappointing low. In this paper it is shown that increasing glycosylation is a powerful strategy for increasing production levels of chymosin in filamentous fungi. Two different concepts based on glycosylation were tested. First, we improved a poorly used N-glycosylation site within the prochymosin molecule. The resulting highly glycosylated chymosin molecule was expressed in Aspergillus niger. It was shown that production of the glycosylated protein was much more efficient, giving a yield increase of more than 100% compared to production of the native chymosin molecule. In an alternative strategy the N-glycosylation site was located outside of the native chymosin molecule, on a linker separating prochymosin from its carrier molecule. Also in this case significantly increased production levels were obtained. This strategy might offer a powerful tool for increasing production levels of other heterologous proteins as well.

Molecular cloning and expression in yeast of caprine prochymosin

We cloned and characterized a preprochymosin cDNA from the abomasum of milk-fed kid goats. This cDNA contained an open reading frame that predicts a polypeptide of 381 amino acid residues, with a signal peptide and a proenzyme region of 16 and 42 amino acids, respectively. Comparison of the caprine preprochymosin sequence with the corresponding sequences of lamb and calf revealed 99 and 94% identity at the amino acid level. The cDNA fragment encoding the mature portion of caprine prochymosin was fused in frame both to the killer toxin signal sequence and to the alpha-factor signal sequence-FLAG in two different yeast expression vectors. The recombinant plasmids were transformed into Kluyveromyces lactis and Saccharomyces cerevisiae cells, respectively. Culture supernatants of both yeast transformants showed milk-clotting activity after activation at acid pH. The FLAG-prochymosin fusion was purified from S. cerevisiae culture supernatants by affinity chromatography. Proteolytic activity assayed toward casein fractions indicated that the recombinant caprine chymosin specifically hydrolysed kappa-casein.

Expression of a synthetic copy of the bovine chymosin gene in Aspergillus awamori from constitutive and pH-regulated promoters and secretion using two different pre-pro sequences

A copy of the bovine chymosin gene (chy) with a codon usage optimized for its expression in Aspergillus awamori was constructed starting from synthetic oligonucleotides. To study the ability of this filamentous fungus to secrete bovine prochymosin, two plasmids were constructed in which the transcriptional, translational, and secretory control regions of the A. nidulans gpdA gene and pepB genes were coupled to either preprochymosin or prochymosin genes. Secretion of a protein enzymatically and immunologically indistinguishable from bovine chymosin was achieved in A. awamori transformants with each of these constructions. In all cases, the primary translation product (40.5 kDa) was self-processed to a mature chymosin polypeptide having a molecular weight of 35.6 kDa. Immunological assays indicated that most of the chymosin was secreted to the extracellular medium. Hybridization analysis of genomic DNA from chymosin transformants showed chromosomal integration of prochymosin sequences and, in some transformants, multiple copies of the expression cassettes were observed. Expression from the gpdA promoter was constitutive, whereas expression from the pepB promoter was strongly influenced by pH. A very high expression from the pepB promoter was observed during the growth phase. The A. awamori pepB gene terminator was more favorable for chymosin production than the S. cerevisiae CYC1 terminator.

[Rennet production by Rhizomucor miehei NRRL 3169]

The production of rennet was studied, using different strains of the fungus Rhizomucor miehei. The selection and preservation of strains, type of growth, media design and operation conditions were evaluated. The experiments were carried out in Erlenmeyer flasks in rotary shaker at 250 rpm and 2.5 eccentricity, and in mechanically stirred fermentors of the New Brunswick type, at 30 degrees C. In the studies concerning strain selection, the best strain was Rhizomucor miehei NRRL 3169. The major titles of enzyme were obtained in batch process at 168 h, with 884 SU/ml, whereas in mechanically stirred fermentors the best value was 1160 SU/ml. These values were far more superior to former ones published by various experts.

Isolation and characterisation of a calnexin homologue, clxA, from Aspergillus niger

We describe the isolation of a gene (clxA) encoding calnexin from laboratory and industrial strains of Aspergillus niger. Calnexin is a chaperone, which specifically recognises monoglucosylated glycoproteins in the endoplasmic reticulum, and is thus an essential component of the process that assesses the folded state of nascent secreted glycoproteins. Manipulation of chaperones has previously been adopted in attempts to overcome some of the problems associated with the secretion of heterologous proteins from filamentous fungi. The A. niger clxA gene encodes a 562-residue protein with strong homology to the calnexin of Schizosaccharomyces pombe. The clxAgene product complements a S. pombe cnx1 mutant. Motifs associated with genes controlled via the Unfolded Protein Response (UPR) were identified by sequence homology in the promoter of clxA. Steady-state levels of clxA mRNA were elevated in a strain expressing bovine prochymosin fused to the catalytic domain of glucoamylase. The ORF is punctuated by four introns, and contains two sets of four repeated peptide motifs that are characteristic of the calnexin family, together with a putative membrane-spanning domain. Deletion studies indicate that clxA is not an essential gene in A. niger.

Strong hybrid promoters and integrative expression/secretion vectors for quasi-constitutive expression of heterologous proteins in the yeast Yarrowia lipolytica

The industrial yeast Yarrowia lipolytica secretes high amounts of an alkaline extracellular protease encoded by the XPR2 gene. The industrial use of the XPR2 promoter was however hindered by its complex regulation. We designed hybrid promoters, based on tandem copies of the XPR2 promoter UAS1 region. In contrast to native XPR2 promoter, these hybrid promoters were not repressed by the preferred carbon and nitrogen sources, nor by acidic conditions, and they did not require the presence of peptones in the culture medium. They exhibited a strong quasi-constitutive activity, similar when carried on either integrative or replicative plasmids. We used these hybrid promoters to direct the production of bovine prochymosin, using XPR2 secretion signals. The production of active chymosin was several fold higher than with previously available Y. lipolytica promoters (up to 160 mg/l). Integrative vectors based on the hybrid promoters, allowing the easy insertion of a heterologous gene and its expression or expression/secretion in Y. lipolytica, were designed. We also designed new Y. lipolytica recipient strains with good secreting abilities, able to grow on sucrose, and devoid of extracellular proteases. These new tools will add to the interest of Y. lipolytica as a host for heterologous protein production.

Molecular cloning of neonate/infant-specific pepsinogens from rat stomach mucosa and their expressional change during development

To clarify the nature of rat neonate/infant-specific pepsinogens, we carried out their purification and molecular cloning. Prochymosin was found to be the major neonatal pepsinogen. The general proteolytic activity of its active form, chymosin, was, however, lower than those of pepsins A and C which are predominant in adult animals. Molecular cloning of rat prochymosin cDNA was achieved along with cDNA for another neonate-specific pepsinogen, pepsinogen F, although determination of pepsinogen F in neonatal gastric mucosa was unsuccessful, presumably due to its lack of proteolytic activity or different proteolytic specificity. Northern blot analysis confirmed that genes for prochymosin and pepsinogen F are expressed only at neonatal/infant stages and the switching of gene expression to that of pepsinogen C occurred at late infant stages. A phylogenetic tree based on nucleotide sequences showed clearly that pepsinogens fall into four major groups, namely prochymosin and pepsinogen F of the neonate/infant and pepsinogens A and C of adult animals. Although, to date, prochymosin and pepsinogen F were believed to be expressed in only a limited number of mammals, the present results suggest that they might be expressed at the neonatal/infant stage in a variety of mammals.

Oxidative refolding of recombinant prochymosin

The disulphide-coupled refolding of recombinant prochymosin from Escherichia coli inclusion bodies was investigated. Prochymosin solubilized from inclusion bodies is endowed with free thiol groups and disulphide bonds. This partially reduced form undergoes renaturation more efficiently than the fully reduced form, suggesting that some native structural elements existing in inclusion bodies and remaining after denaturation function as nuclei to initiate correct refolding. This assumption is supported by the finding that in the solubilized prochymosin molecule the cysteine residues located in the N-terminal domain of the protein are not incorrectly paired with the other cysteines in the C-terminal domain. Addition of GSH/GSSG into the refolding system facilitates disulphide rearrangement and thus enhances renaturation, especially for the fully reduced prochymosin. Based on the results described in this and previous papers [Tang, Zhang and Yang (1994) Biochem. J. 301, 17-20], a model to depict the refolding process of prochymosin is proposed. Briefly, the refolding process of prochymosin consists of two stages: the formation and rearrangement of disulphide bonds occurs at the first stage in a pH11 buffer, whereas the formation and adjustment of tertiary structure leading to the native conformation takes place at the second stage at pH8. The pH11 conditions help polypeptides to refold in such a way as to favour the formation of native disulphide bonds. Disulphide rearrangement, the rate-limiting step during refolding, can be achieved by thiol/disulphide exchange initiated by free thiol groups present in the prochymosin polypeptide, GSH/GSSG or protein disulphide isomerase.

Effects of temperature and novobiocin on the expression of calf prochymosin gene and on plasmid copy number in recombinant Escherichia coli

Escherichia coli strain HB101 harboring an expression plasmid bearing calf prochymosin gene under the control of the tac promoter was grown in the presence of IPTG with or without novobiocin at 28 and 40 degrees C, respectively. The differential rates of synthesis of prochymosin inclusions, and, for comparison, of beta-lactamase and beta-galactosidase, as well as plasmid copy number, were determined during the first hours of steady state growth. At 28 degrees C the induced expression of prochymosin gene was almost blocked. Addition of novobiocin did not alleviate this effect. In fact, it strengthened it, and we conclude that both these additive inhibitory effects are a consequence of the decrease in negative superhelical tension of plasmid DNA to an insufficient level. At 40 degrees C the differential rate of prochymosin synthesis was markedly enhanced. Since the copy number of the expression plasmid increased approximately to the same extent, we conclude that an increase in gene dose is the cause. The stimulation of cloned heterologous gene expression at 40 degrees C and inhibition at 28 degrees C may be conveniently used in biotechnological-scale cultivations of some recombinant bacteria.

Protein engineering loops in aspartic proteinases: site-directed mutagenesis, biochemical characterization and X-ray analysis of chymosin with a replaced loop from rhizopuspepsin

The loop exchange mutant chymosin 155-164 rhizopuspepsin was expressed in Trichoderma reesei and exported into the medium to yield a correctly folded and active product. The biochemical characterization and crystal structure determination at 2.5 A resolution confirm that the mutant enzyme adopts a native fold. However, the conformation of the mutated loop is unlike that in native rhizopuspepsin and involves the chelation of a water molecule in the loop. Kinetic analysis using two synthetic peptide substrates (six and 15 residues long) and the natural substrate, milk, revealed a reduction in the activity of the mutant enzyme with respect to the native when acting on both the long peptide substrate and milk. This may be a consequence of the different charge distribution of the mutated loop, its increased size and/or its different conformation.

High-level expression of prochymosin in Escherichia coli: effect of the secondary structure of the ribosome binding site

Regulation of the expression of prochymosin cDNA in Escherichia coli at the translational level was studied by mutating the regions between the Shine-Dalgarno (SD) sequence and the initiation codon and upstream of the SD signal. Results revealed that expression plasmids with a distance of 7-10 bp from SD to ATG have the potential to be expressed at higher levels. However, an approximately 20-fold variation in expression was observed with plasmids harboring different base composition but identical distance in the spacer. Analysis of the predicted secondary structure of ribosome binding sites (RBS) indicates that the control of expression by base composition is mediated by the secondary structure of the RBS. An unfolded state of the RBS is required for high expression. Therefore, a vector for enhanced translation can be designed and constructed via prediction of the secondary structure of the proposed RBS and mutagenesis. Based on this strategy, high-level expression of prochymosin, up to 39% of the total cellular proteins, was achieved. The 9-base sequence proposed by Olins and Rangwala as a translational enhancer did not exhibit an additive effect on prochymosin expression. This is probably because the affinity of the SD sequence used in this study to 16S rRNA is strong enough that no additional element is required to facilitate the formation of the translation initiation complex.

Strain improvement of chymosin-producing strains of Aspergillus niger var. awamori using parasexual recombination

Parasexual recombination was used to obtain improved chymosin-producing strains and to perform genetic analysis on existing strains. Chlorate resistance was used to select for a variety of spontaneous nitrate assimilation pathway mutations in strains previously improved for chymosin production using classical strain improvement methods including mutation and screening, and selection for 2-deoxyglucose resistance (dgr). Diploids of these improved strains were generated via parasexual recombination and were isolated on selective media by complementation of nitrate assimilation mutations. A preliminary genetic analysis of diploid and haploid segregants indicated that the dgr trait, resulting in overexpression of chymosin, was recessive. Also, mutations in two different dgr genes resulted in an increased level of chymosin production. When these mutations were combined via parasexual recombination, the resulting haploid segregants produced about 15% more chymosin than either parental strain. CHEF gel electrophoresis was used to determine the chromosomal location of the integrated chymosin DNA sequences, and to verify diploidy in one case where the chromosome composition of two haploid parents differed.

Production of chymosin for the dairy industry by recombinant DNA technology

The increasing world production of cheese, coupled with a decline in the number of slaughtered calves, has stimulated a search for alternative sources of chymosin. This article briefly reviews microbial alternatives to chymosin and discusses chymosins produced using recombinant DNA technology. Recombinant chymosin represents one of the first successful applications of recombinant DNA technology in the food industry.

Secretion of calf chymosin from the filamentous fungus Aspergillus oryzae

Active calf chymosin was secreted from Aspergillus oryzae transformants when the chymosin cDNA was expressed under the control of glucoamylase gene (glaA) promoter. Secreted prochymosin was autocatalytically activated to the chymosin (0.07-0.16 mg/l). Western blot analysis showed that a secreted protein immunoreactive with an anti-chymosin antibody was of similar size to authentic chymosin. Northern blot analysis revealed that mRNA of the chymosin cDNA was expressed at as high level as that of the glaA gene. The size and the level of the transcript were different among transformants, due to the integration position of the plasmid on the chromosome.

Use of Aspergillus overproducing mutants, cured for integrated plasmid, to overproduce heterologous proteins

Aspergillus niger var. awamori was previously transformed with a vector designed to express a fused glucoamylase-prochymosin gene and bearing the Neurospora crassa pyr4 gene as a selectable marker. Mutant strains that overproduced the glucoamylase-prochymosin fusion protein were derived from one of the transformants. Despite the fact that the expression vector was integrated into the genome of these strains it was possible to obtain strains from which the vector sequences had been removed. This was performed by selection against the pyr4 gene present on the expression vector using 5-fluoroorotic acid. The cured strains were retransformed in order to investigate production of heterologous proteins using other expression vectors. In addition to the glucoamylase-prochymosin fusion protein, the mutant Aspergillus strains also overproduced Rhizomucor miehei aspartic proteinase but not preprochymosin produced as a non-fusion protein. The ability to select for loss of integrated plasmid from Aspergillus transformants may prove to be important for a variety of applications.

The yeast Kluyveromyces lactis as an efficient host for heterologous gene expression

Several different yeast species have been developed into systems for efficient heterologous gene expression. In this paper we review foreign gene expression in the dairy yeast Kluyveromyces lactis. This yeast presents several advantageous properties in comparison to other yeast species. These include its impressive secretory capacities, its excellent fermentation characteristics on large scale, its food grade status and the availability of both episomal and integrative expression vectors. Moreover, in contrast to the methylotrophic yeasts that are frequently used for the expression of foreign genes, K. lactis does not require explosion-proof fermentation equipment. Here, we present an overview of the available tools for heterologous gene expression in K. lactis (available promoters, vector systems, etc). Also, the production of prochymosin, human serum albumin and pancreatic phospholipase by K. lactis is discussed in more detail, and used to rate the achievements of K. lactis with respect to other micro-organisms in which these proteins have been produced.

Production of prochymosin, pepsinogen and progastricsin, and their cellular and intracellular localization in bovine abomasal mucosa

A brief overview of research is presented on the production, cellular and intracellular localization of prochymosin, pepsinogen and progastricsin in bovine abomasal mucosa from fetus to adult. Prochymosin is produced early during gestation (10th week) and is significantly related to milk-feeding. Pepsinogen and progastricsin start to be produced later during gestation (20th week) and are produced in low amounts as long as the calf is fed milk. With age, pepsinogen becomes the dominating zymogen in the abomasal mucosa. Most of the cell types in the fundic gland have the ability to produce all three zymogens and are also found in the same individual secretory granules of these cells.

Development of gastric proteases in fetal pigs and pigs from birth to thirty six days of age. The effect of adrenocorticotropin (ACTH)

Development of the synthesis and secretion of gastric proteases was studied in 55 Large White x Landrace pigs from 22 days before birth (93 days gestation) to 36 days of age. The pigs came from eight litters and were 0.4 - 13.5 kg body weight. Littermate pairs were treated with either saline or adrenocorticotropin (ACTH) from three days of age. Secretion of protease activity (milk-clotting and general proteolytic activity) was investigated in anaesthetized pigs by a gastric perfusion technique using intravenous infusion of pentagastrin at dose rates of 4 and 8 micrograms/h per kg body weight. In addition, concentrations of protease zymogens (prochymosin, pepsinogen A, progastricsin) were measured in fundic tissue extracts by rocket immunoelectrophoresis. Prochymosin was present in fundic tissue at 22 days before birth, reached peak concentrations at birth and decreased in concentration during the subsequent 36 days. Pepsinogen A and progastricsin were absent or present in trace amounts in the first week after birth, but thereafter concentrations of both zymogens increased rapidly. Development of the pentagastrin-stimulated secretion of protease activity reflected the changes of zymogen concentrations in fundic tissue. Chronic treatment of pigs with ACTH from three days of age significantly increased the concentration of prochymosin in fundic tissue at 9-11 days and the concentrations of pepsinogen A and progastricsin at 34-36 days of age. Hormones such as ACTH and glucocorticoids may therefore play a regulatory role in the ontogeny of porcine gastric proteases.

Analysis of the haemolysin transport process through the secretion from Escherichia coli of PCM, CAT or beta-galactosidase fused to the Hly C-terminal signal domain

Secretion of haemolysin (HlyA) is secA independent, but depends upon two accessory membrane proteins, HlyB and HlyD, encoded by the hly determinant. A fourth (cytoplasmic) protein, HlyC, is required to activate HlyA post-translationally, but has no role in export. Deletion studies have previously shown that the HlyA molecule contains a targeting signal close to the C-terminus which specifically directs its secretion to the medium. This targeting signal has been variously located within the terminal 27, 53, 60 or 113 amino acids. In this paper, we have sought to confirm the presence of a C-terminal targeting signal and to analyse the specificity of the Hly transport system through fusion of C-terminal fragments of HlyA to heterologous polypeptides. A C-terminal fragment (23 kDa) of HlyA, when fused at the C-terminus, efficiently promoted the secretion of the eukaryotic protein prochymosin (PCM) to the medium via HlyB and HlyD. This result is in contrast to previous findings that prochymosin, preceded by the alkaline phosphatase signal sequence, cannot be translocated across the Escherichia coli inner membrane. The HlyA targeting domain was also used to secrete to the medium varying portions of chloramphenicol acetyltransferase (CAT) and 98 per cent of the beta-galactosidase (LacZ) molecule (both E. coli cytoplasmic proteins). In the case of the PCM and CAT fusions the efficiency of secretion was reduced as the proportion of the PCM and CAT molecule increased. This result is consistent with inhibition of secretion through the irreversible folding of the larger passenger protein fragments, or the occlusion of the HlyA targeting signal by upstream sequences. Analysis of the nature of the C-terminal domain promoting secretion of prochymosin, demonstrated that shortening the signal domain from 218 to 113 amino acids significantly reduced the efficiency of secretion. This result may also reflect the importance of maintaining an independently folded signal motif well separated from a passenger domain.

Commercial Levels of Chymosin Production by Aspergillus

We have increased the production of bovine chymosin in Aspergillus niger var. awamori to more than one gram per liter of secreted authentic enzyme by combining a mutagenesis protocol with a novel robotic screening program. Analysis of the superior chymosin producing strains indicated that they have enhanced capabilities to secrete extracellular proteins.

Expression of recombinant calf prochymosin in mammalian cell culture

The calf preprochymosin cDNA was cloned into an extrachromosomal mammalian cell expression vector containing Epstein-Barr virus sequences using polymerase chain reaction. Transfection of HeLa cells yielded Hygromycin B resistant cell clones, expressing immunoreactive prochymosin, which was quantitatively secreted into the culture medium. Based on Western blotting we estimated that selected cell clones produced about 10-20 mg prochymosin per liter in 20 h. The biological activity of the secreted chymosin was confirmed by milk clotting assay.

Use of ars18 based vectors to increase protein production in Yarrowia lipolytica

The isolation of ars sequence from the yeast Yarrowia lipolytica has recently been reported (Fournier et al., 1991). Vectors containing ars18 have been used to increase homologous and heterologous protein production. Examples presented are the Yarrowia lipolytica alkaline extracellular protease (AEP), the porcine alpha 1-interferon and the bovine prochymosin. A 2- to 6-fold increase in the corresponding protein production was observed and in several cases it was established that it corresponded to the copy number of plasmid in the cell.

Prochymosin expression in Bacillus subtilis

Prochymosin (PC) sequence was cloned in Bacillus subtilis using two kinds of plasmid constructions. In plasmid pSM316 the cDNA was inserted to obtain the intracellular expression of the enzyme. The enzyme turned out to be expressed in an insoluble form which could be converted to native enzyme under proper denaturing and refolding conditions. The levels of intracellular expression of PC were further enhanced by modifying the 5' region of the gene in a way that a two-cistron expression system was created. For the PC secretion, the cDNA was fused to the subtilisin leader sequence and expressed under the control of the B. subtilis neutral protease promoter. A properly folded PC was secreted by the cells, although to low levels.

Improved production of chymosin in Aspergillus by expression as a glucoamylase-chymosin fusion

We have extended the work on chymosin production in Aspergillus by constructing an expression vector in which the cDNA encoding bovine prochymosin B was fused in frame immediately following the codon for the last amino acid of the A. awamori glucoamylase (glaA) gene. Transformation of A. awamori with this plasmid led to the secretion of considerably higher amounts of chymosin than obtained with previous chymosin expression vectors. We present evidence that mature chymosin is autocatalytically released from the glucoamylase-chymosin fusion protein after secretion.

Effects of glycosylation on the secretion and enzyme activity of Mucor rennin, an aspartic proteinase of Mucor pusillus, produced by recombinant yeast

The Mucor rennin gene encoding a prepro form of the fungal aspartic proteinase from Mucor pusillus was expressed under the control of the yeast GAL7 promoter in Saccharomyces cerevisiae. The mature M. pusillus rennin secreted efficiently by yeast was a highly glycosylated protein. Analysis by a combination of site-directed mutagenesis of each of the three possible glycosylation sites and treatment of the secreted M. pusillus rennins with endo-beta-N-acetylglucosaminidase H revealed that the mature yeast M. pusillus rennin contained two asparagine-linked glycosylation sites among the three possible glycosylation sites. A mutation of the 2 glycosylated asparagine residues of M. pusillus rennin resulted in significant decreases in the level of secretion by yeast cells. In addition, the extent of glycosylation of M. pusillus rennin was found to affect the enzyme properties such as milk-clotting and proteolytic activities.

Kluyveromyces as a Host for Heterologous Gene Expression: Expression and Secretion of Prochymosin

We have developed the yeast Kluyveromyces lactis as a host organism for the production of the milk-clotting enzyme chymosin. In contrast to Saccharomyces cerevisiae, we found that this yeast is capable of the synthesis and secretion of fully active prochymosin. Various signal sequences could be used to efficiently direct the secretion of prochymosin in Kluyveromyces, but not in S. cerevisiae. We conclude that the efficient synthetic and secretory capacity of this heterologous protein is a property of the yeast Kluyveromyces. These results have led to the development of a large scale production process for chymosin.

Prochymosin activation by non-aspartic proteinases

Prochymosin can be converted into chymosin by an action of external proteinases. Thus, thermolysin at pH 5.05 converts calf prochymosin into active Phe-chymosin, which is one amino acid longer than chymosin from the N-terminus with a yield of 73%. Even better results were achieved with prochymosin activation by Legionella pneumophila metalloproteinase. Apparently the stretch of prochymosin polypeptide chain adjacent to the normally observed activation point becomes available for an attack by an external proteinase at pH 5.0-6.0. These data indicate that the intermolecular activation pathway might be of physiological importance.

A short N-proximal region of prochymosin inhibits the secretion of hybrid proteins from Escherichia coli

A gene encoding bovine prochymosin (PC) was fused to the coding sequence (phoA) for the Escherichia coli alkaline phosphatase (AP) signal peptide and expressed in E. coli under the control of the phoA promoter. Upon induction, an AP-PC fusion protein was produced which was neither processed nor exported into the periplasm. We investigated this lack of secretion by constructing a series of gene fusions in which different regions of the PC gene were inserted between the coding regions of the AP leader and mature protein. Analysis of the cellular location of the proteins encoded by these fusions revealed that a region of PC (between amino acids 6 and 29) prevented processing and secretion of an AP-PC fusion when inserted near to the AP signal peptide. In contrast, when this 'blocking sequence' was inserted elsewhere in AP the hybrid proteins were efficiently processed and translocation was initiated.

Secretion by yeast of the zymogen form of Mucor rennin, an aspartic proteinase of Mucor pusillus, and its conversion to the mature form

The Mucor rennin gene encoding a prepro-form of the fungal aspartic proteinase from Mucor pusillus was expressed under the control of the yeast GAL7 promoter in Saccharomyces cerevisiae. An inactive zymogen of the enzyme with the 44-amino-acid pro-sequence was identified in the medium during the initial stage of cultivation. Processing of the purified zymogen to the mature enzyme proceeded autocatalytically under the acidic conditions. The rate of processing was accelerated by an increase in the concentration of the zymogen or addition of the mature enzyme. The in vitro processing was inhibited by inhibitors for the aspartic proteinases. The zymogen with no proteinase activity due to a mutation at the active site residue, Asp, was still processed at a relatively slower rate in a wild-type strain of yeast, but no processing occurred in the pep4-3 mutant strain of S. cerevisiae deficient in yeast proteinase A. Thus, Mucor rennin is excreted in a form of zymogen, which is then processed in the yeast secretion pathway mainly by the autocatalytic proteolysis but, alternatively, by a proteinase of yeast.

Alteration of catalytic properties of chymosin by site-directed mutagenesis

Artificial mutations of chymosin by recombinant DNA techniques were generated to analyze the structure--function relationship in this characteristic aspartic proteinase. In order to prepare the mutant enzymes in their active form, we established procedures for purification of correctly refolded prochymosin from inclusion bodies produced in Escherichia coli transformants and for its subsequent activation. Mutagenesis by linker insertion into cDNA produced several mutants with an altered ratio of milk clotting activity to proteolytic activity and a different extent of stability. In addition to these mutants, several mutants with a single amino acid exchange were also constructed by site-directed mutagenesis and kinetic parameters of these mutant enzymes were determined by using synthetic hexa- and octa-peptides as substrates. Exchange of Tyr75 on the flap of the enzyme to Phe caused a marked change of substrate specificity due to the change of kcat or Km, depending on the substrate used. Exchange of Val110 and Phe111 also caused a change of kinetic parameters, which indicates functional involvement of these hydrophobic residues in both the catalytic function and substrate binding. The mutant Lys220----Leu showed a marked shift of the optimum pH to the acidic side for hydrolysis of acid-denatured haemoglobin along with a distinct increase in kcat for the octa-peptide in a wide pH range.

Complete secretion of activable bovine prochymosin by genetically engineered L forms of Proteus mirabilis

To circumvent problems encountered in the synthesis of active chymosin in a number of bacteria and fungi, a recombinant DNA L-form expression system that directed the complete secretion of fully activable prochymosin into the extracellular culture medium was developed. The expression plasmid constructions involved the in-frame fusion of prochymosin cDNA minus codons 1 to 4 to streptococcal pyrogenic exotoxin type A gene (speA') sequences, including the speA promoter, ribosomal binding site, and signal sequence and five codons of mature SpeA. Secretion of fusion prochymosin enzymatically and immunologically indistinguishable from bovine prochymosin was achieved after transformation of two stable protoplast type L-form strains derived from Proteus mirabilis. The secreted proenzyme was converted by autocatalytic processing to chymosin showing milk-clotting activity. In controlled laboratory fermentation processes, a maximum specific rate of activable prochymosin synthesis of 0.57 x 10(-3)/h was determined from the time courses of biomass dry weight and product formation. Yields as high as 40 +/- 10 micrograms/ml were obtained in the cell-free culture fluid of strain L99 carrying a naturally altered expression plasmid of increased segregational stability. The expression-secretion system described may be generally useful for production of recombinant mammalian proteins synthesized intracellularly as aberrantly folded insoluble aggregates.

Fragments of prochymosin produced in Escherichia coli form insoluble inclusion bodies

Bovine prochymosin produced in Escherichia coli has been used as a model system to investigate factors which may cause a recombinant protein to accumulate as insoluble inclusion bodies. A series of plasmids was constructed to investigate the effect of deletions within the prochymosin-coding sequence on protein inclusion body formation. The results demonstrated that as much as 70% of the prochymosin-coding sequence could be deleted with no significant reduction in the accumulation of insoluble protein. The smallest deletion product identified (11,000 molecular weight) retained only one cysteine, yet this product still accumulated as an insoluble product in E. coli.

Milk-feeding maintains the prochymosin production in cells of bovine abomasal mucosa

The effect of milk-feeding on the development of prochymosin-producing cells in bovine abomasal mucosa was studied using immunochemical methods. As long as the calf was fed milk both mucous neck, and chief cells in the gastric glands, were immunoreactive for prochymosin, the precursor form of a specific milk-clotting enzyme. After weaning, the prochymosin immunoreactivity in chief cells in the lower base of the gastric gland declined and finally disappeared. Mucous neck cells and chief cells in the upper base of the gland, however, continued to be immunoreactive for prochymosin. All these cell types contained pepsinogen immunoreactivity both before and after weaning. Exposure to milk components thus seem to be of importance for the production of prochymosin in the abomasal mucosa. The present findings suggest that dietary components influence the cell differentiation in the gastrointestinal mucosa.

Expression of calf prochymosin in Saccharomyces cerevisiae

A yeast strain which synthesizes activatable calf prochymosin (also known as prorennin) has been constructed by transformation with a vector carrying the methionyl-prochymosin coding sequence attached to efficient yeast transcriptional promoter and terminator sequences. Cloned preprochymosin cDNA was altered by restriction endonuclease cleavage and addition of a synthetic oligonucleotide to yield a DNA sequence encoding methionyl-prochymosin. This methionyl-prochymosin gene was ligated to a yeast chromosomal fragment containing the GAL1 promoter, and the construction was placed in an Escherichia coli-Saccharomyces cerevisiae shuttle vector with or without a transcriptional terminator DNA fragment from the yeast SUC2 gene. In yeast the two constructions result in equal amounts of prochymosin protein and mRNA. The prochymosin from yeast is activatable to chymosin by incubation at low pH and exhibits milk-clotting activity indistinguishable from calf chymosin.

Cloning in Escherichia coli of the structural gene of prorennin, the precursor of calf milk-clotting enzyme renin

Double-stranded cDNA was prepared from prorennin-specific mRNA by sequential actions of reverse transcriptase, DNA polymerase and S1 nuclease, and inserted into the Sa/I site of pBR322 by the poly(dG)-(dC) annealing method. Transformation of Escherichia coli C600 r- m- by the hybrid plasmid yielded transformants containing prorennin cDNA. The presence of the cDNA sequence in these clones was confirmed by both colony hybridization and hybrid-arrested translation of the mRNA in vitro. The largest size of the cloned cDNA was 1,020 bp.

Production and isolation of milk-clotting enzyme from Aspergillus versicolor

The production of a milk-clotting enzyme by Aspergillus versicolor in 19 different culture media was investigated. Considerable milk-clotting activity was achieved by supplying corn steep liquor with either glucose of maltose. Dephytinization of corn steep liquor had an adverse effect on the production of milk-clotting enzyme. The results indicated that complex organic compounds favoured the production of the enzyme. Precipitation with acetone or tannin was unsuitable, but ammonium sulphate and ethanol above certain concentration produced active fractions.