Enzymatic properties and processing of bovine prochymosin synthesized in Escherichia coli

Fermentation conditions for high-level expression of the tac-promoter-controlled calf prochymosin cDNA in Escherichia coli HB101

Escherichia coli HB101 harboring an expression plasmid that bears the calf prochymosin gene controlled by the tac promoter was cultivated under different conditions in order to find an optimal fermentation arrangement that would lead to maximal prochymosin yield. Our results indicate that it is advantageous to use lactose in the double role of inducer and carbon/energy source when foreign gene expression is controlled by the tac promoter and the gene product is only moderately toxic owing to its accumulation in the form of an intracellular body. Glucose, on the other hand, may be used when expression should be repressed. Growth temperature substantially influenced the specific rate of prochymosin and beta-lactamase gene expression and the plasmid copy number. Three phases were distinguished in the time course of the fermentation on lactose: exponential growth practically without prochymosin synthesis, linear growth with prochymosin synthesis, and prochymosin synthesis without growth of biomass. The synthesis of prochymosin in the form of intracellular inclusion body was accompanied by the loss of respiratory activity of the cell and the loss of its ability to multiply. Sixteen hours cultivation at 37 degrees C in a complex medium with lactose as inducer and carbon/energy source resulted in up to 30% of the volume and 48% of the total protein of biomass being accumulated for as prochymosin inclusion bodies. The concentration of extractable enzymatically active chymosin in the culture reached 12 mg/L.

Recombinant production and purification of novel antisense antimicrobial peptide in Escherichia coli

A fusion protein was genetically engineered that contains an antimicrobial peptide, designated P2, at its carboxy terminus and bovine prochymosin at its amino terminus. Bovine prochymosin was chosen as the fusion partner because of its complete insolubility in Escherichia coli, a property utilized to protect the cells from the toxic effects of the antimicrobial peptide. This fusion protein was purified by centrifugation as an insoluble inclusion body. A methionine linker between prochymosin and the P2 peptide enabled P2 to be released by digestion with cyanogen bromide. Cation exchange HPLC followed by reversed-phase HPLC were used to purify the P2 peptide. The recombinant P2 peptide's molecular mass was confirmed by mass spectrometry to within 0.1% of the theoretical value (2480.9 Da), and the antimicrobial activity of the purified recombinant P2 against E. coli D31 was determined to be identical to that of the chemically synthesized peptide (minimal inhibitory concentration of 5 mg/mL). Although the yield of the fusion protein after expression by the cells was high (16% of the total cell protein), the percentage recovery of the P2 peptide in the inclusion bodies was relatively low, which appears to be due to losses in the cyanogen bromide digestion step.

Prochymosin polymorphism in calves of black-and-white cattle and their crosses with Simental bulls

Polymorphism of prochymosin was observed in individual calf abomasa, using agarose gel electrophoresis followed by detection of proteolytic activity. Abomasum samples were randomly collected during slaughtering from 239 and 146 calves (3-5 weeks old) of Black-and-White cattle and their crosses with Simental bulls, respectively. Four distinct prochymosins were found and, according to their decreasing electrophoretic mobility in alkaline agarose gel, termed as prochymosin A, D, B and C which occurred singly and in pairs (then with equal proteolytic activities of both components). Prochymosin A, B and C (designation according to FOLTMANN, 1966) activated at pH 4.7 was transformed into electrophoretically distinct chymosin. When prochymosin D was activated at this pH, chymosin D showed similar mobility as chymosin B both at alkaline and acidic pHs. Prochymosin variants occurred at genetical equilibrium in nine and ten phenotypes in the first and second genetic group. The distribution of phenotypes in the two groups differed significantly (P < 0.05). The gene frequencies of prochymosin A, D, B and C were 0.35, 0.11, 0.52 and 0.02 in Black-and-White calves, and 0.39, 0.08, 0.47 and 0.06 in crosses, respectively. These prochymosins were controlled by four pairs of codominant alleles. A possible correlation of the results obtained by FOLTMANN (1966) with ours and those of ASATO and RAND (1972, 1977) was discussed.

Isolation of extracytoplasmic proteins from Serpulina hyodysenteriae B204 and molecular cloning of the flaB1 gene encoding a 38-kilodalton flagellar protein

Extracytoplasmic proteins were released from Serpulina (Treponema) hyodysenteriae (strain B204) by treatment of whole cells with a nonionic detergent (Tween 20). Centrifugation of the Tween 20-released proteins at 100,000 x g sedimented 10 major extracytoplasmic proteins with approximate molecular masses of 44, 43.5, 42, 39, 38, 34, 33.5, 33, 31, and 29 kDa. Treatment of the sedimented fraction with 6 M urea solubilized all of the proteins except the 39-kDa protein. Peptide sequences were obtained for the purified 42-, 39-, 38-, 34-, 31-, and 29-kDa proteins. The peptide sequences of the 42-, 38-, and 31-kDa proteins indicate that they likely are components of the periplasmic flagella. The amino-terminal peptide sequence of the 38-kDa protein was used to design an oligonucleotide probe and to clone an S. hyodysenteriae DNA fragment containing the gene encoding this protein. The predicted 290-amino-acid protein sequence derived from the cloned gene was highly homologous to those of several other bacterial flagellar proteins and is preceded by consensus sigma D nucleotide sequences found upstream of other flagellar genes. On the basis of its similarity to the FlaB proteins of other spirochetes, we propose to designate the cloned S. hyodysenteriae gene flaB1 and its encoded protein FlaB1. Vaccination of pigs with FlaB1 or its recombinant counterpart did not protect them from an experimental challenge.

Occurrence of prochymosin variants in the abomasum of bovine foetuses and calves

Prochymosin variants were analysed in single abomasa from 67 foetuses (3-9 months of gestation) and 33 calves (about 6 weeks old) of Black-and-White cattle collected in a slaughter house. The method of agarose gel electrophoresis followed by detection of proteolytic activity was used. Three distinct prochymosins A, B and C that occurred singly or in pairs (with equal proteolytic activities of both components) were found. Chymosin A, B and C obtained after conversion of corresponding prochymosins, demonstrated similar electrophoretical mobilities like the three chymosin fractions contained in commercial rennin (Sigma, USA). Our chymosin B showed identical mobility as the amidated form of recombined chymosin B contained in Chymogen (Chr. Hansen's Lab., Denmark A/S). Prochymosin A, B and C in the examined animals were precursors of corresponding chymosins and were controlled by three separate codominant alleles. The following prochymosin phenotypes were found: AA (30), AB (32), AC (4), BB (29), BC (4) and CC (1). Chi-square analysis demonstrated significant differences between the observed and expected numbers of phenotypes. The gene frequencies of prochymosin A, B and C were 0.48, 0.47 and 0.05, respectively.

Expression and characterisation of chymosin pH optima mutants produced in Trichoderma reesei

The production of chymosin mutants designed to have altered pH optima using the cellulolytic filamentous fungus Trichoderma reesei is described. The strong promoter of the gene encoding the major cellulase, cellobiohydrolase I (CBHI) has been used for the expression and secretion of active calf chymosin. Structural analysis of the hydrogen bonding network around the two active site aspartates 32 and 215 in chymosin have suggested that residues Thr 218 and Asp 303 may influence the rate and pH optima for catalysis. The chymosin mutants Thr218Ala and the double mutant Thr218Ala/Asp303Ala have been made by site-directed mutagenesis and expressed in T. reesei. Enzyme kinetics of the active enzyme T218A indicate a pH optimum of 4.2 compared to 3.8 for native chymosin B using a synthetic octa-peptide substrate, confirming the previous analysis undertaken in E. coli. The double mutant T218A/D303A exhibits a similar optimum of 4.4 to that reported for the D303A, indicating that the combination of these changes is not additive. The application of protein engineering in the rational design of specific modifications to tailor the properties of enzymes offers a new approach to the development of industrial processes.

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.

Expression and properties of prochymosin derivatives containing extensions of various length in the pro-part

A series of hybrid prochymosin derivatives containing portions of the simian virus 40 small-t antigen in the pro-part was constructed. Portions comprising 93, 63, 47, 12, and 1 amino acid (aa) from the N terminus of the small-t antigen were separately fused via eight polylinker-encoded amino acids to a prochymosin product commencing with the 5th aa of the pro-part. All the DNAs coding for the hybrid proteins were put under pL-promoter control in the expression constructs. Expression revealed that only fusion of the 47-aa or 12-aa stretch of the small-t antigen to prochymosin gave stable protein products and that only the latter one allowed the hybrid prochymosin to be activated to chymosin. The products containing 93 aa and 63 aa of small-t antigen were unstable and degraded. Complete removal of the small-t antigen portion led to mRNA instability, probably owing to inefficient initiation of translation.

Characterization of a surface antigen of Eimeria tenella sporozoites and synthesis from a cloned cDNA in Escherichia coli

An antigenic surface protein of Eimeria tenella sporozoites has been identified that is the target of two neutralizing monoclonal antibodies Ptn 7.2A4/4 and Ptn 9.9D12. The antigen as isolated from the parasite is composed of a 17 kDa polypeptide and a 8 kDa polypeptide linked by a disulfide bridge. De novo synthesis of the antigen does not begin until approximately 16-20 h after the initiation of oocyst sporulation. A cDNA library was constructed using mRNA from sporulated oocysts and a clone encoding the antigen was isolated. The Ta4 gene encodes a single polypeptide of 25 kDa which contains the 17 and 8 kDa polypeptides. The protein has been synthesized in Escherichia coli either directly or as part of a beta-galactosidase fusion protein. The products synthesized in E. coli are single polypeptides and are not cleaved to two polypeptides as is seen in the parasite. The products accumulate in bacteria in an insoluble form which can be solubilized and renatured to an immunoreactive form.

Unusual zymogen-processing properties of a mutated form of prochymosin

Site-specific mutagenesis of the gene encoding bovine prochymosin was used to produce a mutated zymogen in which seven contiguous amino acids of the N-terminal propeptide had been deleted and an eighth residue had been substituted. This altered region spans the normal site of autocatalytic proteolysis that occurs at the same time as (enzymatic) activation of prochymosin at acidic pH. Activation of the mutated zymogen at pH 4.5 was extremely slow, and cleavage occurred at an unusual Ser-Lys bond in the propeptide of the zymogen. The mutated prochymosin incubated at pH 2 generated the usual pseudochymosin by cleavage of the normal Phe-Leu bond, but at a rate severalfold slower than the authentic zymogen. These results indicate that even after deletion of seven of 42 amino acids of the propeptide the mutant protein could still assume a prochymosin (zymogen) structure, although these changes did result in striking differences in acid-catalyzed activation and processing reactions at one but not the other of the two processing sites of prochymosin.

The nucleotide sequence of the pepN gene and its over-expression in Escherichia coli

The complete nucleotide sequence has been determined for the pepN gene of Escherichia coli K-12. The product of this gene, peptidase N, is apparently 870 amino acids in length. The coding sequence is followed by a tandem pair of stop codons and then a sequence capable of forming a stem-and-loop structure in the pepN mRNA. In the process of subcloning the pepN gene we constructed a plasmid which causes peptidase N to be produced at a level of 50% of total protein. The peptidase is fully active and completely soluble and these overproducing cells appear otherwise normal.