Cloning and expression of Hormoconis resinae glucoamylase P cDNA in Saccharomyces cerevisiae

Cloning and expression of Penicillium minioluteum dextranase in Saccharomyces cerevisiae and its exploitation as a reporter in the detection of mycotoxins

A dextranase gene from Penicillium minioluteum (strain IMI068219) has been cloned, sequenced and expressed in Saccharomyces cerevisiae via fusion of the DNA segment encoding the mature dextranase protein with alpha-factor signal sequence, and insertion into the GAL1-controlled expression vector pYES2/CT. Galactose-induced expression yielded extracellular dextranase activity of 0.63 units/ml and cell-associated dextranase activity of 0.48 units/ml, after 24 h incubation. The dextranase construct was introduced into a strain of S. cerevisiae expressing the human cytochrome P450 3A4 (CYP3A4) and the cognate reductase, which was then used to develop a microplate toxicity bioassay. Toxicity was signalled as inhibition of dextranase activity, assayed fluorimetrically. This novel bioassay was assessed using six economically significant mycotoxins.

Expression of Aureobasidium pullulans xynA in, and secretion of the xylanase from, Saccharomyces cerevisiae

A previous report dealt with the cloning in Escherichia coli and sequencing of both the cDNA and genomic DNA encoding a highly active xylanase (XynA) of Aureobasidium pullulans (X.-L. Li and L. G. Ljungdahl, Appl. Environ. Microbiol. 60:3160-3166, 1994). Now we show that the gene was expressed in Saccharomyces cerevisiae under the GAL1 promoter in pYES2 and that its product was secreted into the culture medium. S. cerevisiae clone pCE4 with the whole open reading frame of xynA, including the part coding for the signal peptide, had xylanase activity levels of 6.7 U ml-1 in the cell-associated fraction and 26.2 U ml-1 in the culture medium 4 h after galactose induction. Two protein bands with sizes of 25 and 27 kDa and N-terminal amino acid sequences identical to that of APX-II accounted for 82% of the total proteins in the culture medium of pCE4. These proteins were recognized by anti-APX-II antibody. The results suggest that the XynA signal peptide supported the posttranslational processing of xynA product and the efficient secretion of the active xylanase from S. cerevisiae. Clones pCE3 and pGE3 with inserts of cDNA and genomic DNA, respectively, containing only the mature enzyme region attached by a Met codon had low levels of xylanase activity in the cell-associated fractions (1.6 U ml-1) but no activity in the culture media. No xylanase activity was detected in clone pGE4, which was the same as pCE4, except that pGE4 had a 59-bp intron in the signal peptide region. A comparison of the A. pullulans and S. cerevisiae signal peptides demonstrated that the XynA signal peptide was at least three times more efficient than those of S. cerevisiae invertase or mating alpha-factor pheromone in secreting the heterologous xylanase from S. cerevisiae cells.

Efficient expression and secretion of Aspergillus niger RH5344 polygalacturonase in Saccharomyces cerevisiae

An Aspergillus niger endopolygalacturonase (EC 3.2.1.15) cDNA was expressed in the yeast Saccharomyces cerevisiae. Secretion of the protein into the growth medium was efficiently directed by the fungal leader sequence, and processing occurred at the same site as in Aspergillus. The expression level was significantly enhanced by using a "short" version of the yeast ADHI promoter. An additional increase in the yield of heterologous protein was due to a higher plasmid stability and a rise in plasmid copy number. This was achieved by deleting most of the bacterial sequences from the expression vector. The yeast-derived enzyme showed the same enzymatic and biochemical properties as the fungal polygalacturonase, such as substrate specificity, pH and temperature optima and pI value. The yeast-derived enzyme, however, showed a higher degree of glycosylation and exhibited a more pronounced temperature stability than the fungal enzyme.

Construction by one-step gene replacement of Trichoderma reesei strains that produce the glucoamylase P of Hormoconis resinae

Two one-step gene replacement vectors containing either the Hormoconis resinae glucoamylase P (gamP) genomic gene or the corresponding cDNA, each under the control of the promoter of the Trichoderma reesei cellobiohydrolase 1 gene (cbh1), were constructed and used to replace the cbh1 gene in a T. reesei strain. In both vectors the cbh1 promoter is precisely fused to the gamP protein coding region. Both the gamP cDNA and the genomic gene direct the secretion of the active glucoamylase P (GAMP) enzyme from T. reesei, which indicates that the intron sequences in the genomic gamP gene are processed in T. reesei. According to the results, a T. reesei transformant strain, in which the cbh1 gene has been replaced by a single copy of the gamP genomic gene, secretes more active GAMP than does a transformant strain having three copies of the cDNA clone in tandem orientation at the cbh1 locus.

Effect of upstream sequences of the ADH1 promoter on the expression of Hormoconis resinae glucoamylase P by Saccharomyces cerevisiae

The effect of the upstream sequences of the yeast ADH1 promoter on the expression of Hormoconis resinae glucoamylase P by Saccharomyces cerevisiae was studied. Sequence analysis of the 5'-terminal region of the promoter revealed sequence patterns resembling a transcription start point and the binding site for the regulatory protein ADR1. A short promoter was constructed by deleting all the promoter sequences upstream of nucleotide -409, including the upstream activating sequence UASRPG. A medium-length promoter was constructed by deleting a fragment of 558 bp containing the putative upstream transcription start point but not the UAS. The short promoter increased the glucoamylase expression level 1.6-fold compared with the long promoter, but the beginning of secretion was delayed by about 10 h probably because of the absence of the UAS. The medium-length promoter directed expression of the glucoamylase without an initial delay, with the enzyme activity lying between the activities produced under the long and short promoters. Northern blot analysis confirmed the secretion patterns of the strains with different promoters but failed to reveal any transcripts starting at the putative upstream start point.

Purification and specificity of recombinant Hormoconis resinae glucoamylase P and endogenous glucoamylase from Trichoderma reesei

Hormoconis resinae glucoamylase P of high debranching activity was purified from a recombinant Trichoderma reesei strain. Four different purified fractions were obtained. Three had the same amino terminal sequence as the wild-type enzyme and about the same specific activity, and yielded the same single band on SDS-PAGE after deglycosylation. Presumably they resulted from different glycosylation patterns of the recombinant glucoamylase P. One fraction had a much lower specific activity and yielded tryptic peptides that identified it as the host cellobiohydrolase I contaminated with glucoamylase P. The different glycosylation patterns of recombinant glucoamylase P had only minor effects on its thermal inactivation. During purification of the recombinant glucoamylase, a protein with lower debranching activity was found and purified by chromatofocusing to homogeneity as assessed by SDS-PAGE. It had a pI of about 4.0 and a ratio of pullulan- to starch-degrading activity of 15%. Its amino terminal sequence showed 60% identity to the amino terminal sequence of glucoamylases P and S from Hormoconis resinae. Presumably this enzyme is the endogenous glucoamylase of Trichoderma reesei.

Secretion of the Hormoconis resinae glucoamylase P enzyme from Trichoderma reesei directed by the natural and the cbh1 gene secretion signal

Secretion of the Hormoconis resinae glucoamylase P (GAMP) enzyme from Trichoderma reesei using either the natural N-terminal extension of the premature glucoamylase P or the cellobiohydrolase I (CBHI) signal peptide was examined. The expression conditions for the heterologous glucoamylase P (gamP) gene in T. reesei were standardized by targeting one copy of a plasmid fragment, containing the gamP gene, to the cbh1 locus of the host. The results showed that the transient N-terminal extension of the premature GAMP acts as an efficient secretion signal in T. reesei and leads to a higher yield of extracellular glucoamylase activity than does the signal peptide of CBHI.