Induction and glucose repression of xylanase from a color variant strain ofAureobasidum pullulans

Production of xylanolytic enzymes by Moesziomyces spp. using xylose, xylan and brewery's spent grain as substrates

Xylanases play a crucial role in the hydrolysis of xylan-rich hemicelluloses and have wide industrial applications in the fuel, food, feed and pulp and paper industries. The production of these enzymes at low cost is of paramount importance for their commercial deployment. Moesziomyces antarcticus PYCC 5048^T and M. aphidis PYCC 5535^T were screened for their ability to produce xylanolytic enzymes when grown on d-xylose, xylan (beechwood) and brewery's spent grain (BSG). The extracellular crude extracts produced were characterized and tested in xylan hydrolysis. The yeasts produced xylanolytic enzymes without cellulolytic activity on all the substrates tested. The highest xylanase volumetric activity was obtained with M. aphidis PYCC 5535^T grown on BSG, reaching 518.2 U/ml, a value 8.4- and 4.7-fold higher than those achieved on xylan and d-xylose, respectively. The xylanase activities were characterized in relation to pH and temperature with optima at 4.5 and 50 °C, respectively. The extracts from both M. antarcticus PYCC 5048^Tand M. aphidis PYCC 5535^T were used in xylan hydrolysis, producing d-xylose as the major end product (0.43 and 0.34-0.47 g_D-xylose/g_xylan, respectively, at 50 °C) and relatively low or no xylobiose accumulation (from no detection to 0.12 g_D-xylobiose/g_xylan at 50 °C).

Effect of cultivation pH and agitation rate on growth and xylanase production by Aspergillus oryzae in spent sulphite liquor

The effects of cultivation pH and agitation rate on growth and extracellular xylanase production by Aspergillus oryzae NRRL 3485 were investigated in bioreactor cultures using spent sulphite liquor (SSL) and oats spelts xylan as respective carbon substrates. Xylanase production by this fungus was greatly affected by the culture pH, with pH 7.5 resulting in a high extracellular xylanase activity in the SSL-based medium as well as in a complex medium with xylan as carbon substrate. This effect, therefore, was not solely due to growth inhibition at the lower pH values by the acetic acid in the SSL. The xylanase activity in the SSL medium peaked at 199 U ml(-1) at pH 7.5 with a corresponding maximum specific growth rate of 0.39 h(-1). By contrast, the maximum extracellular beta-xylosidase activity pf 0.36 U ml(-1) was recorded at pH 4.0. Three low molecular weight xylanase isozymes were secreted at all pH values within the range of pH 4-8, whereas cellulase activity on both carbon substrates was negligible. Impeller tip velocities within the range of 1.56-3.12 m s(-1) had no marked effect, either on the xylanase activity, or on the maximum volumetric rate of xylanase production. These results also demonstrated that SSL constituted a suitable carbon feedstock as well as inducer for xylanase production in aerobic submerged culture by this strain of A. oryzae.

β-Methyl-xyloside: positive effect on xylanase induction in Cellulomonas flavigena

Synthesis of extracellular xylanase in Cellulomonas flavigena is induced in the presence of xylan and sugarcane bagasse as substrates. The essential factors for efficient production of xylanase are the appropriate medium composition and an inducing substrate. The increase in xylanase production levels in C. flavigena were tested with a number of carbon sources and different culture conditions. Xylose, arabinose, glycerol and glucose did not induce xylanase production in this microorganism. beta-Methyl-xyloside (beta-mx), a structural analog of xylobiose, also did not induce xylanase when used as the sole carbon source, but when xylan or sugar cane bagasse was supplemented with beta-mx, extracellular xylanase production increased by 25 or 46%, respectively. The response of C. flavigena to xylan plus beta-mx was accompanied by a significant accumulation of reducing sugar, an effect not observed with the combination sugarcane bagasse plus beta-mx as substrate. To our knowledge, this is the first report on the effect of beta-mx on the induction of xylanase in C. flavigena.

Regulated expression of green fluorescent protein under the control of Aureobasidium pullulans xylanase gene xynA

A mutant form of the jellyfish cDNA encoding green fluorescent protein (GFP) was fused to the promoter of the Aureobasidium pullulans xylanase gene xynA and the expression vector pxynEGFP was introduced into A. pullulans. In a manner consistent with regulation of the native xynA gene, gfp activity was induced by xylose and repressed by glucose. The marker may be useful for monitoring populations of A. pullulans in situ and for identifying transcriptional control elements of xynA.

Disaccharides permeases: constituents of xylanolytic and mannanolytic systems of Aureobasidium pullulans

Aureobasidium pullulans, a yeast-like microorganism was found to produce mannobiose permease and xylobiose permease, transporting beta-1,4-mannobiose or beta-1,4-xylobiose into the cells from extracellular media. Both permeases are induced by the same inducers as the corresponding hemicellulolytic enzyme systems. Mannobiose permease is induced by beta-1,4-mannobiose or is formed in the cells growing on mannan (inducers of beta-mannanolytic enzymes) and xylobiose permease is induced by d-xylose, beta-1,4-xylobiose or during the growth on xylan (inducers of xylanolytic enzymes). The permeases are energy dependent, synthesized de novo and their activities are inhibited by d-glucose. Since mannobiose permease transports beta-1,4-mannobiose, xylobiose permease appears to be less specific and transports beta-1,4-mannobiose, beta-1,4-xylobiose and methyl beta-d-xylopyranoside. Methyl beta-d-mannopyranoside or methyl beta-d-xylopyranoside serve as less efficient inducers of the corresponding permeases than beta-1,4-mannobiose or beta-1, 4-xylobiose.

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.

Cloning, sequencing, and regulation of a xylanase gene from the fungus Aureobasidium pullulans Y-2311-1

Aureobasidium pullulans Y-2311-1 growing on xylan secretes four major xylanases with different masses and isoelectric points. Two of these enzymes, named APX-I and APX-II, have been purified previously. Their N-terminal amino acid sequences are identical except that APX-I has Asp and APX-II has Asn at position 7. An 83-bp DNA region was amplified by PCR and used as a probe for the xylanase gene cloning. The longest cDNA (xynA) obtained by cDNA cloning and PCR amplification consisted of 895 bp. A. pullulans xynA had an open reading frame encoding a polypeptide of 221 amino acids with a calculated mass of 23,531 Da and contained a putative 34-amino-acid signal peptide in front of the amino terminus of the mature enzyme. Strong homology was found between the deduced amino acid sequence of XynA and some xylanases from bacterial and fungal sources. It is suggested that A. pullulans XynA belongs to the family G glycanases. Northern (RNA blot) analysis revealed that only one transcript of 900 bases was present in cultures grown in medium containing D-xylose or oat spelt xylan. Transcription was completely repressed in the presence of glucose in the medium. Southern blot analysis indicated that A. pullulans xynA was present as a single copy in the genome. Comparison between the genomic and cDNA sequences revealed that one intron of 59 bp was present in the coding region. The data presented suggest that the highly active xylanases, APX-I and APX-II, secreted by A. pullulans are encoded by the same gene.

Regulation of the xylanase-encoding xlnA gene of Aspergillus tubigensis

A gene encoding an endo-1,4-beta-xylanase from Aspergillus tubigensis was cloned by oligonucleotide screening using oligonucleotides derived from amino acid sequence data obtained from the purified protein. The isolated gene was functional as it could be expressed in the very closely related fungus Aspergillus niger. The xylanase encoded by this gene is synthesized as a protein of 211 amino acids. After cleavage of the presumed prepropeptide this results in a mature protein of 184 amino acids with a molecular weight of 19 kDa and an isoelectric point of 3.6. The regulatory region of the xlnA gene was studied with respect to the response to xylan induction and carbon catabolite repression. By deletion analysis of the 5' upstream region of the gene a 158 bp region involved in the xylan specific induction was identified. To study this regulatory element a reporter system for transcriptional activating sequences was developed that is based on the A. niger glucose oxidase-encoding gene. From the results with this reporter system it is concluded that this 158 bp fragment not only contains the information required for induction of transcription but that it also plays a role in carbon catabolite repression of the xlnA gene. The region directly upstream of this fragment contains four potential CREA target sites; deletion of this region leads to an increase in the level of transcription. These results suggest that carbon catabolite repression of the xlnA gene is controlled at two levels, directly by repression of xlnA gene transcription and indirectly by repression of the expression of a transcriptional activator. This type of mechanism would be similar to the double lock mechanism for the regulation of gene expression of alcA in Aspergillus nidulans. The reporter system was also used to study the regulation of expression via the functions located on this fragment in A. niger and in A. nidulans. Essentially the same pattern of regulation was found in both of these hosts. Therefore, regulation of xylanase gene expression is basically conserved in all three aspergilli.