MULTIPLICITY OF XYLANASE

Multiplicity is a common phenomenon in microbial xylanases. Different isoforms may have diverse physicochemical properties, structures, specific activities and yields, as well as overlapping but dissimilar specificities, thereby increasing the efficiency and extent of hydrolysis. The most outstanding case regarding multiple forms of xylanase was expression of more than 30 different protein bands by Phanerochaete chrysosporium, when grown on avicel. Sachslehner et al., (1998) using analytical isoelectric focusing detected at least six distinct xylanase bands with RBB xylan from the crude culture extract of Sclerotium rolfsii. Similarly, Saraswat and Bisaria (2000), reported production of seven different extracellular xylanase isoenzymes by Ascomycetous fungus Melanocarpus albomyces. Earlier, Gomez de Segura et al., (1998) reported production of five different xylanase by rumen anaerobic Neocallimastix frontalis. Nihira et al., (2001) purified three distinct endoxylanases components derived from filamentous fungus Acremonium cellulolyticus. Xylanase multiplicity has also been reported in Trichoderma sp. (Xu et al., 1998), Penicillium purpurogenum (Chavez et al., 2002), Clostridium stercorarium (Adelsberger et al., 2004), Myceliophthora sp. (Badhan et al., 2004), Schizophyllum commune (Kolenova et al., 2005) and Scytalidium thermophilum and Melanocarpus sp. MTCC 3922 (Jatinder et al., 2005).

Another interesting observation comes from the xylanase gene knockout studies of the rice blast fungus M. grisea (Wu et al., 1997) where the gene disruption of one of the major xylanase (xyl2) released three additional xylanases in the mutant strain that have not been detected in the parent strain. This indicates the complex nature of the phenomenon of xylanase multiplicity in fungi (Apel-Birkhold and Walton, 1996). Wong et al., (1986) studied the functional importance of three xylanases isoforms from the saprophytic fungus Trichoderma harzianum and reported a high degree of complementation of the three iso-xylanases in the hydrolysis of aspen xylan. They further concluded that, the three iso-xylanases are not redundant enzymes since each contributes significantly and uniquely to the hydrolysis of the xylan. In plant pathogenic fungi, it was reported that some of the xylanases are induced only during infection (Apel-Birkhold and Walton, 1996) suggesting that different sets of endoxylanases function in saprophytic and pathogenic growth of fungi. It is also speculated that isozymes of cell wall degrading enzymes are produced at different stages during infection of plant tissue (Annis and Goodwin, 1997) possibly following biochemical changes in the host environment.

Thomson (1993) suggested various mechanisms that could account for the multiplicity of function and specificity of the xylan degrading enzymes. Electrophoretically distinct xylanases could arise from post-translational modification (Ruiz-Arribas et al., 1997) of a gene product such as differential glycosylation or proteolysis. The detection of minor xylanases may also be an artifact of the growth and /or purification conditions or these enzymes may have functions, which are not required in large amounts e.g., hydrolysis of linkages not found frequently (Wong & Saddler, 1992). Multiple xylanases may be allozymes, products of different alleles of the same gene, or they could be distinct gene products produced by a fungus to enhance its utilization of xylan (Hazlewood and Gilbert, 1993; Uffen, 1997). Recent studies from our lab have shown that the multiple xylanases produced by Myceliophthora sp. were functionally diverse and these xylanases were not produced due to proteolytic modification (Badhan et al., 2007). More information about the extent and nature of the multiplicity, as well as the functional importance and regulation of this phenomenon in fungal xylanolytic systems would be useful for better understanding of the system.