Cloning of clusteredStreptomyces viridosporusT7A lignocellulose catabolism genes encoding peroxidase and endoglucanase and their extracellular expression inPichia pastoris

Abstract

A 4.1-kb fragment of chromosomal DNA from the lignocellulose-decomposing actinomycete Streptomyces viridosporus T7A was previously found to encode a lignin peroxidase gene. However, when cloned into Escherichia coli in pBSKS+, peroxidase activity was not expressed. When cloned in pIJ702 in Streptomyces lividans, the gene was expressed in a peroxidase positive background, owing to the production by S. lividans of its own extracellular peroxidases. To circumvent these problems, the DNA was cloned into the commercial expression vector pIC9 for extracellular expression in the yeast Pichia pastoris. Yeast transformants, however, expressed two activities, extracellular peroxidase and an extracellular endoglucanase. The enzymes were not expressed by the yeast cells alone or by yeast cells with pIC9 without the insert. Expression of the enzymes by only those transformants expressing the 4.1-kb DNA was confirmed by Western blot analyses, by nondenaturing activity gel staining, and by spectrophotometric enzyme assays of extracellular culture filtrates. Activity gel staining showed that the two activities resided in different proteins and the peroxidase expressed was similar to ALip-P3, one of the isoenzymes of lignin peroxidase of the S. viridosporus T7A wildtype. Other evidence indicated that in the transformants, the peroxidase and endoglucanase genes in the 4.1-kb insert were controlled by the methanol-inducible AOX1 yeast promoter in pIC9, since their expression was induced by methanol. In the best transformants, extracellular production of peroxidase by recombinant P. pastoris cultures was significantly higher than typically observed in S. viridosporus. The results also indicate that lignocellulose catabolism genes may be clustered on the S. viridosporus chromosome.Key words: lignocellulose, degradation, Streptomyces, peroxidase, endoglucanase, cloning, pIC9, Pichia pastoris.