PARTIAL CHARACTERIZATION OF THE STREPTOMYCES-LIVIDANS XLNB PROMOTER AND ITS USE FOR EXPRESSION OF A THERMOSTABLE XYLANASE FROM THERMOTOGA-MARITIMA

Xylanase activity assays were used to screen a Streptomyces coelicolor genomic library in Escherichia coil, and a xylanase gene that is 99% identical to the xylanase B gene (xlnB) of S. lividans (GenBank access ion no. M64552) was identified. The promoter region of this gene was i dentified by using a transcriptional fusion between the upstream regio n of the S. coelicolor xlnB gene and the xylE reporter gene. Transcrip tion from the xlnB promoter was found to be induced by xylan and repre ssed by glucose. A single apparent transcription start site was identi fied by both primer extension analysis and in vitro run off transcript ion assays. Analysis of deletions of the promoter identified a region required for glucose repression. By using the transcriptional and prot ein localization signals of the Streptomyces xlnB gene, an in-frame tr anslational fusion between the end of the xlnB signal sequence and the ATG of the Thermotoga maritima xynA gene was constructed. The xynA ge ne encodes a thermostable xylanase that has been demonstrated to be us eful in the bleaching of Kraft pulp. The xlnB=xynA gene fusion was exp ressed in Streptomyces, and the activity of the protein produced was t hermostable and was localized to the supernatant fraction of harvested cells.