Production of a cloned xylanase in Bacillus cereus and its performance in kraft pulp prebleaching

Xylanase production from a Bacillus subtilis gene cloned into a strain of Escherichia coli was monitored. Although this gene was expressed in E. coli at several temperatures, efficient xylanase secretion did not occur; the observed protein release apparently depended on cell leakage or lysis. Screening for a better microbial protein secretor free of cellulase selected Bacillus cereus No. 518. A bidirectional vector plasmid (pMK3) was employed to carry the cloned gene into this B. cereus strain. Transformation was carried out by electroporation. Total xylanase production by the new pMK3-borne gene in B. cereus was similar to that from E. coli but the xylanase was shown to be normally secreted. The xylanase gene products from the E. coli and B. cereus hosts were shown to function identically. Both xylanases improved the delignification of unbleached softwood and hardwood kraft pulps, thus reducing the Cl2 required to achieve a given degree of bleaching, without altering the physical properties of the fibers. Using a target kappa number (lignin content) of 5, xylanase pretreatment of aspen kraft (chemical) pulp led to a 22% savings of chlorine. Adsorbable organic halogens in the bleachery effluent were also lowered by more than 50%.