The most promising application of xylanases (E.C. 3.2.1.8) is in the prebleaching of kraft pulp. The present paper reports bleaching effects of a thermoalkali stable xylanase from Cellulosimicrobium sp. MTCC 10645. The bacterium produced thermo-alkali stable xylanase in a basal medium supplemented with wheat bran (1% w/v), which was optimally active at pH 7.0 and 50°C. The xylanase was stable at temperature 50°C for 1？h and retained up to 86% of the activity. The xylanase was stable in a broad pH range of 6.0–11.0 for 1？h at 50°C. Metal ions Ca+2, Hg+2, and Pb+2 were inhibitory for xylanase retaining 72.3%, 35.07% and 36.7% relative activity at 10？mM concentration, whereas Fe+2, Cu+2, Mn+2, Na+2, Co+2, and Zn+2 were inducers at concentrations of 5？mM and 10？mM. The enzyme exhibited greater binding affinity exclusively for xylans but not for avicel, CMC, cellobiose, starch, or p-nitrophenyl xylopyranoside. Parachloromercuric benzoate and iodoacetamide were found stimulatory, while potassium permanganate, cysteine, and cystine markedly reduced the activity. The xylanase dose of 2.0？U/g dry weight pulp of 10% consistency gave optimum bleach boosting of kraft pulp at pH 8.0 and temperature 50°C for 5？h reaction time. 1. Introduction The most promising application of xylanases (E.C. 3.2.1.8) is in the prebleaching of kraft pulp. The pulp and paper industry is modifying its pulping, bleaching, and effluent treatment technologies to reduce the environmental impact of mill effluents. Prebleaching of kraft pulps with xylanases lowers chlorine charges, which reduce chloroorganic discharges [1–3]. Tremblay and Archibald reported the delignification of unbleached softwood and hardwood kraft pulps [4]. Thus reducing the Cl2 required to achieve a given degree of bleaching [3, 5–7]. The public concern on the impact of pollutants from paper and pulp industries, which use chlorine as the bleaching agent act as strong driving force in developing biotechnology aided techniques for novel bleaching that is biobleaching [8, 9]. The occurrence of cellulase contamination is posing a major threat in applying the xylanases in biobleaching. The cellulases easily result in the hydrolysis of cellulose, which should be the main recovered product in paper industry. However, the enzyme preparations from microorganisms producing higher levels of xylanases with no cellulase activity can be applied in paper industry because the loss of pulp viscosity is at minimum level [10]. Xylanases have been reported from bacteria, fungi, actinomycetes, and yeasts [11–14]. The use of abundantly
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