The glycoside hydrolase family 9 cellulase (Cel9) from Clostridium phytofermentans has a multi-modular structure and is essential for cellulose hydrolysis. In order to facilitate production and purification of Cel9, recombinant Cel9 was functionally expressed in Escherichia coli. Cel9 exhibited maximum activity at pH 6.5 and 65 degrees C on carboxymethyl cellulose in a 10-min reaction period. The hydrolysis products on regenerated amorphous cellulose (RAC) were cellotetraose (a major product), cellotriose, cellobiose and glucose, and 71-80% of the reducing sugars produced by Cel9 were in soluble form, suggesting that Cel9 was a processive endoglucanase. The highest synergy between C. phytofermentans Cel9 and C. phytofermentans cellobiohydrolase Cel48 on Avicel was about 1.8 at a ratio of about 1:5. Cel9 alone was sufficient to solublize filter paper while Cel48 was not; however, it enhanced the solublization process along with Cel9 synergistically. This study provided useful information for understanding of the cellulose hydrolysis mechanism of this cellulolytic bacterium with potential industrial importance. (C) 2010 Elsevier Ltd. All rights reserved.