The nucleotide sequence of the Clostridium thermocellum F1 celQ gene, which codes for the endoglucanase CelQ, consists of 2,130 bp encoding 710 amino acids. The precursor form of CelQ has a molecular weight of 79,809 and is composed of a signal peptide, a family 9 cellulase domain, a family IIIc carbohydrate-binding module (CBM), and a dockerin domain. Truncated derivatives of CelQ were constructed: CelQ Delta doc consisted of the catalytic domain and the CBM; CelQcat consisted of the catalytic domain only. CelQ Delta doc showed strong activity toward carboxymethylcellulose (CMC) and barley beta -glucan and low activity toward Avicel, acid-swollen cellulose, lichenan, and xylan. The V-max and K-m values were 235 mu mol/min/mg and 3.3 mg/ml, respectively, for CMC. By contrast, CelQcat, which was devoid of the CBM, showed negligible activity toward CMC, i.e., about 1/1,000 of the activity of CelQ Delta doc, supporting the previously proposed idea that family IIIc CBMs participate in the catalytic function of the enzyme. Immunological analysis using an antiserum raised against CelQ Delta doc confirmed that CelQ is a component of the C. thermocellum cellulosome.