Endo-beta-1,4-glucanase (CMCax; EC 3.2.1.4) from Acetobacter xylinum ATCC23769 was expressed as a 6xHis-tagged fusion protein in Escherichia coli. The optimal temperature, pH, K. and V-max of the purified His-tagged CMCax toward carboxymethyl cellulose were 50degreesC, 4.5, 20 mg/ml and 37.2 muM/min, respectively. The number of recognition residues of cello-oligosaccharide by this enzyme were five (cellopentaose) or longer, and the stereochemical course of hydrolysis was of the inverting type. Addition of a small amount (1.5 mg/l) of His-tagged CMCax into a culture medium enhanced cellulose production 1.2-fold. CMCax overproduction in A. xylinum also enhanced the yield of cellulose production. Transmission electron microscopic analysis revealed that the cellulose ribbons secreted from the CMCax overproducing strain were dispersed compared with those from the wild type strain in the same manner as by carboxymethyl cellulose addition. These results could suggest that CMCax from A. xylinum influences in cellulose ribbon assembly, which is considered to be a rate-determined process in cellulose synthesis.