Two adjacent, highly homologous endoglucanase genes, celD and celE from Fibrobacter succinogenes S85, which were separated by an AT-rich 223-nucleotide intergenic region were characterized. The celD gene codes for endoglucanase D (EGD), a protein of 668 residues with a molecular mass of 71.7 kDa, while the celE gene encodes endoglucanase E, a protein of 467 amino acids with a molecular mass of 50.7 kDa. Both gene products belong to family 9 of glycosyl hydrolases. EGD displays an array of serine-rich periodic sequences (SRPS) near its C terminus which separate the catalytic domain from a basic terminal domain (BTD) rich in positively charged amino acids. Endoglucanase E has a BTD which is homologous to that of EGD, but it lacks the SRPS and 151 residues present at the N terminus of EGD. The SRPS structures may function as flexible linkers which facilitate interactions between the BTDs and acidic membrane proteins from F. succinogenes S85. The recombinant EGD showed pH and temperature optima of 5.5 and 35 degrees C, respectively. The enzyme cleaved barley-beta-glucan, carboxymethyl cellulose, and acid-swollen cellulose with specific activities of 19.1, 11.5 and 1.7 micromol x min-1 x mg of protein-1, respectively. There was a rapid drop in viscosity during hydrolyses of carboxymethyl cellulose, which is characteristic of an endoglucanase. Glucose was the main hydrolysis product of acid-swollen cellulose. Monospecific polyclonal antibodies against EGD detected the expression of a 68-kDa cellulose-inducible protein corresponding in size to the recombinant EGD in the culture fluid of F. succinogenes S85 and several larger proteins. The celE gene appeared to have little activity when expressed from the beta-galactosidase promoter in pBluescript in Escherichia coli; however, reverse transcriptase PCR analysis with internal primers for the gene revealed that a cellulose-inducible message was made in F. succinogenes, thereby documenting expression of the gene.