celA from the cellulolytic bacterium Bacillus lautus PL236 encodes EG-A, an endo-beta-1,4-glucanase. An open reading frame of 2,100 bp preceded by a ribosome-binding site encodes a protein with a molecular mass of 76,863 Da with a typical signal sequence. The NH2-terminal active domain of EG-A is not homologous to any reported cellulase or xylanase and may represent a new family of such enzymes. A 150-amino-acid COOH-terminal peptide is homologous to noncatalytic domains in several other cellulases (A. Meinke, N.R. Gilkes, D.G. Kilburn, R.C. Miller, Jr., and R.A.J. Warren, J. Bacteriol. 173:7126-7135, 1991). Upstream of celA, a partial open reading frame encodes a 145-amino-acid peptide which also belongs to the family mentioned. Zymogram analysis of extracts from Escherichia coli and supernatants of Bacillus subtilis and B. megaterium, including protease-deficient mutants thereof, which express celA, revealed two active proteins, EG-A-L and EG-A-S, with Mrs of 74,000 and 57,000, respectively. The proportion of EG-A-L to EG-A-S depends on the extracellular proteolytic activity of the host organism, indicating that EG-A-S arises from posttranslational proteolytic modification of EG-A-L. Since EG-A-S has an NH2 terminus corresponding to the predicted NH2-terminal sequence of EG-A, processing appears to take place between the catalytic and noncatalytic domains described. EG-A-L and EG-A-S were purified to homogeneity and shown to have almost identical characteristics with respect to activity against soluble substrates and pH and temperature dependency. EG-A-L binds strongly to cellulose, in contrast to EG-A-S, and has higher activity against insoluble substrates than the latter. We conclude that the COOH-terminal 17,000-Mr peptide of EG-A-L constitutes a cellulose-binding domain.