CelA is the most abundant enzyme secreted by Caldicellulosiruptor bescii and has been shown to outperform mixtures of commercially available exo- and endoglucanases in vitro. CelA contains both a glycoside hydrolase family 9 endoglucanase and a glycoside hydrolase family 48 exoglucanase known to be synergistic in their activity, connected by three cellulose-binding domains via linker peptides. Here, repeated aspartate residues were introduced into the N-terminal ends of CelA GH9 and GH48 domains to improve secretion efficiency and/or catalytic efficiency of CelA. Among several constructs, the highest activity on carboxymethylcellulose (CMC), 0.81 +/- 0.03mg/mL was observed for the C. bescii strain containing CelA with 5-aspartate tag at the N-terminal end of GH9 domainan 82% increase over wild type CelA. In addition, expression of CelA with N-terminal repeated aspartate residues in C. bescii results in a dramatic increase in its ability to grow on Avicel. Biotechnol. Bioeng. 2017;114: 945-950. (C) 2016 Wiley Periodicals, Inc.