Transcriptional induction and expression of the endoglucanase celA gene from a ruminal Clostridium sp. ("C. longisporum")

Cloning, sequencing, and expression of a Eubacterium cellulosolvens 5 gene encoding an endoglucanase (Cel5A) with novel carbohydrate-binding modules, and properties of Cel5A

A novel Eubacterium cellulosolvens 5 gene encoding an endoglucanase (Cel5A) was cloned and expressed in Escherichia coli, and its enzymatic properties were characterized. The cel5A gene consists of a 3,444-bp open reading frame and encodes a 1,148-amino-acid protein with a molecular mass of 127,047 Da. Cel5A is a modular enzyme consisting of an N-terminal signal peptide, two glycosyl hydrolase family 5 catalytic modules, two novel carbohydrate-binding modules (CBMs), two linker sequences, and a C-terminal sequence with an unknown function. The amino acid sequences of the two catalytic modules and the two CBMs are 94% and 73% identical to each other, respectively. Two regions that consisted of one CBM and one catalytic module were tandemly connected via a linker sequence. The CBMs did not exhibit significant sequence similarity with any other CBMs. Analyses of the hydrolytic activity of the recombinant Cel5A (rCel5A) comprising the CBMs and the catalytic modules showed that the enzyme is an endoglucanase with activities with carboxymethyl cellulose, lichenan, acid-swollen cellulose, and oat spelt xylan. To investigate the functions of the CBMs and the catalytic modules, truncated derivatives of rCel5A were constructed and characterized. There were no differences in the hydrolytic activities with various polysaccharides or in the hydrolytic products obtained from cellooligosaccharides between the two catalytic modules. Both CBMs had the same substrate affinity with intact rCel5A. Removal of the CBMs from rCel5A reduced the catalytic activities with various polysaccharides remarkably. These observations show that CBMs play an important role in the catalytic function of the enzyme.

Regulation of expression of cellulosomes and noncellulosomal (hemi)cellulolytic enzymes in Clostridium cellulovorans during growth on different carbon sources

Cellulosomes and noncellulosomal (hemi)cellulolytic enzymes are produced by Clostridium cellulovorans to degrade plant cell walls. To understand their synergistic relationship, changes in mRNA and protein expression in cellulosomes and noncellulosomal (hemi)cellulolytic enzymes (hereafter called noncellulosomal enzymes) of cultures grown on cellobiose, cellulose, pectin, xylan, and corn fiber or mixtures thereof were examined. Cellulase expression, favored particularly by the presence of Avicel, was found with all substrates. Comparison of cellulosome and noncellulosomal enzymes showed that expression profiles were strongly affected by the carbon source. High xylanase or pectate lyase expression was observed when C. cellulovorans was grown on xylan or pectin, respectively. Mixed carbon substrates (cellulose-pectin-xylan mixture or corn fiber) induced a wider variety of enzymes than a single carbon source, such as cellobiose, pectin, or xylan. Cellulosomal proteome profiles were more affected by the carbon source than the noncellulosomal enzymes. Transcription and protein analyses revealed that cellulosomes and noncellulosomal enzymes were expressed simultaneously on mixed carbon sources, but their degree of inducibility varied when the substrate was either cellulose or cellobiose. Cellulosomes and noncellulosomal enzymes had synergistic activity on various carbon substrates. These results indicated that expression of plant cell wall-degrading enzymes is highly influenced by the available carbon source and that synergy between cellulosomes and noncellulosomal enzymes contribute to plant cell wall degradation.