Cloning and characterization of two novel ß-glucosidase genes encoding isoenzymes of the cellobiase complex from Cellulomonas biazotea.

Enzymatic degradation of cellulosic waste to generate renewable biofuels has offered an attractive solution to the energy problem. Synergistic hydrolysis of cellulose residues requires the participation of three different types of cellulases - endoglucanases, exoglucanases, and ß-glucosidases (Bgl). Our group has been interested in using Bgl of Cellulomonas biazotea in studies designed to investigate cooperative action among different cellulases. We previously have cloned bgl genes encoding Cba and Cba3, which are C. biazotea Bgl isozymes representing two different Bgl families, respectively; specifically, Glycoside Hydrolase Family 3 (GH3) and Glycoside Hydrolase Family 1 (GH1). To gain an understanding of the complexity of Bgl in C. biazotea, we analyzed E. coli clones containing plasmids into which C. biazotea DNA had been inserted; these clones could hydrolyze 4-methylumbelliferyl ß-d-glucopyranoside (MUG) supplemented in solid agar media, suggesting they might contain bgl genes. Through restriction analysis and DNA sequencing, two novel bgl genes, designated cba4 and cba5 and encoding Cba4 (484 amino acids) and Cba5 (758 amino acids) were identified. Cba4 and Cba5 appear to be members of GH1 and GH3, respectively. Both Cba4 and Cba5 were concluded to be genuine cellobiases as each was found to enable their E. coli hosts to survive on media in which cellobiose was the sole carbon source. Despite lacking a typical secretory signal sequence, Cba4 and Cba5 are secretory proteins. Although they are isoenzymes, Cba, Cba3, Cba4, and Cba5 were shown to possess distinct substrate specificities. These four Bgl members may play important roles in hydrolyzing a wide variety of ß-glucosides including cellobiose and non-cellulosic substrates.

Engineering versatile protein expression systems mediated by inteins in Escherichia coli.

We have recently employed an intein, Saccharomyces cerevisiae vascular membrane ATPase (VMA), in conjunction with efficient expression and secretory functions formed between the ompA leader sequence and the human epidermal growth factor (EGF) gene (fused at the 5' end of VMA), and the human basic fibroblast growth factor (bFGF) gene (fused at the 3' end of VMA), to engineer an efficient intein-based Escherichia coli system for high-level co-expression of EGF and bFGF as authentic mature products. Both products were found not only excreted to the culture medium but also located, surprisingly, in the cytoplasm (Kwong and Wong 2013). In this study, we employed two structurally varied inteins, VMA and Mycobacterium xenopi GyraseA (GyrA), and further demonstrated that despite acting alone, both VMA and GyrA were able to mediate successful co-expression of two widely different proteins, EGF and an endoglucanase (Eng) in E. coli. Although EGF and Eng were initially expressed as large precursors/intermediates, they were soluble and auto-cleavable to finally yield the desired products in both the cytoplasm and culture media. The results further substantiate our postulation that the aforementioned intein/E. coli approach might lead to the development of cost-effective and versatile host systems, wherein all culture fractions are involved in producing the target proteins.