Cellulolytic enzymes in Microbulbifer sp. Strain GL-2, a marine fish intestinal bacterium, with emphasis on endo-1,4-β-glucanases Cel5A and Cel8.

Cellulose is an abundant biomass on the planet. Various cellulases from environmental microbes have been explored for industrial use of cellulose. Marine fish intestine is of interest as one source of new enzymes. Here, we report the discovery of genes encoding two β-glucosidases (Bgl3A and Bgl3B) and four endo-1,4-β-glucanases (Cel5A, Cel8, Cel5B, and Cel9) as part of the genome sequence of a cellulolytic marine bacterium, Microbulbifer sp. Strain GL-2. Five of these six enzymes (excepting Cel5B) are presumed to localize to the periplasm or outer membrane. Transcriptional analysis demonstrated that all six genes were highly expressed in stationary phase. The transcription was induced by cello-oligosaccharides rather than by glucose, suggesting that the cellulases are produced primarily for nutrient acquisition following initial growth, facilitating the secondary growth phase. We cloned the genes encoding two of the endo-1,4-β-glucanases, Cel5A and Cel8, and purified the corresponding recombinant enzymes following expression in Escherichia coli. The activity of Cel5A was observed across a wide range of temperatures (10-40 ˚C) and pHs (6-8). This pattern differed from those of Cel8 and the commercial cellulase Enthiron, both of which exhibit decreased activities below 30 ˚C and at alkaline pHs. These characteristics suggest that Cel5A might find use in industrial applications. Overall, our results reinforce the hypothesis that marine bacteria remain a possible source of novel cellulolytic activities.