Regulation of endoglucanase gene (cmcax) expression in Acetobacter xylinum

Although cellulose is the most abundant biopolymer in nature, the detailed mechanisms of cellulose biosynthesis remain unknown. Acetobacter xylinum is one of the best-studied model organisms for cellulose biosynthesis. Interestingly, the over-expression of the cmcax gene cause enhancement of cellulose production in A. xylinum, while its product (CMCax) has cellulose degradation activity. The addition of CMCax into medium also promotes cellulose production, suggesting that CMCax is involved in cellulose synthetic pathway. In the present study, we reveal the regulation mechanism of cmcax expression in A. xylinum. First, we treated cells with four kinds of beta-glucodisaccharide. Using an enzyme assay and real-time quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), we observed an increase in CMCax activity and an induction of cmcax expression by gentiobiose treatment. Therefore, we concluded that gentiobiose induced cmcax expression. Although gentiobiose does not originally exist in the cultivation medium, we have revealed that membrane and intra-cellular proteins extracted from A. xylinum produce gentiobiose from glucose, which is one of the components in the cultivation medium. Furthermore, we confirmed that cmcax expression in a wild-type strain increased gradually after 5 d cultivation using real-time qRT-PCR. These results have led us to conclude that the increase in cmcax expression after 5 d cultivation is caused by the increase in gentiobiose, which could be synthesized by a condensation reaction in A. xylinum. Since CMCax plays a pivotal role in the cellulose production system, our results will contribute to the elucidation of mechanisms of cellulose biosynthesis.