β-glucosidase genes differentially expressed during composting

Abstract

Background: Cellulose degradation by cellulase is brought about by complex communities of interacting microorganism, which significantly to the cycling of carbon on a global scale. β-Glucosidase is the rate-limiting enzyme of degradation of cellulose. Thus, analysis of expression of genes involved in cellulose degradation and regulation of β-glucosidase gene expression in composting is beneficial to a better understanding of cellulose degradation mechanism. According to our previous researches, we present the hypothesis that "microbial functional communities differentially regulate the expression of glucose-tolerant β-glucosidase and glucose sensitive βglucosidase (up or down regulation) to adapt to the changes in cellulose degradation."Results: Here, the functional microbial community structure and function change in association with cellulose degradation during the process of natural and inoculated composts was investigated by metatranscriptome and DNA clone library. Compared with inoculated compost, cellulose degradation was obviously inhibited during natural composting. Especially, the cooling phase of natural compost exhibited carbon catabolite repression (CCR) effect due to high concentration of glucose and cellobiose. The expression of genes encoding endoglucanase and exoglucanase were significantly down-regulation, while the CCR has no effect on β-glucosidase genes expression levels. But functional microbial community composition changed significantly, the composition of glucose-tolerant βglucosidase increased.Conclusions: These results indicated that microbial functional communities differentially regulate the expression of glucose tolerant β-glucosidase (up regulation) and non-glucose tolerant β-glucosidase (down regulation) under CCR. This work provides a frame work to predict how functional microbial communities will respond to cellulose degradation conditions changes.

Mechanism of differential expression of β-glucosidase genes in functional microbial communities in response to carbon catabolite repression

“…These results indicated that under the general condition that no CCR effect occurs during cellulose degradation, the functional microbial communities mainly expressed non-glucose tolerant β-glucosidase participate in the process of cellulose degradation. This result was consistent with Zhang [ 7 ] and Mathew [ 53 ], which is a prevalent economic microbial behavior. Meanwhile, the ratio of glucose-tolerant β-glucosidase genes in all treatments at T2 phase was lower than that at the T1 phase in the DNA library (Additional file 2 : Table S2), indicating that the non-glucose β-glucosidase genes are the main components of β-glucosidase genes at T2 phase.…”

“…In G H C H treatment, the microbial community composition shifted; the abundance of Firmicutes increased, while Actinobacteria decreased. Considering that community structure is closely related to the function of microbial communities [ 7 ]. Actinobacteria were known to play an important role in decomposing organic materials in composts [ 34 , 37 ].…”

“…The cellulase gene was regulated by glucose and cellobiose [ 7 , 35 ]. Previous research reported that although cellobiose is considered a poor inducer [ 31 ], cellulase expression was induced by suitable concentration cellobiose and inhibited by high concentration cellobiose [ 35 ].…”

“…β-Glucosidase completes the final step of hydrolysis by converting the cellobiose (an intermediate product of cellulose hydrolysis) and some oligosaccharides to glucose, alleviating the negative feedback regulation of exoglucanase and endoglucanase biosynthesis caused by elevated cellobiose levels [ 4 , 5 ]. However, in some cases, glucose produced by β-glucosidase accumulates and inhibits cellulase production [ 6 , 7 ]. In addition, β-glucosidase is known to play an important role in regulating the synthesis of other enzyme components and is recognized as a rate-limiting enzyme in the microbial degradation of cellulose [ 8 – 10 ].…”

“…Previous study revealed that glucose/non-glucose tolerant β-glucosidases had opposite transcriptional regulation patterns in the cooling phase of natural compost (CCR occurred) [ 7 ]. In addition, the hypothesis that “β-glucosidase-producing microbial communities differentially regulate expression of glucose/non-glucose-tolerant β-glucosidase genes to adapt to changes in cellulose degradation” was verified.…”