School of Biotechnology, Jiangnan University, Wuxi, 214122, China.
College of Biochemical Engineering, Anhui Polytechnic University, Wuhu, 241000, China.
Sci Rep. 2017 Aug 31;7(1):10161. doi: 10.1038/s41598-017-10964-0.
Cellulolytic fungi have evolved a sophisticated genetic regulatory network of cellulase synthesis to adapt to the natural environment. Even in the absence of lignocellulose, it still secretes low levels of "constitutive" cellulase for standby application. However, the mechanisms of this constitutive expression remain incompletely understood. Here we identified a cellobiose synthetase (CBS) from Rhizopus stolonifer, which has the capacity to catalyse the synthesis of cellobiose from uridine diphosphate glucose (UDPG). Through the construction of R. stolonifer Δcbs strain, we found that CBS plays a key role in the synthesis of cellulase. Further analysis of cellulase synthesis under glucose culture reveals that the cellobiose-responsive regulator CLR1 was activated by CBS-synthesized cellobiose, thereby promoting the expression of CLR2 and finally opening the transcription of cellulase genes. Our results suggest that R. stolonifer can be induced by self-synthesized cellobiose to produce cellulase, which can be used to reconstruct the expression regulation network to achieve rapid production of cellulase using simple carbon source. Based on our data, the "constitutive expression" of cellulase actually derives from the induction of cellobiose that synthesized by CBS from carbohydrate metabolites, which updates our knowledge of cellulase, and provides a novel insight into the regulation of cellulase synthesis.
纤维素分解真菌已经进化出了一套复杂的纤维素合成基因调控网络，以适应自然环境。即使在没有木质纤维素的情况下，它仍然会低水平分泌“组成型”纤维素酶以备不时之需。然而，这种组成型表达的机制仍不完全清楚。在这里，我们从根霉（Rhizopus stolonifer）中鉴定出一种纤维二糖合成酶（CBS），它能够催化尿苷二磷酸葡萄糖（UDPG）合成纤维二糖。通过构建根霉Δcbs 菌株，我们发现 CBS 在纤维素酶的合成中起着关键作用。进一步分析在葡萄糖培养下的纤维素酶合成，我们发现 CBS 合成的纤维二糖激活了纤维二糖响应调控因子 CLR1，从而促进了 CLR2 的表达，最终打开了纤维素酶基因的转录。我们的结果表明，根霉可以被自身合成的纤维二糖诱导产生纤维素酶，这可以用来重建表达调控网络，以使用简单的碳源实现纤维素酶的快速生产。基于我们的数据，纤维素酶的“组成型表达”实际上来源于 CBS 从碳水化合物代谢物中合成的纤维二糖的诱导，这更新了我们对纤维素酶的认识，并为纤维素酶合成的调控提供了新的见解。
