Gene Technology and Applied Biochemistry, Institute of Chemical Engineering, Vienna University of Technology, Gumpendorfer Str. 1a, A-1060 Vienna, Austria.
Appl Environ Microbiol. 2011 Sep;77(17):5988-94. doi: 10.1128/AEM.05427-11. Epub 2011 Jul 8.
The saprophytic fungus Hypocrea jecorina (anamorph, Trichoderma reesei) is an important native producer of hydrolytic enzymes, including xylanases. Regarding principles of sustainability, cheap and renewable raw materials, such as d-xylose (the backbone monomer of xylan), have been receiving increasing attention from industries. Recently, it was demonstrated that small (0.5 to 1 mM) amounts of d-xylose induce the highest expression of xylanase in H. jecorina. However, it was also reported that active metabolism of d-xylose is necessary for induction. In this report, we demonstrate that xylitol, the next intermediate in the pentose pathway after d-xylose, does not trigger transcription of xylanase-encoding genes in H. jecorina QM9414. The highest level of transcription of xylanolytic enzyme-encoding genes occurred in an xdh1 (encoding a xylitol dehydrogenase) deletion strain cultured in the presence of 0.5 mM d-xylose, suggesting that a metabolite upstream of xylitol is the inducer. The expression levels of xylanases in an xdh1-lad1 double-deletion strain were lower than that of an xdh1 deletion strain. This observation suggested that l-xylulose is not an inducer and led to the hypothesis that l-arabitol is the actual inducer of xylanase expression. A direct comparison of transcript levels following the incubation of the H. jecorina parental strain with various metabolites of the pentose pathway confirmed this hypothesis. In addition, we demonstrate that xyr1, the activator gene, is not induced in the presence of pentose sugars and polyols, regardless of the concentration used; instead, we observed low constitutive expression of xyr1.
腐生真菌 Hypocrea jecorina（无性型，里氏木霉）是一种重要的天然水解酶产生菌，包括木聚糖酶。从可持续性原则出发，廉价且可再生的原料（如木糖，木聚糖的骨架单体）越来越受到各行业的关注。最近，研究表明，少量（0.5 至 1mM）的 d-木糖可以诱导 H. jecorina 中木聚糖酶的最高表达。然而，也有报道称，d-木糖的活性代谢对于诱导是必需的。在本报告中，我们证明木糖醇，即 d-木糖之后戊糖途径的下一个中间产物，不会触发 H. jecorina QM9414 中木聚糖酶编码基因的转录。在 0.5mM d-木糖存在的情况下，xdh1（编码木糖醇脱氢酶）缺失菌株中木聚糖裂解酶编码基因的转录水平最高，这表明木糖醇上游的代谢物是诱导物。xdh1-lad1 双缺失菌株中木聚糖酶的表达水平低于 xdh1 缺失菌株。这一观察结果表明 l-阿拉伯糖醇不是诱导物，并导致了 l-山梨糖醇是木聚糖酶表达的实际诱导物的假设。xdh1 缺失菌株与戊糖途径的各种代谢物孵育后，对木聚糖酶基因的转录水平进行直接比较，证实了这一假设。此外，我们还证明了 xyr1，即激活基因，在戊糖和多元醇存在的情况下不会被诱导，无论使用何种浓度；相反，我们观察到 xyr1 低组成型表达。
