Fungal Physiology, Westerdijk Fungal Biodiversity Institute & Fungal Molecular Physiology, Utrecht University, Uppsalalaan 8, 3584, CT, Utrecht, The Netherlands.
BMC Microbiol. 2017 Nov 6;17(1):214. doi: 10.1186/s12866-017-1118-z.
The genes of the non-phosphorylative L-rhamnose catabolic pathway have been identified for several yeast species. In Schefferomyces stipitis, all L-rhamnose pathway genes are organized in a cluster, which is conserved in Aspergillus niger, except for the lra-4 ortholog (lraD). The A. niger cluster also contains the gene encoding the L-rhamnose responsive transcription factor (RhaR) that has been shown to control the expression of genes involved in L-rhamnose release and catabolism.
In this paper, we confirmed the function of the first three putative L-rhamnose utilisation genes from A. niger through gene deletion. We explored the identity of the inducer of the pathway regulator (RhaR) through expression analysis of the deletion mutants grown in transfer experiments to L-rhamnose and L-rhamnonate. Reduced expression of L-rhamnose-induced genes on L-rhamnose in lraA and lraB deletion strains, but not on L-rhamnonate (the product of LraB), demonstrate that the inducer of the pathway is of L-rhamnonate or a compound downstream of it. Reduced expression of these genes in the lraC deletion strain on L-rhamnonate show that it is in fact a downstream product of L-rhamnonate.
This work showed that the inducer of RhaR is beyond L-rhamnonate dehydratase (LraC) and is likely to be the 2-keto-3-L-deoxyrhamnonate.
已鉴定出几种酵母物种非磷酸化 L-鼠李糖分解代谢途径的基因。在酿酒酵母中，所有 L-鼠李糖途径基因都组织在一个簇中，该簇在黑曲霉中保守，除了 lra-4 直系同源物（lraD）。黑曲霉簇还包含编码 L-鼠李糖响应转录因子（RhaR）的基因，该基因已被证明控制与 L-鼠李糖释放和分解代谢相关的基因的表达。
在本文中，我们通过基因缺失证实了来自黑曲霉的前三个假定 L-鼠李糖利用基因的功能。我们通过在 L-鼠李糖和 L-鼠李糖酸盐转移实验中生长的缺失突变体的表达分析，探索了途径调节剂（RhaR）的诱导物的身份。在 lraA 和 lraB 缺失菌株中，L-鼠李糖诱导基因的表达降低，但在 L-鼠李糖酸盐（LraB 的产物）上没有，这表明该途径的诱导物是 L-鼠李糖酸盐或其下游化合物。在 lraC 缺失菌株中，这些基因在 L-鼠李糖酸盐上的表达降低表明，它实际上是 L-鼠李糖酸盐的下游产物。
这项工作表明，RhaR 的诱导物不仅是 2-酮-3-L-去氧鼠李糖脱水酶（LraC），而且很可能是 2-酮-3-L-去氧鼠李糖。
