Instituto de Agroquímica y Tecnología de Alimentos (IATA), Consejo Superior de Investigaciones Científicas (CSIC), c/ Catedrático Agustín Escardino Benlloch 7, Paterna, Valencia, 46980, Spain.
Environ Microbiol. 2021 May;23(5):2420-2432. doi: 10.1111/1462-2920.15439. Epub 2021 Mar 14.
l-rhamnose is found in nature mainly as a component of structural plant polysaccharides and can be used as a carbon source by certain microorganisms. Catabolism of this sugar in bacteria, archaea and fungi occurs by two routes involving either phosphorylated or non-phosphorylated intermediates. Unlike the corresponding pathway in yeasts, the metabolic details of the non-phosphorylated pathway in filamentous fungi are not fully defined. The first three genes (lraA, lraB and lraC) of the non-phosphorylated pathway in Aspergillus nidulans have recently been studied revealing dependence on lraA function for growth on l-rhamnose and α-l-rhamnosidase production. In the present work, two genes encoding the subsequent steps catalysed by l-2-keto-3-deoxyrhamnonate (l-KDR) aldolase (AN9425) and l-lactaldehyde dehydrogenase (AN0554) are identified. Loss-of-function mutations cause adverse growth effects on l-rhamnose. Akin to genes lraA-C and those encoding rhamnosidases (rhaA, rhaE), their expression is induced on l-rhamnose via the transcriptional activator RhaR. Interestingly, the aldolase belongs to the ftablamily of bacterial l-KDR aldolases (PF03328/COG3836) and not that of yeasts (PF00701/COG0329). In addition, AN0554 corresponds to the previously characterized aldA gene (encodes aldehyde dehydrogenase involved in ethanol utilization) thus revealing a previously unknown role for this gene in the catabolism of l-rhamnose.
L-鼠李糖主要作为结构性植物多糖的组成部分存在于自然界中，某些微生物可以将其作为碳源利用。细菌、古菌和真菌中这种糖的分解代谢通过两条途径进行，涉及磷酸化或非磷酸化中间体。与酵母中相应的途径不同，丝状真菌中非磷酸化途径的代谢细节尚未完全确定。最近研究了粗糙脉孢菌中非磷酸化途径的前三个基因（lraA、lraB 和 lraC），揭示了该途径对 lraA 功能的依赖性，lraA 功能对于在 l-鼠李糖和 α-l-鼠李糖苷酶上的生长是必需的。在本工作中，鉴定了编码 l-2-酮-3-脱氧鼠李糖（l-KDR）醛缩酶（AN9425）和 l-乳醛脱氢酶（AN0554）催化的后续步骤的两个基因。功能丧失突变导致在 l-鼠李糖上的生长受到不利影响。与 lraA-C 基因和编码鼠李糖苷酶（rhaA、rhaE）的基因类似，它们的表达通过转录激活因子 RhaR 在 l-鼠李糖上被诱导。有趣的是，醛缩酶属于细菌 l-KDR 醛缩酶的 ftabl 家族（PF03328/COG3836），而不是酵母的（PF00701/COG0329）。此外，AN0554 对应于先前表征的 aldA 基因（编码参与乙醇利用的醛脱氢酶），从而揭示了该基因在 l-鼠李糖分解代谢中的未知作用。
