Identification of the genes encoding the catalytic steps corresponding to LRA4 (L-KDR aldolase) and LADH (L-lactaldehyde dehydrogenase) in Aspergillus nidulans: evidence for involvement of the loci AN9425/lraD and AN0544/aldA in the L-rhamnose catabolic pathway
Environ Microbiol. 2021 Feb 21. doi: 10.1111/1462-2920.15439. Online ahead of print.
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 family 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. This article is protected by copyright. All rights reserved.