Identification of the genes encoding the catalytic steps corresponding to LRA4 (l-2-keto-3-deoxyrhamnonate aldolase) and l-lactaldehyde dehydrogenase in Aspergillus nidulans: evidence for involvement of the loci AN9425/lraD and AN0544/aldA in the l-rhamnose catabolic pathway

Abstract

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.