The gene cluster containing four genes of the pathway for L-rhamnose catabolism in the yeast Pichia stipitis

For the catabolism of L-rhamnose in microorganisms two distinctly different pathways are known. One pathway has phosphorylated intermediates and an isomerase as the first enzyme and the other is an oxidative pathway without phosphorylated intermediates. In fungi only the oxidative pathway is used. In this pathway L-rhamnose is oxidised to L-rhamnono-gamma-lactone followed by a lactonase and a dehydratation reaction. The resulting 3,6-Dideoxy-L-erythro-hexulosonic acid(2-keto-3-deoxy-L-rhamnonic acid) is split by an aldolase to pyruvate and L-lactaldehyde and the L-lactaldehyde is oxidised to L-lactic acid. In the yeast Pichia stipitis the first four genes of this pathway are organized in a cluster. We show that the four genes of this cluster are upregulated on L-rhamnose but not on any other carbon source. We also characterized the L-rhamnonate dehydratase after expression in a heterologous host and identified a second gene in the P. stipitis genome coding for a 3,6-Dideoxy-L-erythro-hexulosonic acid aldolase. In eukaryotic microorganisms the genes of the cluster were found to be conserved in Pezizomycotina, in Saccharomycotina species closely related to P. stipitis and in some Basidiomycota.