l-Rhamnose catabolism in archaea.

The halophilic archaeon Haloferax volcanii utilizes l-rhamnose as a sole carbon and energy source. It is shown that l-rhamnose is taken up by an ABC transporter and is oxidatively degraded to pyruvate and l-lactate via the diketo-hydrolase pathway. The genes involved in l-rhamnose uptake and degradation form a l-rhamnose catabolism (rhc) gene cluster. The rhc cluster also contains a gene, rhcR, that encodes the transcriptional regulator RhcR which was characterized as an activator of all rhc genes. 2-keto-3-deoxy-l-rhamnonate, a metabolic intermediate of l-rhamnose degradation, was identified as inducer molecule of RhcR. The essential function of rhc genes for uptake and degradation of l-rhamnose was proven by the respective knockout mutants. Enzymes of the diketo-hydrolase pathway, including l-rhamnose dehydrogenase, l-rhamnonolactonase, l-rhamnonate dehydratase, 2-keto-3-deoxy-l-rhamnonate dehydrogenase and 2,4-diketo-3-deoxy-l-rhamnonate hydrolase, were characterized. Further, genes of the diketo-hydrolase pathway were also identified in the hyperthermophilic crenarchaeota Vulcanisaeta distributa and Sulfolobus solfataricus and selected enzymes were characterized, indicating the presence of the diketo-hydrolase pathway in these archaea. Together, this is the first comprehensive description of l-rhamnose catabolism in the domain of archaea.