Our model bacterium R. cellulolyticum can catabolize the cellulose and the major polysaccharides forming the hemicellulose in plant cell walls. These polymers are degraded extracellularly by an arsenal of enzymes gathered in cellulosomes. Nevertheless, this degradation generally does not lead to the release of the elementary monosaccharides composing these polysaccharides, but rather to oligosaccharides which are imported “en bloc” via specific ABC-transporters. For instance, the group has shown that when R. cellulolyticum grows on xyloglucan, this hemicellulosic polysaccharide is degraded extracellularly into xyloglucandextrins which are imported by an ABC-transporter, thus allowing the simultaneous uptake of up to nine monosaccharides at the cost of single ATP. No PTS-system was found in R. cellulolyticum but its genome encodes many other uncharacterized ABC-transporters predicted to import sugars, whose substrate(s) is(are) not yet known. The identification of their ligand(s) will help us to establish a map of sugars that can be potentially taken up and processed by our model bacterium, and create a useful database for microbiologists, as similar ABC-transporters are found in many other Gram-positive bacteria. Are some of these ABC-transporters redundant in terms of substrate? Are they gathered in patches or scattered at the surface of the bacterium? These are some of the questions we also plan to address.
Beyond sugar uptake, the recent studies show that the central carbon metabolism in R. cellulolyticum appears to be unconventional and particularly energy saving. We thus plan to explore further the central carbon metabolism of our model bacterium, and determine whether it is GTP- or ATP-driven. This study may also lead to the discovery of novel metabolic enzymes displaying uncommon traits (like the GTP-dependent and reversible hexokinase) that could inspire novel metabolic engineering applied to bacteria of industrial interest.