RbsD is the only protein whose biochemical function is unknown among the six gene products of the rbs operon involved in the active transport of ribose. FucU, a paralogue of RbsD conserved from bacteria to human, is also the only protein whose function is unknown among the seven gene products of the L-fucose regulon. Here we report the crystal structures of Bacillus subtilis RbsD, which reveals a novel decameric toroidal assembly of the protein. Nuclear magnetic resonance and other studies on

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... sD reveal that the intersubunit cleft of the protein binds specific forms of D-ribose, but it does not have an enzyme activity toward the sugar. Likewise, FucU binds L-fucose but lacks an enzyme activity toward this sugar. We conclude that RbsD and FucU are cytoplasmic sugar-binding proteins, a novel class of proteins whose functional role may lie in helping influx of the sugar substrates. D-Ribose is the most abundant and important sugar that is a component of nucleic acids and many other biomolecules as well as an energy source. The Escherichia coli high affinity ribose transport system consists of six proteins encoded by the rbs operon (rbsD, -A, -C, -B, -K, and -R). Ribose-binding protein (RBP 1 ; encoded by rbsB) binds D-ribose at the periplasmic space (1). The sugar-bound form of RBP interacts with the membrane-bound permease (encoded by rbsC) that transports ribose across the inner membrane (2) coupled with the action of the ATP-binding cassette component (encoded by rbsA) (3). The transported D-ribose is phosphorylated into D-ribose 5-phosphate by ribokinase (encoded by rbsK), which is believed to be the first step for the utilization of the sugar in biosynthetic or metabolic pathways (4). RbsR is the repressor that binds to the transcriptional start site of the rbs operon. Among the six components, RbsD is the only protein whose function is un-*