Probably a riboflavin-binding protein that interacts with the energy-coupling factor (ECF) ABC-transporter complex. Unlike classic ABC transporters this ECF transporter provides the energy necessary to transport a number of different substrates. The substrates themselves are bound by transmembrane, not extracytoplasmic soluble proteins (By similarity). Mediates riboflavin uptake, probably also transports FMN and roseoflavin. Inhibited by FMN and roseoflavin. Forms a stable energy-coupling factor (ECF) transporter complex composed of a membrane-embedded substrate-binding protein (S component), 2 ATP-binding proteins (A component) and 2 transmembrane proteins (T component). May be able to interact with more than 1 S component at a time (By similarity). Induced by riboflavin deficiency. Riboflavin is stacked with one or more Trp residues in the binding pocket of RibU. Mutant exhibits altered transcriptional control of the riboflavin biosynthesis operon in response to riboflavin and FMN, and does not consume riboflavin from the growth medium. Riboflavin, FMN and roseoflavin, but not FAD, bind to RibU with high affinity and 1:1 stoichiometry. Belongs to the prokaryotic riboflavin transporter (P-RFT) (TC 2.A.87) family.