Substrate-binding (S) component of an energy-coupling factor (ECF) ABC-transporter complex (By similarity). Mediates uptake of riboflavin (PubMed:27672192). Can also transport roseoflavin (RoF), and probably 8-demethyl-8-aminoriboflavin (AF), toxic riboflavin analogs (PubMed:27672192). In addition, during intracellular growth, binds and imports FMN and FAD directly from the cytosol of host cells (PubMed:35316134). Essential for virulence and intracellular growth (PubMed:35316134). Forms a stable energy-coupling factor (ECF) transporter complex composed of 2 membrane-embedded substrate-binding proteins (S component), 2 ATP-binding proteins (A component) and 2 transmembrane proteins (T component). Mutant lacking this gene is completely avirulent in mice but has no detectable growth defect in nutrient-rich media (PubMed:35316134). Deletion mutant shows only a slight defect in growth in media containing riboflavin, but does not replicate in chemically defined media with FMN or FAD (PubMed:35316134). Belongs to the prokaryotic riboflavin transporter (P-RFT) (TC 2.A.87) family.