May be directly involved in the regulation of the rib genes. C-terminal part of RibR specifically binds to RFN of the rib leader of the riboflavin biosynthetic operon. The RFN element is a sequence within the rib-leader mRNA reported to serve as a receptor for an FMN-dependent riboswitch. Possibly, RibR produces the comodulator FMN through its own N-terminal flavokinase activity. FMN-activated RibR may stabilize the anti-anti terminator structure of RFN mRNA, causing transcription termination of the rib genes in trans. ATP + riboflavin = ADP + FMN + H(+) Repressed by the HTH-type transcriptional regulator CymR. Mutants lacking this gene are deregulated with respect to flavin (riboflavin, FMN and FAD) biosynthesis. Overexpression of RibR suppresses the riboflavin-overproducing ribC-defective phenotype, which is the major riboflavin kinase responsible of FMN biosynthesis by riboflavin phosphorylation (PubMed:17590224 and PubMed:15668000). Belongs to the RibR family.