Catalyzes the formation of 6,7-dimethyl-8-ribityllumazine by condensation of 5-amino-6-(D-ribitylamino)uracil with 3,4-dihydroxy-2-butanone 4-phosphate (PubMed:10482294, PubMed:16165152, PubMed:20195542). This is the penultimate step in the biosynthesis of riboflavin. The isozyme RibH2 but not RibH1 is essential for Brucella intracellular survival and replication inside macrophages or in mice (PubMed:20195542). Displays low catalytic activity in comparison with the isozyme RibH1 (PubMed:16165152). Is a highly immunogenic protein (PubMed:14500496). Activates dendritic cells (DCs) in vitro, increasing the levels of costimulatory molecules and the secretion of pro-inflammatory cytokines, and recruits DCs, B cells and CD8+ T cells in vivo, both effects in a TLR4-dependent manner (PubMed:16455994). Induces the cross presentation of covalently attached peptides and generates a strong and long-lasting humoral immune response without adjuvants; TLR4 signaling is necessary for the induction of the cytotoxic response but not for antigen cross presentation (PubMed:23029192). Elicits a TLR4-mediated protective response against B16 melanoma in mice, slowing tumor growth and prolonging mice survival (PubMed:25973756). (2S)-2-hydroxy-3-oxobutyl phosphate + 5-amino-6-(D-ribitylamino)uracil = 6,7-dimethyl-8-(1-D-ribityl)lumazine + H(+) + 2 H2O + phosphate Cofactor biosynthesis; riboflavin biosynthesis; riboflavin from 2-hydroxy-3-oxobutyl phosphate and 5-amino-6-(D-ribitylamino)uracil: step 1/2. Homodecamer, arranged as a dimer of pentamers. The two ribH genes may be differentially expressed during the Brucella infection cycle. Brucella would use RibH1 for flavin biosynthesis during the extracellular phase and RibH2 during intracellular growth. Cells lacking this gene are not auxotrophic for riboflavin and grow at wild-type rates in both rich and minimal media. But simultaneous disruption of ribH1 and ribH2 is lethal. The ribH2 mutant shows a decrease in survival and replication in macrophages at all time-points and is attenuated in mice. Could be useful as a specific antigen for the serological diagnosis of active infection of both human and bovine brucellosis (PubMed:10482294). Has been used as a protein carrier of foreign peptides and proteins (PubMed:15390265). The described characteristics of BLS make this protein an ideal antigen carrier for vaccine development (PubMed:14660615, PubMed:15390265, PubMed:16455994, PubMed:23029192, PubMed:14500496). The antitumor effect of BLS could lead to a therapeutic strategy utilizing a TLR4 ligand; as the expression of TLR4 has been reported on a large number of tumors, BLS signaling via TLR4 could make a notable contribution to the success of cancer treatment when coadministered with other cancer vaccines or treatments like radiation or chemotherapy (PubMed:25973756). Belongs to the DMRL synthase family.