Riboflavin synthase (RS-α) catalyzes the biosynthesis of riboflavin (vitamin B2) by dismutation of 6,7-dimethyl-8-(1'-D-ribityl)lumazine (Lum) (EC 2.5.1.9). Riboflavin synthases of bacteria and fungi are structurally and evolutionary related to antenna proteins involved in bioluminescence of marine bacteria [1,2]. These proteins seem to have evolved from the duplication of a domain of about 100 residues, the lumazine-binding repeat.

The 3D structure of RS-α, which is an asymmetric homotrimer, shows that both domains form a 6-stranded antiparallel β-barrel (see <PDB:1PKV>) [3], while a C-terminal helix is involved in trimerization. The Lum-binding domain of RS-α forms two Greek-key folds with the topology BBHBBBHB, where most of the substrate binding sites are located in β-strands (B) 4 and 5 and in helix (H) 2 [3,4,5].

Some proteins known to contain a lumazine-binding repeat:

• Riboflavin synthase α chain (EC 2.5.1.9) (gene ribC in Escherichia coli, ribB in Bacillus subtilis and Photobacterium leiognathi, RIB5 in yeast). This enzyme synthesizes riboflavin from two molecules of Lum, a pteridine-derivative.
• Photobacterium phosphoreum lumazine protein (LumP) (gene luxL). LumP is a protein that modulates the color of the bioluminescence emission of bacterial luciferase. In the presence of LumP, light emission is shifted to higher energy values (shorter wavelength). LumP binds non-covalently to 6,7-dimethyl-8-(1'-D-ribityl)lumazine.
• Vibrio fischeri yellow fluorescent protein (YFP) (gene luxY). Like LumP, YFP modulates light emission but towards a longer wavelength. YFP binds non-covalently to FMN.

The profile we developed covers the entire lumazine-binding repeat.