FMN riboswitch

The FMN riboswitch (also known as RFN element) is a highly conserved RNA element which is naturally occurring, and is found frequently in the 5'-untranslated regions of prokaryotic mRNAs that encode for flavin mononucleotide (FMN) biosynthesis and transport proteins.

[4] Regarding the second riboswitch in Bacilius subtilis, premature transcription termination occurs within the 5' untranslated region of the ribDEAHT operon, precluding access to the ribosome-binding site of ypaA mRNA.

FMN's planar isoalloxazine ring system intercalates between A48 and A85 residues on the riboswitch, thereby providing a continuous stacking alignment.

Further, the uracil-like edge of the ring system forms specific Watson–Crick-like hydrogen bonds with a highly conserved A99 residue on the riboswitch.

Special interest is had with FMN riboswitches present in Fusobacterium nucleatum, as this bacterium plays a role in periodontal disease and other human infections, and is considered one of the most pathogenic bacteria of the genus.

Pictured is a FMN molecule bound in the binding sit of the FMN riboswitch. Adenine nucleotides A48, A85, and A99 (depicted with yellow carbons), as well as a positively charged magnesium ion (green) assist in binding. The planar isoalloxazine ring system of FMN ( FMN depicted with green carbons) intercalates between A48 and A85. Uracil-like edges of the FMN ring system form specific Watson–Crick-like hydrogen bonds with the highly conserved A99 residue on the riboswitch. Lastly, the negatively charged phosphate group of FMN interacts with the positively charged magnesium ion, allowing this riboswitch to exhibit selectivity for the FMN . PDB code: 3F2Q
A representation of the 3D structure of the FMN riboswitch. This derived from a crystal structure of the FMN riboswitch bound to FMN. [ 7 ]