SAM-V riboswitch

SAM-V riboswitch is the fifth known riboswitch to bind S-adenosyl methionine (SAM). It was first discovered in the marine bacterium Candidatus Pelagibacter ubique and can also be found in marine metagenomes.[1] SAM-V features a similar consensus sequence and secondary structure as the binding site of SAM-II riboswitch, but bioinformatics scans cluster the two aptamers independently. These similar binding pockets suggest that the two riboswitches have undergone convergent evolution.[2]

SAM-V
Conserved secondary structure of the SAM-V riboswitch.
Identifiers
Symbol	SAM-V
Rfam	RF01826
Other data
RNA type	Cis-reg; Riboswitch;
Domain(s)	Marine metagenome
PDB structures	PDBe

SAM-binding was confirmed using equilibrium dialysis. The riboswitch has been characterised as a 'tandem riboswitch' - it is able to regulate both translation and transcription. When SAM is present in high concentration, SAM-II will bind its ligand and form a terminator stem to halt transcription. If SAM exists in lower concentrations, SAM-V will be transcribed and, if SAM concentration should then increase, it can bind SAM and occlude the Shine-Dalgarno sequence of the downstream open reading frame. This regulation controls parts of the sulfur metabolism of marine bacteria.[2]

The crystal structure of the riboswitch has been solved (PDB 6FZ0). It contains a pseudoknot.[3]