SAM-V riboswitch
SAM-V riboswitch is the fifth known riboswitch to bind S-adenosyl methionine. It was first discovered in the marine bacterium Candidatus Pelagibacter ubique and can also be found in marine metagenomes. 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.
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.
The crystal structure of the riboswitch has been solved. It contains a pseudoknot.
