Collection of regulogs for SAH_riboswitch (RF01057) RNA regulatory element
The SAH riboswitch was originally identified by bioinformatics as a conserved RNA-like structure located upstream of the methionine metabolism genes in various Proteobacteria and in some Actinobacteria . Later, the SAM-IV riboswitch was shown to bind directly to S-adenosylhomocysteine (SAH) . S-adenosylmethionine (SAM) is a common co-substrate involved in methyl group transfers and is synthesized from the methionine amino acid by the SAM synthase MetK. SAH is produced upon SAM-dependent methylation, and the intracellular balance of the two species has to be closely monitored to maintain cellular homeostasis and avoid toxicity .
The SAH riboswitch is important for maintaining the SAM/SAH ratio, as it up-regulates many of the genes involved in recycling of SAH to create more SAM. The up-regulation of downstream genes can be achieved through anti-terminator formation or exposure of SD sequence . The ability of SAH riboswitch to differentiate SAH from SAM is especially remarkable given the larger cellular pool of SAM under normal conditions .
The x-ray crystal structure of the SAH riboswitch complexed with SAH was solved . This structure reveals an unusual pseudoknot topology that creates a shallow groove on the surface of the RNA that binds SAH primarily through interactions with the adenine ring and methionine main chain atoms and discriminates against SAM through a steric mechanism.
|Phylum||Regulog||RNA regulons (studied genomes)||RNA sites|
|Actinobacteria||SAH_riboswitch - Mycobacteriaceae||6 (9)||6|
|Proteobacteria||SAH_riboswitch - Burkholderia||8 (8)||8|
|Proteobacteria||SAH_riboswitch - Pseudomonadaceae||7 (8)||7|
|Proteobacteria||SAH_riboswitch - Ralstonia||6 (6)||6|
|Proteobacteria/Beta||SAH_riboswitch - Comamonadaceae||11 (11)||21|
|Proteobacteria/Beta||SAH_riboswitch - Various betaproteobacteria||11 (12)||14|
|Proteobacteria/Gamma||SAH_riboswitch - Moraxellaceae||2 (4)||2|
|Proteobacteria/Gamma||SAH_riboswitch - Xanthomonadales||3 (4)||3|