A variant riboswitch aptamer class for S-adenosylmethionine common in marine bacteria.
Riboswitches that sense S-adenosylmethionine (SAM) are widely distributed throughout a variety of bacterial lineages. Four classes of SAM-binding riboswitches have been reported to date, constituting the most diverse collection of riboswitch classes that sense the same compound. Three of these classes, termed SAM-I, SAM-II, and SAM-III represent unique structures that form distinct binding pockets for the ligand. SAM-IV riboswitches carry different conserved sequence and structural features compared to other SAM riboswitches, but nucleotides and substructures corresponding to the ligand binding pocket are identical to SAM-I aptamers. In this article, we describe a fifth class of SAM binding aptamer, which we have termed SAM-V. SAM-V was discovered by analyzing GC-rich intergenic regions preceding metabolic genes in the marine alpha-proteobacterium "Candidatus Pelagibacter ubique." Although the motif is nearly unrepresented in cultured bacteria whose genomes have been completely sequenced, SAM-V is prevalent in marine metagenomic sequences. The consensus sequence and structure of SAM-V show some similarities to that of the SAM-II riboswitch, and it is likely that the two aptamers form similar ligand binding pockets. In addition, we identified numerous examples of a tandem SAM-II/SAM-V aptamer architecture. In this arrangement, the SAM-II aptamer is always positioned 5' of the SAM-V aptamer and the SAM-II aptamer is followed by a predicted intrinsic transcription terminator stem. The SAM-V aptamer, however, appears to use a ribosome binding site occlusion mechanism for genetic regulation. This tandem riboswitch arrangement exhibits an architecture that can potentially control both the transcriptional and translational stages of gene expression.
Poiata
Elena
E
Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, Connecticut 06520, USA.
Meyer
Michelle M
MM
Ames
Tyler D
TD
Breaker
Ronald R
RR
D020561:Alphaproteobacteria / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D052157:Aptamers, Nucleotide / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D009690:Nucleic Acid Conformation; D012313:RNA / Q000737:chemistry / Q000235:genetics; D012436:S-Adenosylmethionine / Q000378:metabolism*
D052157:Aptamers, Nucleotide; D012313:RNA; D012436:S-Adenosylmethionine