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. Author manuscript; available in PMC: 2016 Jul 25.
Published in final edited form as: RNA Biol. 2010 Jan 25;7(1):104–110. doi: 10.4161/rna.7.1.10757

Figure 4.

Figure 4

The SMK box riboswitch. Regulation is at the level of translation initiation for all SMK box riboswitches uncovered to date. In the absence of SAM, the SD sequence is available for binding of the 30S ribosomal subunit, and sequences at the 5' end pair to form the P0 helix. Binding of SAM (*) results in disruption of the P0 helix and sequestration of the SD sequence in the ASD-SD helix (P1). SAM makes direct contacts with residues within the SD. Sequestration of the SD sequence prevents ribosome binding, resulting in repression of gene expression.