The ykoK RNA motif, or M-box, is a magnesium-sensing RNA structure that controls the expression of magnesium ion transport genes in Bacteria. The ykoK leader RNA motif was originally identified upstream of the ykoK (mgtE) gene in Bacillus subtilis and genes with related functions in other bacteria . Examples of the conserved M-box RNA structure occur upstream of each of the three major families of Mg2+ transporters (CorA, MgtE and MgtA/MgtB) in various bacterial species . The M-box RNAs control expression of downstream genes via regulated terminator formation, a mechanism commonly referred to as transcription attenuation.
The in vitro probing data suggest that the M-box aptamer domain is dominated by secondary structure in the presence of low or no Mg2+ but is substantially rearranged upon Mg2+ association to include higher order tertiary structure. In the presence of high Mg2+, the M-box aptamer is formed, and thereby allowing a transcription terminator structure to form and repressing the expression of the downstream magnesium uptake transporter gene. The solved crystal structure of the M-box riboswitch showed that this RNA aptamer binds multiple Mg2+ ions [15096624