Keywords with Definitions 1859.4 Complex Riboswitches
Although the majority of natural riboswitches provides a simple yet efficient
means of metabolite‐controlled gene expression, few noncanonical riboswitches
that contain two aptamers have been reported [30–32]. These riboswitches have
been shown or predicted to exhibit functions more complex compared with the
single aptamer riboswitches, such as cooperative response to a ligand [30] or
Boolean logic response to two distinct metabolites [31].
Several synthetic mimics of these complex bacterial riboswitches have been
constructed. Sharma et al. constructed riboswitches that function as AND and
NAND logic gates in response to theophylline and thiamine pyrophosphate
(TPP) by tetA genetic selection [33]. More recently, Muranaka and Yokobayashi
combined two independently optimized TPP riboswitches into the same 5′ UTR
to construct a “band‐pass” riboswitch that activates gene expression within a
limited range of the ligand concentration [34].
In an alternative approach, Klauser et al. recently combined multiple ribozyme‐
based switches to create logic gates [35]. Qi et al. co‐expressed two allosteric
trans‐acting ncRNA regulators to demonstrate a NOR logic gate with a small
molecule and a protein as inputs [26].
9.5 Conclusions
As briefly summarized previously, synthetic riboswitches are attractive tools for
interfacing bacterial synthetic circuits with small molecules of synthetic or natu-
ral origins. In particular, the demonstrated versatility of RNA aptamers to recog-
nize a wide variety of molecules is of practical importance although adapting in
vitro selected aptamers to intracellular applications remains a technical chal-
lenge. It is also noteworthy that complex functions that include molecular recog-
nition, gene regulation, and, in some cases, multiple signal integration can all be
encoded within one or few short segments of RNA, whereas equivalent switches
and circuits based on protein transcription factors would require much larger
genetic information.
Keywords with Definitions
Untranslated region (UTR) The sequences within an mRNA that do not code
for a protein
Aptamer A nucleotide sequence (e.g., RNA) that is capable of binding specific
target molecules such as small molecules and proteins
Expression platform A sequence within a riboswitch that is responsible for
ligand‐mediated structural change resulting in gene regulation
Riboswitch A stretch of RNA sequence mostly found in the 5′ UTR of bacterial
mRNAs that binds a metabolite through an aptamer and regulates expression
of the cis‐gene