Antibacterial drug-mechanism module, not a specific disease. Do NOT create a "Riboswitch Targeting" Disease entry. Disorder entries reference nodes via conforms_to (e.g. "bacterial_riboswitch_targeting#Riboswitch-Gated Essential Gene Expression (Riboswitch Target)") and point their riboswitch-directed treatments at that node via target_mechanisms. Nodes are druggable steps and escape routes, not individual compounds.
Three things to get right when conforming to this module. First, the target is the regulatory RNA, not the enzyme it regulates: the resistance mutations map to the aptamer and expression platform, not to a biosynthetic enzyme, which is how the target was identified in the first place. An entry that models ribocil as a riboflavin-synthesis enzyme inhibitor has the mechanism wrong. Second, the salvage bypass decides the spectrum, so it is a node rather than a caveat in prose - exogenous riboflavin suppresses ribocil activity outright, and whether that rules the FMN riboswitch out for Gram-positive infection is unsettled in the literature (see the discussion attached to that node). Third, the FMN riboswitch is poorly conserved across Gram-negative genera, so activity does not transfer between them: aptamer identity relative to E. coli runs from 90 percent in K. pneumoniae down to 61 percent in A. baumannii, and Ribocil C-PA is active against Enterobacteriaceae but not against A. baumannii or P. aeruginosa. Treat each genus as a separate claim.
Roseoflavin is deliberately annotated as a multitarget compound rather than a clean riboswitch inhibitor. Its flavin derivatives also bind and inactivate flavoenzymes, its activity in E. coli is not fully suppressed by exogenous riboflavin, and the conservation between bacterial and human flavoenzymes is a stated safety concern. Citing it as evidence for riboswitch occupancy alone overstates what the source says.
Ribocil has no class in either vocabulary that therapeutic_agent accepts, so its treatment entry carries a free-text preferred_term with no term binding, per the ontology term contract. Searched on 2026-10-03 via the OLS4 search API. CHEBI with exact=true for "ribocil": no hits, while the same query returns CHEBI:72346 for "roseoflavin", which is bound below. NCIT - the other vocabulary the ChemicalEntityTerm enum admits, through its NCIT:C1909 Pharmacologic Substance branch - with exact=true for "ribocil", "ribocil-C" and "Ribocil C-PA", and again without exact=true for "ribocil": no hits in any of the four. No term was manufactured for any member of the series.
Does environmental riboflavin salvage rule the FMN riboswitch out as an antibacterial target in Gram-positive pathogens, or does simultaneous engagement of the biosynthesis and uptake riboswitches overcome it?
KNOWLEDGE GAP
riboswitch_salvage_bypass_gram_positive_suitability
Attached to:
Exogenous Metabolite Salvage Bypass
Two groups reach opposite conclusions from the same salvage observation, and this is a disagreement over how to read the published evidence rather than an absence of it. The dual-targeting work argues that because one riboswitch class gates both synthesis and transport, a compound engaging both closes the bypass, and reports that ribocil-C and roseoflavin do so in MRSA and E. faecalis; its title places that work in an infectious setting, it claims pharmacological evidence, and it is indexed under the MeSH heading Mice, so in vivo experiments with the dual-engaging compounds exist and the gap does not claim otherwise. The Gram-negative work, published three years later, nonetheless treats the Gram-positive case as unresolved, writing that scavenging tempers enthusiasm for the FMN riboswitch target in S. aureus while citing that earlier work. So the later group read the dual-targeting result and did not take it as settling the question.
What a reader of this module cannot get from the repository is the design of that in vivo experiment - in particular whether the host environment was riboflavin-replete, which is the condition on which the two predictions diverge. The cached record for the dual-targeting paper is abstract-only and its full text was not reachable from the curation environment: the publisher returns HTTP 403, and the article is in neither PMC nor Europe PMC open access. That is a limit of this curation pass, not a finding about the literature, and it is recorded here so the next curator checks the full text rather than inheriting the gap. Until someone does, a Gram-positive conformer should record the salvage node explicitly rather than treating repression of biosynthesis as sufficient.
Proposed experiments:
Dual FMN riboswitch engagement in a Gram-positive infection model under defined host riboflavin availability
Riboswitch-Gated Essential Gene Expression (Riboswitch Target)
therapeutic vulnerability
A riboswitch is a non-coding RNA structure in the untranslated region of a bacterial mRNA, built from an aptamer domain that binds a small-molecule ligand and a downstream expression platform that converts that binding event into a change in expression of the adjacent gene, by attenuating transcription or by occluding the ribosome binding site. Over 24 ligand classes are known, including the enzyme cofactors thiamine pyrophosphate and flavin mononucleotide, several amino acids, the purines, and inorganic ions. This is the druggable element: it is bacterial-specific with no known mammalian counterpart, and it sits upstream of pathways the cell needs to grow. The FMN riboswitch is the worked instance, controlling both de novo riboflavin synthesis and riboflavin transport; the T-box riboswitches of Gram-positive bacteria are a second class, sensing tRNA aminoacylation state to control amino-acid metabolism and transport.
Downstream
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Ligand-Mimetic Riboswitch Occupancy
A synthetic mimic or an antimetabolite analogue of the cognate ligand binds the aptamer domain, which is the drug-target engagement step.
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Riboswitch Target-Site Resistance
Because the target is the RNA element itself, mutations in the aptamer or the expression platform abolish compound binding without disabling the pathway.
Show evidence (6 references)
"Riboswitches are non-coding RNA structures located in messenger RNAs that bind endogenous ligands, such as a specific metabolite or ion, to regulate gene expression."
Establishes the class of element this node represents. Quoted from the paper's framing of riboswitch biology rather than from its own results, hence quote_role BACKGROUND; evidence_source is OTHER because the quoted sentence states established RNA biology rather than reporting a study of any particular evidence type.
"Riboswitches consist of 2 functionally distinct modules an aptamer ligand binding domain and a downstream expression platform."
Supports the two-domain architecture this node describes, which is what makes the element druggable at the aptamer while the regulatory output is produced by the expression platform.
"binding of the cognate ligand to the aptamer domain induces a conformational change in the expression platform that regulates gene expression typically through attenuation of transcription or inhibition of translation"
Supports the two regulatory outputs named in this node and the binding of the GO transcriptional-attenuation term.
+ 3 more references
Ligand-Mimetic Riboswitch Occupancy
central effector
An exogenous compound occupies the aptamer's ligand-binding pocket and holds the expression platform in the conformation that shuts the downstream gene off. The compound need not resemble the natural ligand: ribocil is structurally distinct from flavin mononucleotide yet binds the same pocket and acts as a selective synthetic mimic, which is the finding that opened this target class beyond cofactor analogues. Occupancy is the step a riboswitch ligand, an antimetabolite that is converted intracellularly into a riboswitch-binding derivative, and a riboswitch-directed antisense oligonucleotide all converge on, by different chemistry. The GO term bound on this node is the aptamer's own FMN-binding function, which is what defines the pocket being occupied; it is deliberately not a claim that these compounds are FMN analogues, since the finding that opened this target class is precisely that an occupying compound need not resemble the natural ligand.
Downstream
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Repression of Metabolite Biosynthesis and Uptake
The locked-off expression platform suppresses the biosynthesis and transport genes the riboswitch controls.
Show evidence (2 references)
"ribocil, a highly selective chemical modulator of bacterial riboflavin riboswitches, which was identified in a phenotypic screen and acts as a structurally distinct synthetic mimic of the natural ligand, flavin mononucleotide, to repress riboswitch-mediated ribB gene expression and inhibit..."
The primary report of ligand-mimetic occupancy by a structurally unrelated synthetic compound, which is the claim this node makes. Evidence source is IN_VITRO because the finding rests on a bacterial phenotypic screen and cell-free riboswitch binding rather than on animal or human data.
"Although ribocil is structurally distinct from the natural riboswitch regulatory ligand flavin mononucleotide, ribocil binding to the riboswitch results in efficient repression of ribB expression and inhibition of riboflavin biosynthesis and bacterial growth."
Confirms that aptamer occupancy, rather than enzyme inhibition, is what links compound binding to repression of the downstream gene.
Flavin Cofactor Starvation and Growth Arrest
consequence
Riboflavin is the precursor of flavin mononucleotide and flavin adenine dinucleotide, the cofactors of the flavoenzymes that carry much of intermediary metabolism. Repressing its supply depletes those pools and arrests growth; the measured fall in intracellular flavins on treatment is what confirms that an antibacterial effect runs through this route rather than through an off-target one.
Show evidence (2 references)
"Ribocil C targets the flavin mononucleotide (FMN) riboswitch, ultimately inducing bacterial death through riboflavin starvation."
States the causal route from riboswitch engagement to bacterial death by starvation, which is the claim of this node.
"Using an LC-MS/MS assay to monitor flavin levels in E. coli cultures treated with Ribocil C-PA (at 0.5x the MIC), reduction in flavins was observed"
The measurement behind the cofactor-depletion claim, in treated E. coli cultures, with the inactive enantiomer as control.
Riboswitch Target-Site Resistance
adaptive escape
Resistance arises in the RNA target itself, by mutation of the aptamer domain or of the expression platform, which abolishes compound binding or uncouples binding from repression while leaving the regulated pathway intact. Selection of such mutants is how the target was identified - resistance mapped to the regulatory RNA rather than to any biosynthetic enzyme. The frequency of resistance is high enough to be a stated liability for this target class, and resistant mutants grow at rates close to wild type, so there is little fitness cost to offset it.
Show evidence (3 references)
"resistance mutation selection and whole-genome sequencing showed that the target of ribocil was not an enzyme in the riboflavin biosynthesis pathway, but instead the flavin mononucleotide riboswitch"
Supports both halves of this node: resistance arises at the RNA element, and that is what established the RNA rather than an enzyme as the target.
"After generating and sequencing 20 different E. coli BW25113 clones resistant to Ribocil C-PA, 16 different mutations were observed, all of which mapped back to either the FMN riboswitch aptamer or expression platform"
Localizes resistance mutations to the aptamer and the expression platform, the two sub-domains named in this node.
"While the elevated frequency of resistance that is apparently associated with this target is not ideal"
The authors' own statement that resistance frequency is a liability of this target class, supporting the caveat in this node rather than a claim about any one compound.
FMN Riboswitch Ligand (Ribocil Series)
Agent:
ribocil
Relation: this treatment uses this therapeutic agent
This treatment uses ribocil.
Platform:
Small molecule
Ribocil and its analogues ribocil-B, ribocil-C and the accumulation-optimized derivative Ribocil C-PA are synthetic small molecules that occupy the FMN riboswitch aptamer and repress the riboflavin biosynthesis genes it controls. This is the reference instance of the riboswitch drug-target design pattern for conforming entries. All of it is preclinical: the series has in vitro activity against Enterobacteriaceae clinical isolates and efficacy in mouse pneumonia and septicaemia models, and no member has entered clinical development. Activity does not generalize across Gram-negative genera, because the FMN riboswitch is poorly conserved between them.
Evidence: 2
Riboflavin Antimetabolite (Roseoflavin)
Platform:
Small molecule
Roseoflavin is a natural riboflavin analogue from Streptomyces davawensis. It is a prodrug, converted intracellularly to roseoflavin mononucleotide and roseoflavin adenine dinucleotide, and the mononucleotide binds FMN riboswitches and represses the riboflavin synthesis and transport genes. It is recorded here as a multitarget compound, not a clean riboswitch inhibitor: its flavin derivatives also associate with and inactivate flavoenzymes, its activity in E. coli is not fully suppressed by exogenous riboflavin, and the conservation of flavoenzymes between bacteria and humans is a stated safety concern for developing it as an antibiotic. Preclinical.
Evidence: 3
FMN Riboswitch-Directed Antisense Oligonucleotide
Platform:
Antisense oligonucleotide
RNase H knockdown
Delivery: Ligand conjugate
Targeting: Peptide
RNA target:
FMN riboswitch mRNA (Staphylococcus aureus)
An alternative chemistry against the same target: chimeric antisense oligonucleotides designed against the conserved FMN riboswitch, conjugated to the pVEC cell-penetrating peptide for uptake. Reported activity against MRSA in the low nanomolar range, with no inhibition of E. coli, which lacks the targeted riboswitch sequence - the negative control that establishes specificity. A conforming entry whose treatment is an oligonucleotide should also consider the antisense_oligonucleotide_therapy module, which covers the RNase H effector step this exploits. Preclinical.
Evidence: 1
T-box Riboswitch Inhibitor
Platform:
Small molecule
T-box riboswitches sense the aminoacylation state of their cognate tRNAs and control amino-acid metabolism and transport genes in Gram-positive bacteria. A virtual screen against T-box crystal structures produced T-box-i, which disrupts T-box-mediated transcription in vivo, showed no cytotoxicity in mammalian cells, and did not induce resistance in S. aureus cultures - the last being notable against the high resistance frequency seen with FMN riboswitch ligands. Preclinical, and included here to keep the module from being read as FMN-only.
Evidence: 2