A novel N,N-8-amino-8-demethyl-D-riboflavin Dimethyltransferase (RosA) catalyzing the two terminal steps of roseoflavin biosynthesis in Streptomyces davawensis

Isolation, characterisation and description of the roseoflavin producer Streptomyces berlinensis sp. nov

The Gram-positive bacteria Streptomyces davaonensis and Streptomyces cinnabarinus have been the only organisms known to produce roseoflavin, a riboflavin (vitamin B2) derived red antibiotic. Using a selective growth medium and a phenotypic screening, we were able to isolate a novel roseoflavin producer from a German soil sample. The isolation procedure was repeated twice, that is, the same strain could be isolated from the same location in Berlin 6 months and 12 months after its first isolation. Whole genome sequencing of the novel roseoflavin producer revealed an unusual chromosomal arrangement and the deposited genome sequence of the new isolate (G + C content of 71.47%) contains 897 genes per inverted terminal repeat, 6190 genes in the core and 107 genes located on an illegitimate terminal end. We identified the roseoflavin biosynthetic genes rosA, rosB and rosC and an unusually high number of riboflavin biosynthetic genes. Overexpression of rosA, rosB and rosC in Escherichia coli and enzyme assays confirmed their predicted functions in roseoflavin biosynthesis. A full taxonomic analysis revealed that the isolate represents a previously unknown Streptomyces species and we propose the name Streptomyces berlinensis sp. nov. for this roseoflavin producer.

Structure of methyltransferase RedM that forms the dimethylpyrrolinium of the bisindole reductasporine

Bisindoles are biologically active natural products that arise from the oxidative dimerization of two molecules of l-tryptophan. In bacterial bisindole pathways, a core set of transformations is followed by the action of diverse tailoring enzymes that catalyze reactions that lead to diverse bisindole products. Among bisindoles, reductasporine is distinct due to its dimethylpyrrolinium structure. Its previously reported biosynthetic gene cluster encodes two unique tailoring enzymes, the imine reductase RedE and the dimethyltransferase RedM, which were shown to produce reductasporine from a common bisindole intermediate in recombinant E. coli. To gain more insight into the unique tailoring enzymes in reductasporine assembly, we reconstituted the biosynthetic pathway to reductasporine in vitro and then solved the 1.7 Å resolution structure of RedM. Our work reveals RedM adopts a variety of conformational changes with distinct open and closed conformations, and site-directed mutagenesis alongside sequence analysis identifies important active site residues. Finally, our work sets the stage for understanding how RedM evolved to react with a pyrrolinium scaffold and may enable the development of new dimethyltransferase catalysts.

Efficient production of bacterial antibiotics aminoriboflavin and roseoflavin in eukaryotic microorganisms, yeasts

BACKGROUND

Actinomycetes Streptomyces davaonensis and Streptomyces cinnabarinus synthesize a promising broad-spectrum antibiotic roseoflavin, with its synthesis starting from flavin mononucleotide and proceeding through an immediate precursor, aminoriboflavin, that also has antibiotic properties. Roseoflavin accumulation by the natural producers is rather low, whereas aminoriboflavin accumulation is negligible. Yeasts have many advantages as biotechnological producers relative to bacteria, however, no recombinant producers of bacterial antibiotics in yeasts are known.

RESULTS

Roseoflavin biosynthesis genes have been expressed in riboflavin- or FMN-overproducing yeast strains of Candida famata and Komagataella phaffii. Both these strains accumulated aminoriboflavin, whereas only the latter produced roseoflavin. Aminoriboflavin isolated from the culture liquid of C. famata strain inhibited the growth of Staphylococcus aureus (including MRSA) and Listeria monocytogenes. Maximal accumulation of aminoriboflavin in shake-flasks reached 1.5 mg L- 1 (C. famata), and that of roseoflavin was 5 mg L- 1 (K. phaffii). Accumulation of aminoriboflavin and roseoflavin by K. phaffii recombinant strain in a bioreactor reached 22 and 130 mg L- 1, respectively. For comparison, recombinant strains of the native bacterial producer S. davaonensis accumulated near one-order less of roseoflavin while no recombinant producers of aminoriboflavin was reported at all.

CONCLUSIONS

Yeast recombinant producers of bacterial antibiotics aminoriboflavin and roseoflavin were constructed and evaluated.

Roseoflavin, a Natural Riboflavin Analogue, Possesses In Vitro and In Vivo Antiplasmodial Activity

The ability of the human malaria parasite Plasmodium falciparum to access and utilize vital nutrients is critical to its growth and proliferation. Molecules that interfere with these processes could potentially serve as antimalarials. We found that two riboflavin analogues, roseoflavin and 8-aminoriboflavin, inhibit malaria parasite proliferation by targeting riboflavin metabolism and/or the utilization of the riboflavin metabolites flavin mononucleotide and flavin adenine dinucleotide. An additional eight riboflavin analogues were evaluated, but none were found to be more potent than roseoflavin, nor was their activity on target. Focusing on roseoflavin, we tested its antimalarial activity in vivo against Plasmodium vinckei vinckei in mice. We found that roseoflavin decreased the parasitemia by 46-fold following a 4 day suppression test and, on average, increased the survival of mice by 4 to 5 days. Our data are consistent with riboflavin metabolism and/or the utilization of riboflavin-derived cofactors being viable drug targets for the development of new antimalarials and that roseoflavin could serve as a potential starting point.

A second riboflavin import system is present in flavinogenic Streptomyces davaonensis and supports roseoflavin biosynthesis

The antibiotic roseoflavin is produced by Streptomyces davaonensis in the stationary phase of growth. To support biosynthesis of the secondary metabolite roseoflavin, S. davaonensis underwent several genetic adaptations with regard to metabolism of the roseoflavin precursor and primary metabolite riboflavin. In addition to 17 riboflavin biosynthesis genes at different chromosomal locations, S. davaonensis contains the riboflavin transporter gene ribM being part of the riboflavin biosynthetic operon ribE1MAB5H. Deletion of this operon generated riboflavin auxotrophic S. davaonensis strains. The finding that S. davaonensis ΔribE1MAB5H was able to grow in a culture medium containing low levels of riboflavin indicated that in addition to RibM, a second riboflavin transporter is present in this bacterium. The S. davaonensis genes ribXY (former rosXY) represented candidate genes for such a second riboflavin transport system and the results of our experiments now show that RibXY from S. davaonensis is a highly efficient riboflavin importer but not a roseoflavin importer.

Antibacterial Potential of Termite-Associated Streptomyces spp

Twenty-one strains of termite-associated actinomycetes were tested for their activities against three bacteria. The results showed that nine strains showed bacteriostatic activities against at least one tested bacterium, and the actinomycete YH01, which was isolated from the body surface of the queen of Odontotermes formosanus, had potent antibacterial activity. The YH01 was further identified as Streptomyces davaonensis. Two metabolites roseoflavin (1) and 8-methylamino-8-demethyl-d-riboflavin (2) were isolated and purified from S. davaonensis YH01. Their structures were determined by NMR, MS, and the related literature. The metabolite 1 showed strong inhibition activities against Bacillus subtilis (MIC = 1.56 μg/mL) and Staphylococcus aureus (MIC = 3.125 μg/mL), which were comparable to referenced gentamycin sulfate, with MIC values of 1.56 and 1.56 μg/mL, respectively. Furthermore, the anti-MRSA potential of compound 1 was determined against nine kinds of MRSA strains, with inhibition zones in the ranges of 12.7-19.7 mm under a concentration of 15 μg/6 mm discs and 18.3-22.7 mm under a concentration of 30 μg/6 mm discs. However, metabolite 1 had no inhibitory effect on Gram-negative bacteria. These results suggested that roseoflavin produced by YH01 holds promise for use against Gram-positive bacteria, especially to MRSA.

The novel phosphatase RosC catalyzes the last unknown step of roseoflavin biosynthesis in Streptomyces davaonensis

The bacterium Streptomyces davaonensis produces the antibiotic roseoflavin, which is a riboflavin (vitamin B₂ ) analog. The key enzyme of roseoflavin biosynthesis is the 8-demethyl-8-amino-riboflavin-5'-phosphate (AFP) synthase RosB which synthesizes AFP from riboflavin-5'-phosphate. AFP is not a substrate for the last enzyme of roseoflavin biosynthesis the N, N-dimethyltransferase RosA, which generates roseoflavin from 8-demethyl-8-amino-riboflavin (AF). Consequently, the roseoflavin biosynthetic pathway depends on a phosphatase, which dephosphorylates AFP to AF. Here, we report on the identification and characterization of such an AFP phosphatase which we named RosC. The gene rosC is located immediately downstream of rosA and both genes are part of a cluster comprising 10 genes. Deletion of rosC from the chromosome of S. davaonensis led to reduced roseoflavin levels in the corresponding recombinant strain. In contrast to wild-type S. davaonensis, cell-free extracts of the rosC deletion strain did not catalyze dephosphorylation of AFP. RosC was purified from an overproducing Escherichia coli strain. RosC is the fastest enzyme of roseoflavin biosynthesis (k_cat 31.3 ± 1.4 min^-1 ). The apparent K_M for the substrate AFP was 34.5 µM. Roseoflavin biosynthesis is now completely understood--it takes three enzymes (RosB, RosC, and RosA) to convert the flavin cofactor riboflavin-5'-phosphate into a potent antibiotic.

Metabolic engineering of roseoflavin-overproducing microorganisms

Background

Roseoflavin, a promising broad-spectrum antibiotic, is naturally produced by the bacteria Streptomyces davaonensis and Streptomyces cinnabarinus. The key enzymes responsible for roseoflavin biosynthesis and the corresponding genes were recently identified. In this study we aimed to enhance roseoflavin production in S. davaonensis and to synthesize roseoflavin in the heterologous hosts Bacillus subtilis and Corynebacterium glutamicum by (over)expression of the roseoflavin biosynthesis genes.

Results

While expression of the roseoflavin biosynthesis genes from S. davaonensis was not observed in recombinant strains of B. subtilis, overexpression was successful in C. glutamicum and S. davaonensis. Under the culture conditions tested, a maximum of 1.6 ± 0.2 µM (ca. 0.7 mg/l) and 34.9 ± 5.2 µM (ca. 14 mg/l) roseoflavin was produced with recombinant strains of C. glutamicum and S. davaonensis, respectively. In S. davaonensis the roseoflavin yield was increased by 78%.

Conclusions

The results of this study provide a sound basis for the development of an economical roseoflavin production process.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1181-2) contains supplementary material, which is available to authorized users.

The hidden enzymology of bacterial natural product biosynthesis

Bacterial natural products display astounding structural diversity, which, in turn, endows them with a remarkable range of biological activities that are of significant value to modern society. Such structural features are generated by biosynthetic enzymes that construct core scaffolds or perform peripheral modifications, and can thus define natural product families, introduce pharmacophores and permit metabolic diversification. Modern genomics approaches have greatly enhanced our ability to access and characterize natural product pathways via sequence-similarity-based bioinformatics discovery strategies. However, many biosynthetic enzymes catalyse exceptional, unprecedented transformations that continue to defy functional prediction and remain hidden from us in bacterial (meta)genomic sequence data. In this Review, we highlight exciting examples of unusual enzymology that have been uncovered recently in the context of natural product biosynthesis. These suggest that much of the natural product diversity, including entire substance classes, awaits discovery. New approaches to lift the veil on the cryptic chemistries of the natural product universe are also discussed.

Structure-Guided Analyses of a Key Enzyme Involved in the Biosynthesis of an Antivitamin

RosB catalyzes the formation of 8-aminoriboflavin 5'-phosphate (AFP), the key intermediate in roseoflavin biosynthesis, from the metabolic precursors riboflavin 5'-phosphate (RP, also known as FMN) and glutamate. The conversion of the aromatic methyl group at position 8 in RP into the aromatic amine in AFP occurs via two intermediates, namely, the aldehyde 8-formyl-RP and the acid 8-carboxy-RP. To gain insights into the mechanism for this chemically challenging transformation, we utilized a structure-based approach to identify active site variants of RosB that stall the reaction at various points along the reaction coordinate. Crystal structures of individual variants in complex with different reaction intermediates, identified via mass spectroscopic analysis, illuminate conformational changes that occur at the active site during multistep conversion. These studies provide a plausible route for the progression of the reaction and a molecular rationale for the mechanism of this unusual biocatalyst.

Uptake and Metabolism of Antibiotics Roseoflavin and 8-Demethyl-8-Aminoriboflavin in Riboflavin-Auxotrophic Listeria monocytogenes

The riboflavin analogs roseoflavin (RoF) and 8-demethyl-8-aminoriboflavin (AF) are produced by the bacteria Streptomyces davawensis and Streptomyces cinnabarinus Riboflavin analogs have the potential to be used as broad-spectrum antibiotics, and we therefore studied the metabolism of riboflavin (vitamin B₂), RoF, and AF in the human pathogen Listeria monocytogenes, a bacterium which is a riboflavin auxotroph. We show that the L. monocytogenes protein Lmo1945 is responsible for the uptake of riboflavin, RoF, and AF. Following import, these flavins are phosphorylated/adenylylated by the bifunctional flavokinase/flavin adenine dinucleotide (FAD) synthetase Lmo1329 and adenylylated by the unique FAD synthetase Lmo0728, the first monofunctional FAD synthetase to be described in bacteria. Lmo1329 generates the cofactors flavin mononucleotide (FMN) and FAD, whereas Lmo0728 produces FAD only. The combined activities of Lmo1329 and Lmo0728 are responsible for the intracellular formation of the toxic cofactor analogs roseoflavin mononucleotide (RoFMN), roseoflavin adenine dinucleotide (RoFAD), 8-demethyl-8-aminoriboflavin mononucleotide (AFMN), and 8-demethyl-8-aminoriboflavin adenine dinucleotide (AFAD). In vivo reporter gene assays and in vitro transcription/translation experiments show that the L. monocytogenes FMN riboswitch Rli96, which controls expression of the riboflavin transport gene lmo1945, is negatively affected by riboflavin/FMN and RoF/RoFMN but not by AF/AFMN. Treatment of L. monocytogenes with RoF or AF leads to drastically reduced FMN/FAD levels. We suggest that the reduced flavin cofactor levels in combination with concomitant synthesis of inactive cofactor analogs (RoFMN, RoFAD, AFMN, and AFAD) explain why RoF and AF contribute to antibiotic activity in L. monocytogenes IMPORTANCE: The riboflavin analogs roseoflavin (RoF) and 8-demethyl-8-aminoriboflavin (AF) are small molecules which are produced by Streptomyces davawensis and Streptomyces cinnabarinus RoF and AF were reported to have antibacterial activity, and we studied how these compounds are metabolized by the human bacterial pathogen Listeria monocytogenes We found that the L. monocytogenes protein Lmo1945 mediates uptake of AF and RoF and that the combined activities of the enzymes Lmo1329 and Lmo0728 are responsible for the conversion of AF and RoF to toxic cofactor analogs. Comparative studies with RoF and AF (a weaker antibiotic) suggest that the reduction in FMN/FAD levels and the formation of inactive FMN/FAD analogs explain to a large extent the antibiotic activity of AF and RoF.

A Remarkable Oxidative Cascade That Replaces the Riboflavin C8 Methyl with an Amino Group during Roseoflavin Biosynthesis

Roseoflavin is a naturally occurring riboflavin analogue with antibiotic properties. It is biosynthesized from riboflavin in a reaction involving replacement of the C8 methyl with a dimethylamino group. Herein we report the identification of a flavin-dependent enzyme that converts flavin mononucleotide (FMN) and glutamate to 8-amino-FMN via the intermediacy of 8-formyl-FMN. A mechanistic proposal for this remarkable transformation is proposed.

Structural and kinetic studies on RosA, the enzyme catalysing the methylation of 8-demethyl-8-amino-d-riboflavin to the antibiotic roseoflavin

N,N‐8‐demethyl‐8‐amino‐d‐riboflavin dimethyltransferase (RosA) catalyses the final dimethylation of 8‐demethyl‐8‐amino‐d‐riboflavin (AF) to the antibiotic roseoflavin (RoF) in Streptomyces davawensis. In the present study, we solved the X‐ray structure of RosA, and determined the binding properties of substrates and products. Moreover, we used steady‐state and rapid reaction kinetic studies to obtain detailed information on the reaction mechanism. The structure of RosA was found to be similar to that of previously described S‐adenosylmethionine (SAM)‐dependent methyltransferases, featuring two domains: a mainly α‐helical ‘orthogonal bundle’ and a Rossmann‐like domain (α/β twisted open sheet). Bioinformatics studies and molecular modelling enabled us to predict the potential SAM and AF binding sites in RosA, suggesting that both substrates, AF and SAM, bind independently to their respective binding pocket. This finding was confirmed by kinetic experiments that demonstrated a random‐order ‘bi‐bi’ reaction mechanism. Furthermore, we determined the dissociation constants for substrates and products by either isothermal titration calorimetry or UV/Vis absorption spectroscopy, revealing that both products, RoF and S‐adenosylhomocysteine (SAH), bind more tightly to RosA compared with the substrates, AF and SAM. This suggests that RosA may contribute to roseoflavin resistance in S. davawensis. The tighter binding of products is also reflected by the results of inhibition experiments, in which RoF and SAH behave as competitive inhibitors for AF and SAM, respectively. We also showed that formation of a ternary complex of RosA, RoF and SAH (or SAM) leads to drastic spectral changes that are indicative of a hydrophobic environment.

Database

Structural data are available in the Protein Data Bank under accession number 4D7K.

The ribB FMN riboswitch from Escherichia coli operates at the transcriptional and translational level and regulates riboflavin biosynthesis

FMN riboswitches are genetic elements that, in many bacteria, control genes responsible for biosynthesis and/or transport of riboflavin (vitamin B2 ). We report that the Escherichia coli ribB FMN riboswitch controls expression of the essential gene ribB coding for the riboflavin biosynthetic enzyme 3,4-dihydroxy-2-butanone-4-phosphate synthase (RibB; EC 4.1.99.12). Our data show that the E. coli ribB FMN riboswitch is unusual because it operates at the transcriptional and also at the translational level. Expression of ribB is negatively affected by FMN and by the FMN analog roseoflavin mononucleotide, which is synthesized enzymatically from roseoflavin and ATP. Consequently, in addition to flavoenzymes, the E. coli ribB FMN riboswitch constitutes a target for the antibiotic roseoflavin produced by Streptomyces davawensis.

Recent advances in riboflavin biosynthesis

Riboflavin is biosynthesized from GTP and ribulose 5-phosphate. Whereas the early reactions conducing to 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione 5'-phosphate show significant taxonomic variation, the subsequent reaction steps are universal in all taxonomic kingdoms. With the exception of a hitherto elusive phosphatase, all enzymes of the pathway have been characterized in some detail at the structural and mechanistic level. Some of the pathway enzymes (GTP cycloyhdrolase II, 3,4-dihydroxy-2-butanone 4-phosphate synthase, riboflavin synthase) have exceptionally complex reaction mechanisms. The commercial production of the vitamin is now entirely based on highly productive fermentation processes. Due to their absence in animals, the pathway enzymes are potential targets for the development of novel anti-infective drugs.

Bacterial flavin mononucleotide riboswitches as targets for flavin analogs

Roseoflavin is a toxic riboflavin (vitamin B2) analog and naturally is produced by Streptomyces davawensis. Roseoflavin is converted to roseoflavin mononucleotide (RoFMN) by promiscuous flavokinases (EC 2.7.1.26). Flavin mononucleotide (FMN) riboswitches control the expression of genes involved in riboflavin biosynthesis and/or transport. RoFMN triggers FMN riboswitches and negatively (or positively) affects expression of the downstream genes. RoFMN binding to the aptamer portion of FMN riboswitch RNAs occurs in the course of transcription by cellular RNA polymerases. We developed an in vitro test system to functionally characterize the interaction between riboflavin/FMN analogs such as roseoflavin/RoFMN and FMN riboswitches in the context of an actively transcribing RNA polymerase.

Natural riboflavin analogs

Riboflavin analogs have a good potential to serve as basic structures for the development of novel anti-infectives. Riboflavin analogs have multiple cellular targets, since riboflavin (as a precursor to flavin cofactors) is active at more than one site in the cell. As a result, the frequency of developing resistance to antimicrobials based on riboflavin analogs is expected to be significantly lower. The only known natural riboflavin analog with antibiotic function is roseoflavin from the bacterium Streptomyces davawensis. This antibiotic negatively affects flavoenzymes and FMN riboswitches. Another roseoflavin producer, Streptomyces cinnabarinus, was recently identified. Possibly, flavin analogs with antibiotic activity are more widespread than anticipated. The same could be true for flavin analogs yet to be discovered, which could constitute tools for cellular chemistry, thus allowing a further extension of the catalytic spectrum of flavoenzymes.

Flavoproteins are potential targets for the antibiotic roseoflavin in Escherichia coli

The riboflavin analog roseoflavin is an antibiotic produced by Streptomyces davawensis. Riboflavin transporters are responsible for roseoflavin uptake by target cells. Roseoflavin is converted to the flavin mononucleotide (FMN) analog roseoflavin mononucleotide (RoFMN) by flavokinase and to the flavin adenine dinucleotide (FAD) analog roseoflavin adenine dinucleotide (RoFAD) by FAD synthetase. In order to study the effect of RoFMN and RoFAD in the cytoplasm of target cells, Escherichia coli was used as a model. E. coli is predicted to contain 38 different FMN- or FAD-dependent proteins (flavoproteins). These proteins were overproduced in recombinant E. coli strains grown in the presence of sublethal amounts of roseoflavin. The flavoproteins were purified and analyzed with regard to their cofactor contents. It was found that 37 out of 38 flavoproteins contained either RoFMN or RoFAD. These cofactors have different physicochemical properties than FMN and FAD and were reported to reduce or completely abolish flavoprotein function.

The flavoenzyme azobenzene reductase AzoR from Escherichia coli binds roseoflavin mononucleotide (RoFMN) with high affinity and is less active in its RoFMN form

The Gram-positive bacterium Streptomyces davawensis is the only organism known to produce the antibiotic roseoflavin. Roseoflavin is a structural riboflavin analogue and is converted to the flavin mononucleotide (FMN) analogue roseoflavin mononucleotide (RoFMN) by flavokinase. FMN-dependent homodimeric azobenzene reductase (AzoR) (EC 1.7.1.6) from Escherichia coli was analyzed as a model enzyme. In vivo and in vitro experiments revealed that RoFMN binds to the AzoR apoenzyme with an even higher affinity compared to that of the "natural" cofactor FMN. Structural analysis (at a resolution of 1.07 Å) revealed that RoFMN binding did not affect the overall topology of the enzyme and also did not interfere with dimerization of AzoR. The AzoR-RoFMN holoenzyme complex was found to be less active (30% of AzoR-FMN activity) in a standard assay. We provide evidence that the different physicochemical properties of RoFMN are responsible for its reduced cofactor activity.

Genome sequence of the bacterium Streptomyces davawensis JCM 4913 and heterologous production of the unique antibiotic roseoflavin

Streptomyces davawensis JCM 4913 synthesizes the antibiotic roseoflavin, a structural riboflavin (vitamin B(2)) analog. Here, we report the 9,466,619-bp linear chromosome of S. davawensis JCM 4913 and a 89,331-bp linear plasmid. The sequence has an average G+C content of 70.58% and contains six rRNA operons (16S-23S-5S) and 69 tRNA genes. The 8,616 predicted protein-coding sequences include 32 clusters coding for secondary metabolites, several of which are unique to S. davawensis. The chromosome contains long terminal inverted repeats of 33,255 bp each and atypical telomeres. Sequence analysis with regard to riboflavin biosynthesis revealed three different patterns of gene organization in Streptomyces species. Heterologous expression of a set of genes present on a subgenomic fragment of S. davawensis resulted in the production of roseoflavin by the host Streptomyces coelicolor M1152. Phylogenetic analysis revealed that S. davawensis is a close relative of Streptomyces cinnabarinus, and much to our surprise, we found that the latter bacterium is a roseoflavin producer as well.

A highly specialized flavin mononucleotide riboswitch responds differently to similar ligands and confers roseoflavin resistance to Streptomyces davawensis

Streptomyces davawensis is the only organism known to synthesize the antibiotic roseoflavin, a riboflavin (vitamin B₂) analog. Roseoflavin is converted to roseoflavin mononucleotide (RoFMN) and roseoflavin adenine dinucleotide in the cytoplasm of target cells. (Ribo-)Flavin mononucleotide (FMN) riboswitches are genetic elements, which in many bacteria control genes responsible for the biosynthesis and transport of riboflavin. Streptomyces davawensis is roseoflavin resistant, and the closely related bacterium Streptomyces coelicolor is roseoflavin sensitive. The two bacteria served as models to investigate roseoflavin resistance of S. davawensis and to analyze the mode of action of roseoflavin in S. coelicolor. Our experiments demonstrate that the ribB FMN riboswitch of S. davawensis (in contrast to the corresponding riboswitch of S. coelicolor) is able to discriminate between the two very similar flavins FMN and RoFMN and shows opposite responses to the latter ligands.