Balancing of mitochondrial translation through METTL8-mediated m3C modification of mitochondrial tRNAs.

Mitochondria contain a specific translation machinery for the synthesis of mitochondria-encoded respiratory chain components. Mitochondrial tRNAs (mt-tRNAs) are also generated from the mitochondrial DNA and, similar to their cytoplasmic counterparts, are post-transcriptionally modified. Here, we find that the RNA methyltransferase METTL8 is a mitochondrial protein that facilitates 3-methyl-cytidine (m3C) methylation at position C32 of the mt-tRNASer(UCN) and mt-tRNAThr. METTL8 knockout cells show a reduction in respiratory chain activity, whereas overexpression increases activity. In pancreatic cancer, METTL8 levels are high, which correlates with lower patient survival and an enhanced respiratory chain activity. Mitochondrial ribosome profiling uncovered mitoribosome stalling on mt-tRNASer(UCN)- and mt-tRNAThr-dependent codons. Further analysis of the respiratory chain complexes using mass spectrometry revealed reduced incorporation of the mitochondrially encoded proteins ND6 and ND1 into complex I. The well-balanced translation of mt-tRNASer(UCN)- and mt-tRNAThr-dependent codons through METTL8-mediated m3C32 methylation might, therefore, facilitate the optimal composition and function of the mitochondrial respiratory chain.

The Alazami Syndrome-Associated Protein LARP7 Guides U6 Small Nuclear RNA Modification and Contributes to Splicing Robustness.

The La-related protein 7 (LARP7) forms a complex with the nuclear 7SK RNA to regulate RNA polymerase II transcription. It has been implicated in cancer and the Alazami syndrome, a severe developmental disorder. Here, we report a so far unknown role of this protein in RNA modification. We show that LARP7 physically connects the spliceosomal U6 small nuclear RNA (snRNA) with a distinct subset of box C/D small nucleolar RNAs (snoRNAs) guiding U6 2'-O-methylation. Consistently, these modifications are severely compromised in the absence of LARP7. Although general splicing remains largely unaffected, transcriptome-wide analysis revealed perturbations in alternative splicing in LARP7-depleted cells. Importantly, we identified defects in 2'-O-methylation of the U6 snRNA in Alazami syndrome siblings carrying a LARP7 mutation. Our data identify LARP7 as a bridging factor for snoRNA-guided modification of the U6 snRNA and suggest that alterations in splicing fidelity contribute to the etiology of the Alazami syndrome.

Characterization of the nuclear import of the human CHD4-NuRD complex.

Chromatin remodeling enzymes form large multiprotein complexes that play central roles in regulating access to the genome. Here, we characterize the nuclear import of the human CHD4 protein. We show that CHD4 enters the nucleus by means of several importin-α proteins (1, 5, 6 and 7), but independently of importin ß1. Importin α1 directly interacts with a monopartite 'KRKR'-motif in the N-terminus of CHD4 (amino acids 304-307). However, alanine mutagenesis of this motif only leads to an ∼50% reduction in nuclear localization of CHD4, implying that there are additional import mechanisms. Interestingly, we could show that CHD4 was already associated with the nucleosome remodeling deacetylase (NuRD) core subunits, such as MTA2, HDAC1 and RbAp46 (also known as RBBP7), in the cytoplasm, suggesting an assembly of the NuRD core complex before nuclear import. We propose that, in addition to the importin-α-dependent nuclear localization signal, CHD4 is dragged into the nucleus by a 'piggyback' mechanism using the import signals of the associated NuRD subunits.

Heat stress at the bicellular stage inhibits sperm cell development and transport into pollen tubes.

For successful double fertilization in flowering plants (angiosperms), pollen tubes deliver 2 nonmotile sperm cells toward female gametes (egg and central cell, respectively). Heatwaves, especially during the reproduction period, threaten male gametophyte (pollen) development, resulting in severe yield losses. Using maize (Zea mays) as a crop and grass model system, we found strong seed set reduction when moderate heat stress was applied for 2 d during the uni- and bicellular stages of pollen development. We show that heat stress accelerates pollen development and impairs pollen germination capabilities when applied at the unicellular stage. Heat stress at the bicellular stage impairs sperm cell development and transport into pollen tubes. To understand the course of the latter defects, we used marker lines and analyzed the transcriptomes of isolated sperm cells. Heat stress affected the expression of genes associated with transcription, RNA processing and translation, DNA replication, and the cell cycle. This included the genes encoding centromeric histone 3 (CENH3) and α-tubulin. Most genes that were misregulated encode proteins involved in the transition from metaphase to anaphase during pollen mitosis II. Heat stress also activated spindle assembly check point and meta- to anaphase transition genes in sperm cells. In summary, misregulation of the identified genes during heat stress at the bicellular stage results in sperm cell development and transport defects ultimately leading to sterility.

Casein kinase 1 and 2 phosphorylate Argonaute proteins to regulate miRNA-mediated gene silencing.

MicroRNAs (miRNAs) together with Argonaute (AGO) proteins form the core of the RNA-induced silencing complex (RISC) to regulate gene expression of their target RNAs post-transcriptionally. Argonaute proteins are subjected to intensive regulation via various post-translational modifications that can affect their stability, silencing efficacy and specificity for targeted gene regulation. We report here that in Caenorhabditis elegans, two conserved serine/threonine kinases - casein kinase 1 alpha 1 (CK1A1) and casein kinase 2 (CK2) - regulate a highly conserved phosphorylation cluster of 4 Serine residues (S988:S998) on the miRNA-specific AGO protein ALG-1. We show that CK1A1 phosphorylates ALG-1 at sites S992 and S995, while CK2 phosphorylates ALG-1 at sites S988 and S998. Furthermore, we demonstrate that phospho-mimicking mutants of the entire S988:S998 cluster rescue the various developmental defects observed upon depleting CK1A1 and CK2. In humans, we show that CK1A1 also acts as a priming kinase of this cluster on AGO2. Altogether, our data suggest that phosphorylation of AGO within the cluster by CK1A1 and CK2 is required for efficient miRISC-target RNA binding and silencing.

Phosphorylation of the FACT histone chaperone subunit SPT16 affects chromatin at RNA polymerase II transcriptional start sites in Arabidopsis.

The heterodimeric histone chaperone FACT, consisting of SSRP1 and SPT16, contributes to dynamic nucleosome rearrangements during various DNA-dependent processes including transcription. In search of post-translational modifications that may regulate the activity of FACT, SSRP1 and SPT16 were isolated from Arabidopsis cells and analysed by mass spectrometry. Four acetylated lysine residues could be mapped within the basic C-terminal region of SSRP1, while three phosphorylated serine/threonine residues were identified in the acidic C-terminal region of SPT16. Mutational analysis of the SSRP1 acetylation sites revealed only mild effects. However, phosphorylation of SPT16 that is catalysed by protein kinase CK2, modulates histone interactions. A non-phosphorylatable version of SPT16 displayed reduced histone binding and proved inactive in complementing the growth and developmental phenotypes of spt16 mutant plants. In plants expressing the non-phosphorylatable SPT16 version we detected at a subset of genes enrichment of histone H3 directly upstream of RNA polymerase II transcriptional start sites (TSSs) in a region that usually is nucleosome-depleted. This suggests that some genes require phosphorylation of the SPT16 acidic region for establishing the correct nucleosome occupancy at the TSS of active genes.

The archaeal Lsm protein from Pyrococcus furiosus binds co-transcriptionally to poly(U)-rich target RNAs.

Posttranscriptional processes in Bacteria include the association of small regulatory RNAs (sRNA) with a target mRNA. The sRNA/mRNA annealing process is often mediated by an RNA chaperone called Hfq. The functional role of bacterial and eukaryotic Lsm proteins is partially understood, whereas knowledge about archaeal Lsm proteins is scarce. Here, we used the genetically tractable archaeal hyperthermophile Pyrococcus furiosus to identify the protein interaction partners of the archaeal Sm-like proteins (PfuSmAP1) using mass spectrometry and performed a transcriptome-wide binding site analysis of PfuSmAP1. Most of the protein interaction partners we found are part of the RNA homoeostasis network in Archaea including ribosomal proteins, the exosome, RNA-modifying enzymes, but also RNA polymerase subunits, and transcription factors. We show that PfuSmAP1 preferentially binds messenger RNAs and antisense RNAs recognizing a gapped poly(U) sequence with high affinity. Furthermore, we found that SmAP1 co-transcriptionally associates with target RNAs. Our study reveals that in contrast to bacterial Hfq, PfuSmAP1 does not affect the transcriptional activity or the pausing behaviour of archaeal RNA polymerases. We propose that PfuSmAP1 recruits antisense RNAs to target mRNAs and thereby executes its putative regulatory function on the posttranscriptional level.

RBPome identification in egg-cell like callus of Arabidopsis.

RNA binding proteins (RBPs) have multiple and essential roles in transcriptional and posttranscriptional regulation of gene expression in all living organisms. Their biochemical identification in the proteome of a given cell or tissue requires significant protein amounts, which limits studies in rare and highly specialized cells. As a consequence, we know almost nothing about the role(s) of RBPs in reproductive processes such as egg cell development, fertilization and early embryogenesis in flowering plants. To systematically identify the RBPome of egg cells in the model plant Arabidopsis, we performed RNA interactome capture (RIC) experiments using the egg cell-like RKD2-callus and were able to identify 728 proteins associated with poly(A+)-RNA. Transcripts for 97 % of identified proteins could be verified in the egg cell transcriptome. 46 % of identified proteins can be associated with the RNA life cycle. Proteins involved in mRNA binding, RNA processing and metabolism are highly enriched. Compared with the few available RBPome datasets of vegetative plant tissues, we identified 475 egg cell-enriched RBPs, which will now serve as a resource to study RBP function(s) during egg cell development, fertilization and early embryogenesis. First candidates were already identified showing an egg cell-specific expression pattern in ovules.

Cytosolic RGG RNA-binding proteins are temperature sensitive flowering time regulators in Arabidopsis.

mRNA translation is tightly regulated by various classes of RNA-binding proteins (RBPs) during development and in response to changing environmental conditions. In this study, we characterize the arginine-glycine-glycine (RGG) motif containing RBP family of Arabidopsis thaliana representing homologues of the multifunctional translation regulators and ribosomal preservation factors Stm1 from yeast (ScStm1) and human SERBP1 (HsSERBP1). The Arabidopsis genome encodes three RGG proteins named AtRGGA, AtRGGB and AtRGGC. While AtRGGA is ubiquitously expressed, AtRGGB and AtRGGC are enriched in dividing cells. All AtRGGs localize almost exclusively to the cytoplasm and bind with high affinity to ssRNA, while being capable to interact with most nucleic acids, except dsRNA. A protein-interactome study shows that AtRGGs interact with ribosomal proteins and proteins involved in RNA processing and transport. In contrast to ScStm1, AtRGGs are enriched in ribosome-free fractions in polysome profiles, suggesting additional plant-specific functions. Mutant studies show that AtRGG proteins differentially regulate flowering time, with a distinct and complex temperature dependency for each AtRGG protein. In conclusion, we suggest that AtRGGs function in fine-tuning translation efficiency to control flowering time and potentially other developmental processes in response to environmental changes.

Elongation factor 1 is a component of the Arabidopsis RNA polymerase II elongation complex and associates with a subset of transcribed genes.

Elongation factors modulate the efficiency of mRNA synthesis by RNA polymerase II (RNAPII) in the context of chromatin, thus contributing to implement proper gene expression programmes. The zinc-finger protein elongation factor 1 (ELF1) is a conserved transcript elongation factor (TEF), whose molecular function so far has not been studied in plants. Using biochemical approaches, we examined the interaction of Arabidopsis ELF1 with DNA and histones in vitro and with the RNAPII elongation complex in vivo. In addition, cytological assays demonstrated the nuclear localisation of the protein, and by means of double-mutant analyses, interplay with genes encoding other elongation factors was explored. The genome-wide distribution of ELF1 was addressed by chromatin immunoprecipitation. ELF1 isolated from Arabidopsis cells robustly copurified with RNAPII and various other elongation factors including SPT4-SPT5, SPT6, IWS1, FACT and PAF1C. Analysis of a CRISPR-Cas9-mediated gene editing mutant of ELF1 revealed distinct genetic interactions with mutants deficient in other elongation factors. Moreover, ELF1 associated with genomic regions actively transcribed by RNAPII. However, ELF1 occupied only c. 33% of the RNAPII transcribed loci with preference for inducible rather than constitutively expressed genes. Collectively, these results establish that Arabidopsis ELF1 shares several characteristic attributes with RNAPII TEFs.

Critical Role of Transcript Cleavage in Arabidopsis RNA Polymerase II Transcriptional Elongation.

Transcript elongation factors associate with elongating RNA polymerase II (RNAPII) to control the efficiency of mRNA synthesis and consequently modulate plant growth and development. Encountering obstacles during transcription such as nucleosomes or particular DNA sequences may cause backtracking and transcriptional arrest of RNAPII. The elongation factor TFIIS stimulates the intrinsic transcript cleavage activity of the polymerase, which is required for efficient rescue of backtracked/arrested RNAPII. A TFIIS mutant variant (TFIISmut) lacks the stimulatory activity to promote RNA cleavage, but instead efficiently inhibits unstimulated transcript cleavage by RNAPII. We could not recover viable Arabidopsis (Arabidopsis thaliana) tfIIs plants constitutively expressing TFIISmut. Induced, transient expression of TFIISmut in tfIIs plants provoked severe growth defects, transcriptomic changes and massive, transcription-related redistribution of elongating RNAPII within transcribed regions toward the transcriptional start site. The predominant site of RNAPII accumulation overlapped with the +1 nucleosome, suggesting that upon inhibition of RNA cleavage activity, RNAPII arrest prevalently occurs at this position. In the presence of TFIISmut, the amount of RNAPII was reduced, which could be reverted by inhibiting the proteasome, indicating proteasomal degradation of arrested RNAPII. Our findings suggest that polymerase backtracking/arrest frequently occurs in plant cells, and RNAPII-reactivation is essential for correct transcriptional output and proper growth/development.

Retinoschisin and novel Na/K-ATPase interaction partners Kv2.1 and Kv8.2 define a growing protein complex at the inner segments of mammalian photoreceptors.

The RS1 gene on Xp 22.13 encodes retinoschisin which is known to directly interact with the retinal Na/K-ATPase at the photoreceptor inner segments. Pathologic mutations in RS1 cause X-linked juvenile retinoschisis (XLRS), a hereditary retinal dystrophy in young males. To further delineate the retinoschisin-Na/K-ATPase complex, co-immunoprecipitation was performed with porcine and murine retinal lysates targeting the ATP1A3 subunit. This identified the voltage-gated potassium (Kv) channel subunits Kv2.1 and Kv8.2 as direct interaction partners of the retinal Na/K-ATPase. Colocalization of the individual components of the complex was demonstrated at the membrane of photoreceptor inner segments. We further show that retinoschisin-deficiency, a frequent consequence of molecular pathology in XLRS, causes mislocalization of the macromolecular complex during postnatal retinal development with a simultaneous reduction of Kv2.1 and Kv8.2 protein expression, while the level of retinal Na/K-ATPase expression remains unaffected. Patch-clamp analysis revealed no effect of retinoschisin-deficiency on Kv channel mediated potassium ion currents in vitro. Together, our data suggest that Kv2.1 and Kv8.2 together with retinoschisin and the retinal Na/K-ATPase are integral parts of a macromolecular complex at the photoreceptor inner segments. Defective compartmentalization of this complex due to retinoschisin-deficiency may be a crucial step in initial XLRS pathogenesis.

Controlling Enzymatic Activity by Modulating the Oligomerization State via Chemical Rescue and Optical Control.

Selective switching of enzymatic activity has been a longstanding goal in synthetic biology. Drastic changes in activity upon mutational manipulation of the oligomerization state of enzymes have frequently been reported in the literature, but scarcely exploited for switching. Using geranylgeranylglyceryl phosphate synthase as a model, we demonstrate that catalytic activity can be efficiently controlled by exogenous modulation of the association state. We introduced a lysine-to-cysteine mutation, leading to the breakdown of the active hexamer into dimers with impaired catalytic efficiency. Addition of bromoethylamine chemically rescued the enzyme by restoring hexamerization and activity. As an alternative method, we incorporated the photosensitive unnatural amino acid o-nitrobenzyl-O-tyrosine (ONBY) into the hexamerization interface. This again led to inactive dimers, but the hexameric state and activity could be recovered by UV-light induced cleavage of ONBY. For both approaches, we obtained switching factors greater than 350-fold, which compares favorably with previously reported activity changes that were caused by site-directed mutagenesis.

H3K9 demethylases IBM1 and JMJ27 are required for male meiosis in Arabidopsis thaliana.

Dimethylation of histone H3 lysine 9 (H3K9me2), a crucial modification for heterochromatin formation and transcriptional silencing, is essential for proper meiotic prophase progression in mammals. We analyzed meiotic defects and generated genome-wide profiles of H3K9me2 and transcriptomes for the mutants of H3K9 demethylases. Moreover, we also identified proteins interacting with H3K9 demethylases. H3K9me2 is usually found at transposable elements and repetitive sequences but is absent from the bodies of protein-coding genes. In this study, we show that the Arabidopsis thaliana H3K9 demethylases IBM1 and JMJ27 cooperatively regulate crossover formation and chromosome segregation. They protect thousands of protein-coding genes from ectopic H3K9me2, including genes essential for meiotic prophase progression. In addition to removing H3K9me2, IBM1 and JMJ27 interact with the Precocious Dissociation of Sisters 5 (PDS5) cohesin complex cofactors. The pds5 mutant shared similar transcriptional alterations with ibm1 jmj27, including meiosis-essential genes, yet without affecting H3K9me2 levels. Hence, PDS5s, together with IBM1 and JMJ27, regulate male meiosis and gene expression independently of H3K9 demethylation. These findings uncover a novel role of H3K9me2 removal in meiosis and a new function of H3K9 demethylases and cohesin cofactors in meiotic transcriptional regulation.

Hepatocyte expressed chemerin-156 does not protect from experimental non-alcoholic steatohepatitis.

Non-alcoholic steatohepatitis (NASH) is a rapidly growing liver disease. The chemoattractant chemerin is abundant in hepatocytes, and hepatocyte expressed prochemerin protected from NASH. Prochemerin is inactive and different active isoforms have been described. Here, the effect of hepatocyte expressed muChem-156, a highly active murine chemerin isoform, was studied in the methionine-choline deficient dietary model of NASH. Mice overexpressing muChem-156 had higher hepatic chemerin protein. Serum chemerin levels and the capability of serum to activate the chemerin receptors was unchanged showing that the liver did not release active chemerin. Notably, activation of the chemerin receptors by hepatic vein blood did not increase in parallel to total chemerin protein in patients with liver cirrhosis. In experimental NASH, muChem-156 had no effect on liver lipids. Accordingly, overexpression of active chemerin in hepatocytes or treatment of hepatocytes with recombinant chemerin did not affect cellular triglyceride and cholesterol levels. Importantly, overexpression of muChem-156 in the murine liver did not change the hepatic expression of inflammatory and profibrotic genes. The downstream targets of chemerin such as p38 kinase were neither activated in the liver of muChem-156 producing mice nor in HepG2, Huh7 and Hepa1-6 cells overexpressing this isoform. Recombinant chemerin had no effect on global gene expression of primary human hepatocytes and hepatic stellate cells within 24 h of incubation. Phosphorylation of p38 kinase was, however, increased upon short-time incubation of HepG2 cells with chemerin. These findings show that muChem-156 overexpression in hepatocytes does not protect from liver steatosis and inflammation.

Comprehensive analysis of translation from overexpressed circular RNAs reveals pervasive translation from linear transcripts.

Circular RNAs (circRNAs) encompass a widespread and conserved class of RNAs, which are generated by back-splicing of downstream 5' to upstream 3' splice sites. CircRNAs are tissue-specific and have been implicated in diseases including cancer. They can function as sponges for microRNAs (miRNAs) or RNA binding proteins (RBPs), for example. Moreover, some contain open reading frames (ORFs) and might be translated. The functional relevance of such peptides, however, remains largely elusive. Here, we report that the ORF of circZNF609 is efficiently translated when expressed from a circZNF609 overexpression construct. However, endogenous proteins could not be detected. Moreover, initiation of circZNF609 translation is independent of m6A-generating enzyme METTL3 or RNA sequence elements such as internal ribosome entry sites (IRESs). Surprisingly, a comprehensive mutational analysis revealed that deletion constructs, which are deficient in producing circZNF609, still generate the observed protein products. This suggests that the apparent circZNF609 translation originates from trans-splicing by-products of the overexpression plasmids and underline that circRNA overexpression constructs need to be evaluated carefully, particularly when functional studies are performed.

Phosphorylation of Argonaute proteins affects mRNA binding and is essential for microRNA-guided gene silencing in vivo.

Argonaute proteins associate with microRNAs and are key components of gene silencing pathways. With such a pivotal role, these proteins represent ideal targets for regulatory post-translational modifications. Using quantitative mass spectrometry, we find that a C-terminal serine/threonine cluster is phosphorylated at five different residues in human and Caenorhabditis elegans In human, hyper-phosphorylation does not affect microRNA binding, localization, or cleavage activity of Ago2. However, mRNA binding is strongly affected. Strikingly, on Ago2 mutants that cannot bind microRNAs or mRNAs, the cluster remains unphosphorylated indicating a role at late stages of gene silencing. In C. elegans, the phosphorylation of the conserved cluster of ALG-1 is essential for microRNA function in vivo Furthermore, a single point mutation within the cluster is sufficient to phenocopy the loss of its complete phosphorylation. Interestingly, this mutant retains its capacity to produce and bind microRNAs and represses expression when artificially tethered to an mRNA Altogether, our data suggest that the phosphorylation state of the serine/threonine cluster is important for Argonaute-mRNA interactions.

The Arabidopsis condensin CAP-D subunits arrange interphase chromatin.

Condensins are best known for their role in shaping chromosomes. Other functions such as organizing interphase chromatin and transcriptional control have been reported in yeasts and animals, but little is known about their function in plants. To elucidate the specific composition of condensin complexes and the expression of CAP-D2 (condensin I) and CAP-D3 (condensin II), we performed biochemical analyses in Arabidopsis. The role of CAP-D3 in interphase chromatin organization and function was evaluated using cytogenetic and transcriptome analysis in cap-d3 T-DNA insertion mutants. CAP-D2 and CAP-D3 are highly expressed in mitotically active tissues. In silico and pull-down experiments indicate that both CAP-D proteins interact with the other condensin I and II subunits. In cap-d3 mutants, an association of heterochromatic sequences occurs, but the nuclear size and the general histone and DNA methylation patterns remain unchanged. Also, CAP-D3 influences the expression of genes affecting the response to water, chemicals, and stress. The expression and composition of the condensin complexes in Arabidopsis are similar to those in other higher eukaryotes. We propose a model for the CAP-D3 function during interphase in which CAP-D3 localizes in euchromatin loops to stiffen them and consequently separates centromeric regions and 45S rDNA repeats.

The long non-coding RNA LINC00941 and SPRR5 are novel regulators of human epidermal homeostasis.

Several long non-coding RNAs (lncRNAs) act as regulators of cellular homeostasis; however, few of these molecules were functionally characterized in a mature human tissue environment. Here, we report that the lncRNA LINC00941 is a crucial regulator of human epidermal homeostasis. LINC00941 is enriched in progenitor keratinocytes and acts as a repressor of keratinocyte differentiation. Furthermore, LINC00941 represses SPRR5, a previously uncharacterized molecule, which functions as an essential positive regulator of keratinocyte differentiation. Interestingly, 54.8% of genes repressed in SPRR5-deficient epidermal tissue are induced in LINC00941-depleted organotypic epidermis, suggesting a common mode of action for both molecules.

Significance of the Protein Interface Configuration for Allostery in Imidazole Glycerol Phosphate Synthase.

Imidazole glycerol phosphate synthase (ImGPS) from Thermotoga maritima is a model enzyme for studying allostery. The ImGPS complex consists of the cyclase subunit HisF and the glutaminase subunit HisH whose activity is stimulated by substrate binding to HisF in a V-type manner. To investigate the significance of a putative closing hinge motion at the cyclase:glutaminase interface for HisH activity, we replaced residue W123 in HisH with the light-switchable unnatural amino acid phenylalanine-4'-azobenzene (AzoF). Crystal structure analysis employing angle, buried surface area, and distance measurements showed that incorporation of AzoF at this position causes a closing of the interface by ∼18 ± 3%. This slightly different interface configuration results in a much higher catalytic efficiency in unstimulated HisH due to an elevated turnover number. Moreover, the catalytic efficiency of HisH when stimulated by binding of a substrate to HisF was also significantly increased by AzoF incorporation. This was caused by a K-type stimulation that led to a decrease in the apparent dissociation constant for its substrate, glutamine. In addition, AzoF improved the apparent binding of a substrate analogue at the HisF active site. Remarkably, light-induced isomerization of AzoF considerably enhanced these effects. In conclusion, our findings confirm that signal transduction from HisF to HisH in ImGPS involves the closing of the cyclase:glutaminase subunit interface and that incorporation of AzoF at a hinge position reinforces this catalytically relevant conformational change.