A Novel Transcriptional Regulator Related to Thiamine Phosphate Synthase Controls Thiamine Metabolism Genes in Archaea

ABSTRACT Thiamine (vitamin B1) is a precursor of thiamine pyrophosphate (TPP), an essential coenzyme in the central metabolism of all living organisms. Bacterial thiamine biosynthesis and salvage genes are controlled at the RNA level by TPP-responsive riboswitches. In Archaea, TPP riboswitches are restricted to the Thermoplasmatales order. Mechanisms of transcriptional control of thiamine genes in other archaeal lineages remain unknown. Using the comparative genomics approach, we identified a novel family of transcriptional regulators (named ThiR) controlling thiamine biosynthesis and transport genes in diverse lineages in the Crenarchaeota phylum as well as in the Halobacteria and Thermococci classes of the Euryarchaeota. ThiR regulators are composed of an N-terminal DNA-binding domain and a C-terminal ligand-binding domain, which is similar to the archaeal thiamine phosphate synthase ThiN. By using comparative genomics, we predicted ThiR-binding DNA motifs and reconstructed ThiR regulons in 67 genomes representing all above-mentioned lineages. The predicted ThiR-binding motifs are characterized by palindromic symmetry with several distinct lineage-specific consensus sequences. In addition to thiamine biosynthesis genes, the reconstructed ThiR regulons include various transporters for thiamine and its precursors. Bioinformatics predictions were experimentally validated by in vitro DNA-binding assays with the recombinant ThiR protein from the hyperthermophilic archaeon Metallosphaera yellowstonensis MK1. Thiamine phosphate and, to some extent, TPP and hydroxyethylthiazole phosphate were required for the binding of ThiR to its DNA targets, suggesting that ThiR is derepressed by limitation of thiamine phosphates. The thiamine phosphate-binding residues previously identified in ThiN are highly conserved in ThiR regulators, suggesting a conserved mechanism for effector recognition. IMPORTANCE Thiamine pyrophosphate is a cofactor for many essential enzymes for glucose and energy metabolism. Thiamine or vitamin B1 biosynthesis and its transcriptional regulation in Archaea are poorly understood. We applied the comparative genomics approach to identify a novel family of regulators for the transcriptional control of thiamine metabolism genes in Archaea and reconstructed the respective regulons. The predicted ThiR regulons in archaeal genomes control the majority of thiamine biosynthesis genes. The reconstructed regulon content suggests that numerous uptake transporters for thiamine and/or its precursors are encoded in archaeal genomes. The ThiR regulon was experimentally validated by DNA-binding assays with Metallosphaera spp. These discoveries contribute to our understanding of metabolic and regulatory networks involved in vitamin homeostasis in diverse lineages of Archaea.

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Novel Transcriptional Regulons for Autotrophic Cycle Genes in Crenarchaeota

Journal of Bacteriology ◽

10.1128/jb.00249-15 ◽

2015 ◽

Vol 197 (14) ◽

pp. 2383-2391 ◽

Cited By ~ 7

Author(s):

Semen A. Leyn ◽

Irina A. Rodionova ◽

Xiaoqing Li ◽

Dmitry A. Rodionov

Keyword(s):

Carbon Dioxide ◽

Transcriptional Regulation ◽

Comparative Genomics ◽

Dna Binding ◽

Transcriptional Control ◽

Binding Assays ◽

Promoter Regions ◽

Content Type ◽

Genome Context

ABSTRACTAutotrophic microorganisms are able to utilize carbon dioxide as their only carbon source, or, alternatively, many of them can grow heterotrophically on organics. Different variants of autotrophic pathways have been identified in various lineages of the phylumCrenarchaeota. Aerobic members of the orderSulfolobalesutilize the hydroxypropionate-hydroxybutyrate cycle (HHC) to fix inorganic carbon, whereas anaerobicThermoprotealesuse the dicarboxylate-hydroxybutyrate cycle (DHC). Knowledge of transcriptional regulation of autotrophic pathways inArchaeais limited. We applied a comparative genomics approach to predict novel autotrophic regulons in theCrenarchaeota. We report identification of two novel DNA motifs associated with the autotrophic pathway genes in theSulfolobales(HHC box) andThermoproteales(DHC box). Based on genome context evidence, the HHC box regulon was attributed to a novel transcription factor from the TrmB family named HhcR. Orthologs of HhcR are present in allSulfolobalesgenomes but were not found in other lineages. A predicted HHC box regulatory motif was confirmed byin vitrobinding assays with the recombinant HhcR protein fromMetallosphaera yellowstonensis. For the DHC box regulon, we assigned a different potential regulator, named DhcR, which is restricted to the orderThermoproteales. DhcR inThermoproteus neutrophilus(Tneu_0751) was previously identified as a DNA-binding protein with high affinity for the promoter regions of two autotrophic operons. The global HhcR and DhcR regulons reconstructed by comparative genomics were reconciled with available omics data inMetallosphaeraandThermoproteusspp. The identified regulons constitute two novel mechanisms for transcriptional control of autotrophic pathways in theCrenarchaeota.IMPORTANCELittle is known about transcriptional regulation of carbon dioxide fixation pathways inArchaea. We previously applied the comparative genomics approach for reconstruction of DtxR family regulons in diverse lineages ofArchaea. Here, we utilize similar computational approaches to identify novel regulatory motifs for genes that are autotrophically induced in microorganisms from two lineages ofCrenarchaeotaand to reconstruct the respective regulons. The predicted novel regulons in archaeal genomes control the majority of autotrophic pathway genes and also other carbon and energy metabolism genes. The HhcR regulon was experimentally validated by DNA-binding assays inMetallosphaeraspp. Novel regulons described for the first time in this work provide a basis for understanding the mechanisms of transcriptional regulation of autotrophic pathways inArchaea.

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The Iron Deficiency Response of Corynebacterium glutamicum and a Link to Thiamine Biosynthesis

Applied and Environmental Microbiology ◽

10.1128/aem.00065-20 ◽

2020 ◽

Vol 86 (10) ◽

Cited By ~ 1

Author(s):

Andreas Küberl ◽

Aliye Mengus-Kaya ◽

Tino Polen ◽

Michael Bott

Keyword(s):

Corynebacterium Glutamicum ◽

Iron Homeostasis ◽

Essential Element ◽

Iron Limitation ◽

Thiamine Pyrophosphate ◽

Exponential Growth Phase ◽

Iron Starvation ◽

Content Type ◽

Oxoglutarate Dehydrogenase ◽

Thiamine Biosynthesis

ABSTRACT The response to iron limitation of the Gram-positive soil bacterium Corynebacterium glutamicum was analyzed with respect to secreted metabolites, the transcriptome, and the proteome. During growth in glucose minimal medium, iron limitation caused a shift from lactate to pyruvate as the major secreted organic acid complemented by l-alanine and 2-oxoglutarate. Transcriptome and proteome analyses revealed that a pronounced iron starvation response governed by the transcriptional regulators DtxR and RipA was detectable in the late, but not in the early, exponential-growth phase. A link between iron starvation and thiamine pyrophosphate (TPP) biosynthesis was uncovered by the strong upregulation of thiC. As phosphomethylpyrimidine synthase (ThiC) contains an iron-sulfur cluster, limiting activities of the TPP-dependent pyruvate–2-oxoglutarate dehydrogenase supercomplex probably cause the excretion of pyruvate and 2-oxoglutarate. In line with this explanation, thiamine supplementation could strongly diminish the secretion of these acids. The upregulation of thiC and other genes involved in thiamine biosynthesis and transport is presumably due to TPP riboswitches present at the 5′ end of the corresponding operons. The results obtained in this study provide new insights into iron homeostasis in C. glutamicum and demonstrate that the metabolic consequences of iron limitation can be due to the iron dependency of coenzyme biosynthesis. IMPORTANCE Iron is an essential element for most organisms but causes problems due to poor solubility under oxic conditions and due to toxicity by catalyzing the formation of reactive oxygen species (ROS). Therefore, bacteria have evolved complex regulatory networks for iron homeostasis aiming at a sufficient iron supply while minimizing ROS formation. In our study, the responses of the actinobacterium Corynebacterium glutamicum to iron limitation were analyzed, resulting in a detailed view on the processes involved in iron homeostasis in this model organism. In particular, we provide evidence that iron limitation causes TPP deficiency, presumably due to insufficient activity of the iron-dependent phosphomethylpyrimidine synthase (ThiC). TPP deficiency was deduced from the upregulation of genes controlled by a TPP riboswitch and secretion of metabolites caused by insufficient activity of the TPP-dependent enzymes pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase. To our knowledge, the link between iron starvation and thiamine synthesis has not been elaborated previously.

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A New Coactivator Function for Zac1's C2H2 Zinc Finger DNA-Binding Domain in Selectively Controlling PCAF Activity

Molecular and Cellular Biology ◽

10.1128/mcb.00842-08 ◽

2008 ◽

Vol 28 (19) ◽

pp. 6078-6093 ◽

Cited By ~ 16

Author(s):

Anke Hoffmann ◽

Dietmar Spengler

Keyword(s):

Transcription Factors ◽

Dna Binding ◽

Zinc Finger ◽

Transcriptional Control ◽

Zinc Finger Protein ◽

Embryonic Stem ◽

Binding Domain ◽

Dna Binding Domain ◽

Histone H4 ◽

Histone H4 Acetylation

ABSTRACT The generally accepted paradigm of transcription by regulated recruitment defines sequence-specific transcription factors and coactivators as separate categories that are distinguished by their abilities to bind DNA autonomously. The C2H2 zinc finger protein Zac1, with an established role in canonical DNA binding, also acts as a coactivator. Commensurate with this function, p73, which is related to p53, is here shown to recruit Zac1, together with the coactivators p300 and PCAF, to the p21Cip1 promoter during the differentiation of embryonic stem cells into neurons. In the absence of autonomous DNA binding, Zac1's zinc fingers stabilize the association of PCAF with p300, suggesting its scaffolding function. Furthermore, Zac1 regulates the affinities of PCAF substrates as well as the catalytic activities of PCAF to induce a selective switch in favor of histone H4 acetylation and thereby the efficient transcription of p21Cip1. These results are consistent with an authentic coactivator function of Zac1's C2H2 zinc finger DNA-binding domain and suggest coactivation by sequence-specific transcription factors as a new facet of transcriptional control.

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Expression of qa-1F activator protein: identification of upstream binding sites in the qa gene cluster and localization of the DNA-binding domain.

Molecular and Cellular Biology ◽

10.1128/mcb.7.3.1256 ◽

1987 ◽

Vol 7 (3) ◽

pp. 1256-1266 ◽

Cited By ~ 60

Author(s):

J A Baum ◽

R Geever ◽

N H Giles

Keyword(s):

Dna Binding ◽

Gene Cluster ◽

Transcriptional Control ◽

Protein Identification ◽

Regulatory Gene ◽

Quinic Acid ◽

Binding Domain ◽

Dna Binding Domain ◽

Activator Protein

The qa-1F regulatory gene of Neurospora crassa encodes an activator protein required for quinic acid induction of transcription in the qa gene cluster. This activator protein was expressed in insect cell culture with a baculovirus expression vector. The activator binds to 13 sites in the gene cluster that are characterized by a conserved 16-base-pair sequence of partial dyad symmetry. One site is located between the divergently transcribed qa-1F and qa-1S regulatory genes, corroborating prior evidence that qa-1F is autoregulated and controls expression of the qa-1S repressor. Multiple upstream sites located at variable positions 5' to the qa structural genes appear to allow for greater transcriptional control by qa-1F. Full-length and truncated activator peptides were synthesized in vitro, and the DNA-binding domain was localized to the first 183 amino acids. A 28-amino acid sequence within this region shows striking homology to N-terminal sequences from other lower-eucaryotic activator proteins. A qa-1F(Ts) mutation is located within this putative DNA-binding domain.

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Expression of qa-1F activator protein: identification of upstream binding sites in the qa gene cluster and localization of the DNA-binding domain

Molecular and Cellular Biology ◽

10.1128/mcb.7.3.1256-1266.1987 ◽

1987 ◽

Vol 7 (3) ◽

pp. 1256-1266

Author(s):

J A Baum ◽

R Geever ◽

N H Giles

Keyword(s):

Dna Binding ◽

Gene Cluster ◽

Transcriptional Control ◽

Protein Identification ◽

Regulatory Gene ◽

Quinic Acid ◽

Binding Domain ◽

Dna Binding Domain ◽

Activator Protein

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The COUP-TFII variant lacking a DNA-binding domain inhibits the activation of the Cyp7a1 promoter through physical interaction with COUP-TFII

Biochemical Journal ◽

10.1042/bj20121200 ◽

2013 ◽

Vol 452 (2) ◽

pp. 345-357 ◽

Cited By ~ 9

Author(s):

Tomoko Yamazaki ◽

Jun-ichi Suehiro ◽

Hideki Miyazaki ◽

Takashi Minami ◽

Tatsuhiki Kodama ◽

...

Keyword(s):

Dna Binding ◽

Transcriptional Control ◽

Target Genes ◽

Binding Domain ◽

Physical Interaction ◽

Dna Binding Domain ◽

Factor Ii ◽

Huh7 Cells ◽

Direct Binding ◽

Cytochrome P450 Family

The COUP-TFII (chicken ovalbumin upstream promoter-transcription factor II) nuclear receptor, which is composed of a DNA-binding domain and a ligand-binding domain, exerts pleiotropic effects on development and cell differentiation by regulating the transcription of its target genes, including Cyp7a1 (cytochrome P450, family 7, subfamily a, polypeptide 1), which plays important roles in catabolism of cholesterol in the liver. Although multiple variants of COUP-TFII exist, their roles in the regulation of Cyp7a1 expression have not been elucidated. In the present study, we investigated the roles of COUP-TFII-V2 (variant 2), which lacks a DNA-binding domain, in the regulation of the transcriptional control of the Cyp7a1 gene by COUP-TFII in hepatocellular carcinoma cells. We found that COUP-TFII-V2 was significantly expressed in Huh7 cells, in which Cyp7a1 was not expressed. Furthermore, knockdown of COUP-TFII-V2 enhanced endogenous Cyp7a1 expression in Huh7 cells. Although COUP-TFII activates the Cyp7a1 promoter through direct binding to DNA, this activation was affected by COUP-TFII-V2, which physically interacted with COUP-TFII and inhibited its DNA-binding ability. Chromatin immunoprecipitation assays showed that COUP-TFII-V2 inhibited the binding of endogenous COUP-TFII to the intact Cyp7a1 promoter. The results of the present study suggest that COUP-TFII-V2 negatively regulates the function of COUP-TFII by inhibiting its binding to DNA to decrease Cyp7a1 expression.

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Thiamine pyrophosphate riboswitches in Bacteroides species regulate transcription or translation of thiamine transport and biosynthesis genes

10.1101/867226 ◽

2019 ◽

Cited By ~ 1

Author(s):

Zachary A. Costliow ◽

Patrick H. Degnan ◽

Carin K. Vanderpool

Keyword(s):

Gut Microbiome ◽

Transcriptional Response ◽

Thiamine Pyrophosphate ◽

Individual Species ◽

Vitamin B ◽

Transcriptional Level ◽

Thiamine Transport ◽

Branched Chain ◽

Species Specific ◽

Thiamine Biosynthesis

AbstractThiamine (vitamin B1) and its phosphorylated precursors are necessary for decarboxylation reactions required in carbohydrate and branched chain amino acid metabolism. Due to its critical roles in central metabolism, thiamine is essential for human and animal hosts and their resident gut microbes. However, little is known about how thiamine availability shapes the composition of gut microbial communities and the physiology of individual species within those communities. Our previous work has implicated both thiamine biosynthesis and transport activities in the fitness of Bacteroides species. To better understand thiamine-dependent gene regulation in Bacteroides, we examined thiamine biosynthesis and transport genes in three representative species: Bacteroides thetaiotaomicron, Bacteroides uniformis, and Bacteroides vulgatus. All three species possess thiamine biosynthetic operons controlled by highly conserved cis-acting thiamine pyrophosphate (TPP) riboswitches. B. thetaiotaomicron and B. uniformis have additional TPP riboswitch-controlled operons encoding thiamine transport functions. Transcriptome analyses showed that each Bacteroides species had a distinct transcriptional response to exogenous thiamine. Analysis of transcript levels and translational fusions demonstrated that in B. thetaiotaomicron, the TPP riboswitch upstream of biosynthesis genes acts at the level of transcription, while TPP riboswitches upstream of transport operons work at the level of translation. In B. uniformis and B. vulgatus, TPP riboswitches work at the transcriptional level to control downstream operons. The varying responses to exogenous thiamine and use of varied regulatory mechanisms may play an important role in niche establishment by the Bacteroidetes in the complex and constantly shifting gut environment.ImportanceBacteroides species are important and abundant members of human gut microbiome communities. Their activities in the gut are influenced by constant changes in nutrient availability. In this study, we investigated the genetic basis of thiamine (Vitamin B1) uptake and biosynthesis in three representative Bacteroides species. We found species-specific differences in the response to exogenous thiamine, and distinct mechanisms for regulation of uptake and biosynthesis gene expression. Our work implies that gut Bacteroides have evolved distinct strategies for making or acquiring an essential nutrient. These mechanisms may play an important role in the success of Bacteroides in establishing a niche within complex gut microbiome communities.

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The ChiS-Family DNA-Binding Domain Contains a Cryptic Helix-Turn-Helix Variant

mBio ◽

10.1128/mbio.03287-20 ◽

2021 ◽

Vol 12 (2) ◽

Author(s):

Catherine A. Klancher ◽

George Minasov ◽

Ram Podicheti ◽

Douglas B. Rusch ◽

Triana N. Dalia ◽

...

Keyword(s):

Gene Expression ◽

Dna Binding ◽

Vibrio Cholerae ◽

Binding Domain ◽

Structural Basis ◽

Striking Difference ◽

Dna Binding Domain ◽

Structural Domains ◽

Content Type ◽

Domains Of Life

ABSTRACT Sequence-specific DNA-binding domains (DBDs) are conserved in all domains of life. These proteins carry out a variety of cellular functions, and there are a number of distinct structural domains already described that allow for sequence-specific DNA binding, including the ubiquitous helix-turn-helix (HTH) domain. In the facultative pathogen Vibrio cholerae, the chitin sensor ChiS is a transcriptional regulator that is critical for the survival of this organism in its marine reservoir. We recently showed that ChiS contains a cryptic DBD in its C terminus. This domain is not homologous to any known DBD, but it is a conserved domain present in other bacterial proteins. Here, we present the crystal structure of the ChiS DBD at a resolution of 1.28 Å. We find that the ChiS DBD contains an HTH domain that is structurally similar to those found in other DNA-binding proteins, like the LacI repressor. However, one striking difference observed in the ChiS DBD is that the canonical tight turn of the HTH is replaced with an insertion containing a β-sheet, a variant which we term the helix-sheet-helix. Through systematic mutagenesis of all positively charged residues within the ChiS DBD, we show that residues within and proximal to the ChiS helix-sheet-helix are critical for DNA binding. Finally, through phylogenetic analyses we show that the ChiS DBD is found in diverse proteobacterial proteins that exhibit distinct domain architectures. Together, these results suggest that the structure described here represents the prototypical member of the ChiS-family of DBDs. IMPORTANCE Regulating gene expression is essential in all domains of life. This process is commonly facilitated by the activity of DNA-binding transcription factors. There are diverse structural domains that allow proteins to bind to specific DNA sequences. The structural basis underlying how some proteins bind to DNA, however, remains unclear. Previously, we showed that in the major human pathogen Vibrio cholerae, the transcription factor ChiS directly regulates gene expression through a cryptic DNA-binding domain. This domain lacked homology to any known DNA-binding protein. In the current study, we determined the structure of the ChiS DNA-binding domain (DBD) and found that the ChiS-family DBD is a cryptic variant of the ubiquitous helix-turn-helix (HTH) domain. We further demonstrate that this domain is conserved in diverse proteins that may represent a novel group of transcriptional regulators.

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Thiamine Acquisition Strategies Impact Metabolism and Competition in the Gut Microbe Bacteroides thetaiotaomicron

mSystems ◽

10.1128/msystems.00116-17 ◽

2017 ◽

Vol 2 (5) ◽

Cited By ~ 17

Author(s):

Zachary A. Costliow ◽

Patrick H. Degnan

Keyword(s):

Indirect Effects ◽

Water Soluble ◽

Vitamin B ◽

Direct And Indirect Effects ◽

Content Type ◽

Gut Microbes ◽

Acquisition Strategies ◽

Thiamine Transport ◽

Gut Microbe ◽

Thiamine Biosynthesis

ABSTRACT Variation in the ability of gut microbes to transport, synthesize, and compete for vitamin B1 (thiamine) is expected to impact the structure and stability of the microbiota, and ultimately this variation may have both direct and indirect effects on human health. Our study identifies the diverse strategies employed by gut Bacteroidetes to acquire thiamine. We demonstrate how the presence or absence of thiamine biosynthesis or transport dramatically affects the abundance of B. thetaiotaomicron in a competitive environment. This study adds further evidence that altering the presence or concentrations of water-soluble vitamins such as thiamine may be an effective method for manipulating gut community composition. In turn, targeted thiamine delivery could be used therapeutically to alter dysbiotic communities linked to disease. Thiamine (vitamin B1) is an essential cofactor for all organisms. Humans primarily acquire thiamine through their diet, and thiamine deficiencies have adverse neurological effects. However, the role gut microbes play in modulating thiamine availability is poorly understood, and little is known about how thiamine impacts the stability of microbial gut communities. To investigate thiamine’s role in the gut, we utilized the model gut microbe Bacteroides thetaiotaomicron. Transcriptome sequencing (RNA-seq) revealed a global downregulation of thiamine and amino acid biosynthesis, glycolysis, and purine metabolism when thiamine was present. Using genetic mutants with thiamine biosynthesis and transport locus mutations, we determined both systems were critical for growth in thiamine-deficient medium. The defect in the double transport mutant suggests an uncharacterized feedback mechanism between thiamine transport and biosynthesis in B. thetaiotaomicron. Mutant phenotypes were recapitulated during pairwise competitions, reinforcing the importance of encoding versatile thiamine acquisition mechanisms when thiamine concentrations are variable. In addition, liquid chromatography-mass spectrometry (LC-MS) analyses corroborate that exogenous thiamine levels affect the internal thiamine pool of B. thetaiotaomicron. Furthermore, we computationally examined the ability of other gut microbes to acquire thiamine and identified lineage-specific differences in thiamine acquisition strategies. Among the Bacteroidetes, the capacities for both thiamine transport and biosynthesis are common. Together, these data show that thiamine acquisition mechanisms used by B. thetaiotaomicron not only are critical for its physiology and fitness but also provide the opportunity to model how other gut microbes may respond to the shifting availability of thiamine in the gut. IMPORTANCE Variation in the ability of gut microbes to transport, synthesize, and compete for vitamin B1 (thiamine) is expected to impact the structure and stability of the microbiota, and ultimately this variation may have both direct and indirect effects on human health. Our study identifies the diverse strategies employed by gut Bacteroidetes to acquire thiamine. We demonstrate how the presence or absence of thiamine biosynthesis or transport dramatically affects the abundance of B. thetaiotaomicron in a competitive environment. This study adds further evidence that altering the presence or concentrations of water-soluble vitamins such as thiamine may be an effective method for manipulating gut community composition. In turn, targeted thiamine delivery could be used therapeutically to alter dysbiotic communities linked to disease. Author Video: An author video summary of this article is available.

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Control of Recombination Directionality by the Listeria Phage A118 Protein Gp44 and the Coiled-Coil Motif of Its Serine Integrase

Journal of Bacteriology ◽

10.1128/jb.00019-17 ◽

2017 ◽

Vol 199 (11) ◽

Cited By ~ 9

Author(s):

Sridhar Mandali ◽

Kushol Gupta ◽

Anthony R. Dawson ◽

Gregory D. Van Duyne ◽

Reid C. Johnson

Keyword(s):

Listeria Monocytogenes ◽

Dna Binding ◽

Coiled Coil ◽

Binding Domain ◽

Dna Binding Domain ◽

Content Type ◽

Bacterial Chromosomes ◽

Carboxyl Terminal ◽

Recombination Reactions ◽

Serine Integrase

ABSTRACT The serine integrase of phage A118 catalyzes integrative recombination between attP on the phage and a specific attB locus on the chromosome of Listeria monocytogenes, but it is unable to promote excisive recombination between the hybrid attL and attR sites found on the integrated prophage without assistance by a recombination directionality factor (RDF). We have identified and characterized the phage-encoded RDF Gp44, which activates the A118 integrase for excision and inhibits integration. Gp44 binds to the C-terminal DNA binding domain of integrase, and we have localized the primary binding site to be within the mobile coiled-coil (CC) motif but distinct from the distal tip of the CC that is required for recombination. This interaction is sufficient to inhibit integration, but a second interaction involving the N-terminal end of Gp44 is also required to activate excision. We provide evidence that these two contacts modulate the trajectory of the CC motifs as they extend out from the integrase core in a manner dependent upon the identities of the four att sites. Our results support a model whereby Gp44 shapes the Int-bound complexes to control which att sites can synapse and recombine. IMPORTANCE Serine integrases mediate directional recombination between bacteriophage and bacterial chromosomes. These highly regulated site-specific recombination reactions are integral to the life cycle of temperate phage and, in the case of Listeria monocytogenes lysogenized by A118 family phage, are an essential virulence determinant. Serine integrases are also utilized as tools for genetic engineering and synthetic biology because of their exquisite unidirectional control of the DNA exchange reaction. Here, we identify and characterize the recombination directionality factor (RDF) that activates excision and inhibits integration reactions by the phage A118 integrase. We provide evidence that the A118 RDF binds to and modulates the trajectory of the long coiled-coil motif that extends from the large carboxyl-terminal DNA binding domain and is postulated to control the early steps of recombination site synapsis.

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