An ancient polymorphic regulatory region within the BDNF gene associated with obesity modulates anxiety-like behaviour in mice and humans.

Obesity and anxiety are morbidities notable for their increased impact on society during the recent COVID-19 pandemic. Understanding the mechanisms governing susceptibility to these conditions will increase our quality of life and resilience to future pandemics. In the current study, we explored the function of a highly conserved regulatory region (BE5.1) within the BDNF gene that harbours a polymorphism strongly associated with obesity (rs10767664; p = 4.69 × 10-26). Analysis in primary cells suggested that the major T-allele of BE5.1 was an enhancer, whereas the obesity-associated A-allele was not. However, CRISPR/CAS9 deletion of BE5.1 from the mouse genome (BE5.1KO) produced no significant effect on the expression of BDNF transcripts in the hypothalamus, no change in weight gain after 28 days and only a marginally significant increase in food intake. Nevertheless, transcripts were significantly increased in the amygdala of female mice and elevated zero maze and marble-burying tests demonstrated a significant increase in anxiety-like behaviour that could be reversed by diazepam. Consistent with these observations, human GWAS cohort analysis demonstrated a significant association between rs10767664 and anxiousness in human populations. Intriguingly, interrogation of the human GTEx eQTL database demonstrated no effect on BDNF mRNA levels associated with rs10767664 but a highly significant effect on BDNF-antisense (BDNF-AS) gene expression and splicing. The subsequent observation that deletion of BE5.1 also significantly reduced BDNF-AS expression in mice suggests a novel mechanism in the regulation of BDNF expression common to mice and humans, which contributes to the modulation of mood and anxiety in both species.

Context-dependant enhancers as a reservoir of functional polymorphisms and epigenetic markers linked to alcohol use disorders and comorbidities.

Alcohol use disorder (AUD) is one of the major causes of mortality and morbidity world-wide. It is estimated that 50% of the causes of AUD are heritable. Efforts to determine the genetic determinants governing AUD using genome wide association studies (GWAS) show that the most strongly associated SNPs occur within, or in the vicinity of, genes encoding enzymes that metabolise ethanol. However, these studies were not so conclusive in identifying the genes that influenced the choice to drink ethanol or why a proportion of the population become addicted. Most importantly, these studies also found that over 98% of the 1292 SNPs associated with AUD (p<1 × 10-6) were found outside of coding regions and within the poorly understood non-coding genome. Many years of study have shown that functional components of the non-coding genome include enigmatic enhancer elements whose biological role is to modulate levels of gene expression in specific cells, in specific amounts and in response to the correct stimuli. The current short review introduces the functional components of the non-coding genome, such as promoters and enhancers, and critically assesses the latest methods of identifying and characterising their context dependant roles in AUD and mental health disorders. We then go on to examine what is known about the roles of enhancers, such as the GAL5.1 enhancer, in alcohol intake and explore how enhancers are affected by polymorphic variation and epigenetic markers such as DNA-methylation and may influence susceptibility to AUD. The review finishes by discussing the future of AUD genetics and what technologies will need to be brought to bear to understand how genetic and environmentally induced changes in enhancer structure may contribute to the need to drink alcohol to excess.

The anxiety and ethanol intake controlling GAL5.1 enhancer is epigenetically modulated by, and controls preference for, high-fat diet.

Excess maternal fat intake and obesity increase offspring susceptibility to conditions such as chronic anxiety and substance abuse. We hypothesised that environmentally modulated DNA methylation changes (5mC/5hmC) in regulatory regions of the genome that modulate mood and consumptive behaviours could contribute to susceptibility to these conditions. We explored the effects of environmental factors on 5mC/5hmC levels within the GAL5.1 enhancer that controls anxiety-related behaviours and alcohol intake. We first observed that 5mC/5hmC levels within the GAL5.1 enhancer differed significantly in different parts of the brain. Moreover, we noted that early life stress had no significant effect of 5mC/5hmC levels within GAL5.1. In contrast, we identified that allowing access of pregnant mothers to high-fat diet (> 60% calories from fat) had a significant effect on 5mC/5hmC levels within GAL5.1 in hypothalamus and amygdala of resulting male offspring. Cell transfection-based studies using GAL5.1 reporter plasmids showed that 5mC has a significant repressive effect on GAL5.1 activity and its response to known stimuli, such as EGR1 transcription factor expression and PKC agonism. Intriguingly, CRISPR-driven disruption of GAL5.1 from the mouse genome, although having negligible effects on metabolism or general appetite, significantly decreased intake of high-fat diet suggesting that GAL5.1, in addition to being epigenetically modulated by high-fat diet, also actively contributes to the consumption of high-fat diet suggesting its involvement in an environmentally influenced regulatory loop. Furthermore, considering that GAL5.1 also controls alcohol preference and anxiety these studies may provide a first glimpse into an epigenetically controlled mechanism that links maternal high-fat diet with transgenerational susceptibility to alcohol abuse and anxiety.

CRISPR disruption and UK Biobank analysis of a highly conserved polymorphic enhancer suggests a role in male anxiety and ethanol intake.

Excessive alcohol intake is associated with 5.9% of global deaths. However, this figure is especially acute in men such that 7.6% of deaths can be attributed to alcohol intake. Previous studies identified a significant interaction between genotypes of the galanin (GAL) gene with anxiety and alcohol abuse in different male populations but were unable to define a mechanism. To address these issues the current study analysed the human UK Biobank cohort and identified a significant interaction (n = 115,865; p = 0.0007) between allelic variation (GG or CA genotypes) in the highly conserved human GAL5.1 enhancer, alcohol intake (AUDIT questionnaire scores) and anxiety in men. Critically, disruption of GAL5.1 in mice using CRISPR genome editing significantly reduced GAL expression in the amygdala and hypothalamus whilst producing a corresponding reduction in ethanol intake in KO mice. Intriguingly, we also found the evidence of reduced anxiety-like behaviour in male GAL5.1KO animals mirroring that seen in humans from our UK Biobank studies. Using bioinformatic analysis and co-transfection studies we further identified the EGR1 transcription factor, that is co-expressed with GAL in amygdala and hypothalamus, as being important in the protein kinase C (PKC) supported activity of the GG genotype of GAL5.1 but less so in the CA genotype. Our unique study uses a novel combination of human association analysis, CRISPR genome editing in mice, animal behavioural analysis and cell culture studies to identify a highly conserved regulatory mechanism linking anxiety and alcohol intake that might contribute to increased susceptibility to anxiety and alcohol abuse in men.

Perspective: Quality Versus Quantity; Is It Important to Assess the Role of Enhancers in Complex Disease from an In Vivo Perspective?

Sequencing of the human genome has permitted the development of genome-wide association studies (GWAS) to analyze the genetics of a number of complex disorders such as depression, anxiety and substance abuse. Thanks to their ability to analyze huge cohort sizes, these studies have successfully identified thousands of loci associated with a broad spectrum of complex diseases. Disconcertingly, the majority of these GWAS hits occur in non-coding regions of the genome, much of which controls the cell-type-specific expression of genes essential to health. In contrast to gene coding sequences, it is a challenge to understand the function of this non-coding regulatory genome using conventional biochemical techniques in cell lines. The current commentary scrutinizes the field of complex genetics from the standpoint of the large-scale whole-genome functional analysis of the promoters and cis-regulatory elements using chromatin markers. We contrast these large scale quantitative techniques against comparative genomics and in vivo analyses including CRISPR/CAS9 genome editing to determine the functional characteristics of these elements and to understand how polymorphic variation and epigenetic changes within these elements might contribute to complex disease and drug response. Most importantly, we suggest that, although the role of chromatin markers will continue to be important in identifying and characterizing enhancers, more emphasis must be placed on their analysis in relevant in-vivo models that take account of the appropriate cell-type-specific roles of these elements. It is hoped that offering these insights might refocus progress in analyzing the data tsunami of non-coding GWAS and whole-genome sequencing "hits" that threatens to overwhelm progress in the field.

Disease-associated polymorphisms within the conserved ECR1 enhancer differentially regulate the tissue-specific activity of the cannabinoid-1 receptor gene promoter; implications for cannabinoid pharmacogenetics.

Cannabinoid receptor-1 (CB1) represents a potential drug target against conditions that include obesity and substance abuse. However, drug trials targeting CB1 (encoded by the CNR1 gene) have been compromised by differences in patient response. Toward addressing the hypothesis that genetic changes within the regulatory regions controlling CNR1 expression contribute to these differences, we characterized the effects of disease-associated allelic variation within a conserved regulatory sequence (ECR1) in CNR1 intron 2 that had previously been shown to modulate cannabinoid response, alcohol intake, and anxiety-like behavior. We used primary cell analysis of reporters carrying different allelic variants of the human ECR1 and found that human-specific C-allele variants of ECR1 (ECR1(C)) drove higher levels of CNR1prom activity in primary hippocampal cells than did the ancestral T-allele and demonstrated a differential response to CB1 agonism. We further demonstrate a role for the AP-1 transcription factor in driving higher ECR1(C) activity and evidence that the ancestral t-allele variant of ECR1 interacted with higher affinity with the insulator binding factor CTCF. The cell-specific approaches used in our study represent an important step in gaining a mechanistic understanding of the roles of noncoding polymorphic variation in disease and in the increasingly important field of cannabinoid pharmacogenetics.

Disruption of an enhancer associated with addictive behaviour within the cannabinoid receptor-1 gene suggests a possible role in alcohol intake, cannabinoid response and anxiety-related behaviour.

The cannabinoid-1 receptor (CB1) plays a critical role in a number of biological processes including nutrient intake, addiction and anxiety-related behaviour. Numerous studies have shown that expression of the gene encoding CB1 (CNR1) is highly dynamic with changes in the tissue specific expression of CNR1 associated with brain homeostasis and disease progression. However, little is known of the mechanisms regulating this dynamic expression. To gain a better understanding of the genomic mechanisms modulating the expression of CNR1 in health and disease we characterised the role of a highly conserved regulatory sequence (ECR1) in CNR1 intron 2 that contained a polymorphism in linkage disequilibrium with disease associated SNPs. We used CRISPR/CAS9 technology to disrupt ECR1 within the mouse genome. Disruption of ECR1 significantly reduced CNR1 expression in the hippocampus but not in the hypothalamus. These mice also displayed an altered sex-specific anxiety-related behavioural profile (open field test), reduced ethanol intake and a reduced hypothermic response following CB1 agonism. However, no significant changes in feeding patterns were detected. These data suggest that, whilst not all of the expression of CNR1 is modulated by ECR1, this highly conserved enhancer is required for appropriate physiological responses to a number of stimuli. The combination of comparative genomics and CRISPR/CAS9 disruption used in our study to determine the functional effects of genetic and epigenetic changes on the activity of tissue-specific regulatory elements at the CNR1 locus represent an important first step in gaining a mechanistic understanding of cannabinoid regulatory pharmacogenetics.

Sustainable Forest Operations (SFO): A new paradigm in a changing world and climate.

The effective implementation of sustainable forest management depends largely on carrying out forest operations in a sustainable manner. Climate change, as well as the increasing demand for forest products, requires a re-thinking of forest operations in terms of sustainability. In this context, it is important to understand the major driving factors for the future development of forest operations that promote economic, environmental and social well-being. The main objective of this paper is to identify important issues concerning forest operations and to propose a new paradigm towards sustainability in a changing climate, work and environmental conditions. Previously developed concepts of forest operations are reviewed, and a newly developed concept - Sustainable Forest Operations (SFO), is presented. Five key performance areas to ensure the sustainability of forest operations include: (i) environment; (ii) ergonomics; (iii) economics; (iv) quality optimization of products and production; and (v) people and society. Practical field examples are presented to demonstrate how these five interconnected principles are relevant to achieving sustainability, namely profit and wood quality maximization, ecological benefits, climate change mitigation, carbon sequestration, and forest workers' health and safety. The new concept of SFO provides integrated perspectives and approaches to effectively address ongoing and foreseeable challenges the global forest communities face, while balancing forest operations performance across economic, environmental and social sustainability. In this new concept, we emphasize the role of wood as a renewable and environmentally friendly material, and forest workers' safety and utilization efficiency and waste management as additional key elements of sustainability.

A Blind Test of Computational Technique for Predicting the Likelihood of Peptide Sequences to Cyclize.

An in silico computational technique for predicting peptide sequences that can be cyclized by cyanobactin macrocyclases, e.g., PatGmac, is reported. We demonstrate that the propensity for PatGmac-mediated cyclization correlates strongly with the free energy of the so-called pre-cyclization conformation (PCC), which is a fold where the cyclizing sequence C and N termini are in close proximity. This conclusion is driven by comparison of the predictions of boxed molecular dynamics (BXD) with experimental data, which have achieved an accuracy of 84%. A true blind test rather than training of the model is reported here as the in silico tool was developed before any experimental data was given, and no parameters of computations were adjusted to fit the data. The success of the blind test provides fundamental understanding of the molecular mechanism of cyclization by cyanobactin macrocyclases, suggesting that formation of PCC is the rate-determining step. PCC formation might also play a part in other processes of cyclic peptides production and on the practical side the suggested tool might become useful for finding cyclizable peptide sequences in general.

Isolation and anti-HIV-1 integrase activity of lentzeosides A-F from extremotolerant lentzea sp. H45, a strain isolated from a high-altitude Atacama Desert soil.

The extremotolerant isolate H45 was one of several actinomycetes isolated from a high-altitude Atacama Desert soil collected in northwest Chile. The isolate was identified as a new Lentzea sp. using a combination of chemotaxonomic, morphological and phylogenetic properties. Large scale fermentation of the strain in two different media followed by chromatographic purification led to the isolation of six new diene and monoene glycosides named lentzeosides A-F, together with the known compound (Z)-3-hexenyl glucoside. The structures of the new compounds were confirmed by HRESIMS and NMR analyses. Compounds 1-6 displayed moderate inhibitory activity against HIV integrase.

Accurate quantification of modified cyclic peptides without the need for authentic standards.

There is a growing interest in the use of cyclic peptides as therapeutics, but their efficient production is often the bottleneck in taking them forward in the development pipeline. We have recently developed a method to synthesise azole-containing cyclic peptides using enzymes derived from different cyanobactin biosynthetic pathways. Accurate quantification is crucial for calculation of the reaction yield and for the downstream biological testing of the products. In this study, we demonstrate the development and validation of two methods to accurately quantify these compounds in the reaction mixture and after purification. The first method involves the use of a HPLC coupled in parallel to an ESMS and an ICPMS, hence correlating the calculated sulfur content to the amount of cyclic peptide. The second method is an NMR ERETIC method for quantifying the solution concentration of cyclic peptides. These methods make the quantification of new compounds much easier as there is no need for the use of authentic standards when they are not available.

An efficient method for the in vitro production of azol(in)e-based cyclic peptides.

Heterocycle-containing cyclic peptides are promising scaffolds for the pharmaceutical industry but their chemical synthesis is very challenging. A new universal method has been devised to prepare these compounds by using a set of engineered marine-derived enzymes and substrates obtained from a family of ribosomally produced and post-translationally modified peptides called the cyanobactins. The substrate precursor peptide is engineered to have a non-native protease cleavage site that can be rapidly cleaved. The other enzymes used are heterocyclases that convert Cys or Cys/Ser/Thr into their corresponding azolines. A macrocycle is formed using a macrocyclase enzyme, followed by oxidation of the azolines to azoles with a specific oxidase. The work is exemplified by the production of 17 macrocycles containing 6-9 residues representing 11 out of the 20 canonical amino acids.

Gated rotation mechanism of site-specific recombination by ϕC31 integrase.

Integrases, such as that of the Streptomyces temperate bacteriophage ϕC31, promote site-specific recombination between DNA sequences in the bacteriophage and bacterial genomes to integrate or excise the phage DNA. ϕC31 integrase belongs to the serine recombinase family, a large group of structurally related enzymes with diverse biological functions. It has been proposed that serine integrases use a "subunit rotation" mechanism to exchange DNA strands after double-strand DNA cleavage at the two recombining att sites, and that many rounds of subunit rotation can occur before the strands are religated. We have analyzed the mechanism of ϕC31 integrase-mediated recombination in a topologically constrained experimental system using hybrid "phes" recombination sites, each of which comprises a ϕC31 att site positioned adjacent to a regulatory sequence recognized by Tn3 resolvase. The topologies of reaction products from circular substrates containing two phes sites support a right-handed subunit rotation mechanism for catalysis of both integrative and excisive recombination. Strand exchange usually terminates after a single round of 180° rotation. However, multiple processive "360° rotation" rounds of strand exchange can be observed, if the recombining sites have nonidentical base pairs at their centers. We propose that a regulatory "gating" mechanism normally blocks multiple rounds of strand exchange and triggers product release after a single round.

Is ICRP guidance on the use of reference levels consistent?

In ICRP 103, which has replaced ICRP 60, it is stated that no fundamental changes have been introduced compared with ICRP 60. This is true except that the application of reference levels in emergency and existing exposure situations seems to be applied inconsistently, and also in the related publications ICRP 109 and ICRP 111. ICRP 103 emphasises that focus should be on the residual doses after the implementation of protection strategies in emergency and existing exposure situations. If possible, the result of an optimised protection strategy should bring the residual dose below the reference level. Thus the reference level represents the maximum acceptable residual dose after an optimised protection strategy has been implemented. It is not an 'off-the-shelf item' that can be set free of the prevailing situation. It should be determined as part of the process of optimising the protection strategy. If not, protection would be sub-optimised. However, in ICRP 103 some inconsistent concepts have been introduced, e.g. in paragraph 279 which states: 'All exposures above or below the reference level should be subject to optimisation of protection, and particular attention should be given to exposures above the reference level'. If, in fact, all exposures above and below reference levels are subject to the process of optimisation, reference levels appear superfluous. It could be considered that if optimisation of protection below a fixed reference level is necessary, then the reference level has been set too high at the outset. Up until the last phase of the preparation of ICRP 103 the concept of a dose constraint was recommended to constrain the optimisation of protection in all types of exposure situations. In the final phase, the term 'dose constraint' was changed to 'reference level' for emergency and existing exposure situations. However, it seems as if in ICRP 103 it was not fully recognised that dose constraints and reference levels are conceptually different. The use of reference levels in radiological protection is reviewed. It is concluded that the recommendations in ICRP 103 and related ICRP publications seem to be inconsistent regarding the use of reference levels in existing and emergency exposure situations.

A phage protein that binds φC31 integrase to switch its directionality.

The serine integrase, Int, from the Streptomyces phage φC31 mediates the integration and excision of the phage genome into and out of the host chromosome. Integrases usually require a recombination directionality factor (RDF) or Xis to control integration and excision and, as φC31 Int only mediates integration in the absence of other phage proteins, we sought to identify a φC31 RDF. Here we report that the φC31 early protein, gp3 activated attL x attR recombination and inhibited attP x attB recombination. Gp3 binds to Int in solution and when Int is bound to the attachment sites. Kinetic analysis of the excision reaction suggested that gp3 modifies the interactions between Int and the substrates to form an active recombinase. In the presence of gp3, Int assembles an excision synaptic complex and the accumulation of the integration complex is inhibited. The structure of the excision synaptic complex, like that of the hyperactive mutant of Int, IntE449K, appeared to be biased towards one that favours the production of correctly joined products. The functional properties of φC31 gp3 resemble those of the evolutionarily unrelated RDF from phage Bxb1, suggesting that these two RDFs have arisen through convergent evolution.

Zinc is essential for high-affinity DNA binding and recombinase activity of ΦC31 integrase.

The mechanism through which the large serine recombinases bind DNA is poorly understood. Alignments of C31 integrase (Int) and its relatives indicate the presence of a conserved motif containing four cysteines resembling a zinc finger. Inductively coupled plasma-mass spectrometry (ICP-MS) confirmed that an Int monomer contains one atom of zinc. Pre-incubation of Int with ethylenediaminetetraacetic acid (EDTA) was detrimental for both recombination activity and DNA binding affinities but full activity could be restored by adding back Zn(2+). Mutations in the cysteines and other highly conserved residues yielded proteins that were hypersensitive to proteases, suggesting that without zinc the domain is unfolded. Substitutions in the highly charged region between the conserved cysteines led to lowered DNA binding affinities while circular dichroism revealed that these variant Ints were not greatly affected in overall folding. Int was protected from inhibition by EDTA when DNA containing an attachment site was present suggesting that the zinc finger and the DNA are in close proximity. A truncated mutant of Int, hInt V371S(UGA), lacking the putative zinc finger could bind DNA with low affinity. The data are consistent with there being at least two DNA binding motifs in Int one of which is the zinc finger-like motif.

Site-specific recombination by phiC31 integrase and other large serine recombinases.

Most temperate phages encode an integrase for integration and excision of the prophage. Integrases belong either to the lambda Int family of tyrosine recombinases or to a subgroup of the serine recombinases, the large serine recombinases. Integration by purified serine integrases occurs efficiently in vitro in the presence of their cognate (~50 bp) phage and host attachment sites, attP and attB respectively. Serine integrases require an accessory protein, Xis, to promote excision, a reaction in which the products of the integration reaction, attL and attR, recombine to regenerate attP and attB. Unlike other directional recombinases, serine integrases are not controlled by proteins occupying accessory DNA-binding sites. Instead, it is thought that different integrase conformations, induced by binding to the DNA substrates, control protein-protein interactions, which in turn determine whether recombination proceeds. The present review brings together the evidence for this model derived from the studies on phiC31 integrase, Bxb1 integrase and other related proteins.

DNA binding and synapsis by the large C-terminal domain of phiC31 integrase.

The integrase (Int) from phage C31 acts on the phage and host-attachment sites, attP and attB, to form an integrated prophage flanked by attL and attR. Excision (attL x attR recombination) is prevented, in the absence of accessory factors, by a putative coiled-coil motif in the C-terminal domain (CTD). Int has a serine recombinase N-terminal domain, required for synapsis of recombination substrates and catalysis. We show here that the coiled-coil motif mediates protein-protein interactions between CTDs, but only when bound to DNA. Although the histidine-tagged CTD (hCTD) was monomeric in solution, hCTD bound cooperatively to three of the recombination substrates (attB, attL and attR). Furthermore, when provided with attP and attB, hCTD brought these substrates together in a synaptic complex. Substitutions in the coiled-coil motif that greatly reduce Int integration activity, L460P and Y475H, prevented CTD-CTD interactions and led to defective DNA binding and no detectable DNA synapsis. A substitution, E449K, in full length Int confers the ability to perform excision in addition to integration as it has gained the ability to synapse attL x attR. hCTD(E449K) was similar to hCTD in DNA binding but unable to form the CTD synapse suggesting that the CTD synapse is not essential but could be part of the mechanism that controls directionality.

Characterization and crystal structure of Escherichia coli KDPGal aldolase.

2-Keto-3-deoxy-6-phosphogluconate (KDPG) and 2-keto-3-deoxy-6-phosphogalactonate (KDPGal) aldolases catalyze an identical reaction differing in substrate specificity in only the configuration of a single stereocenter. However, the proteins show little sequence homology at the amino acid level. Here we investigate the determinants of substrate selectivity of these enzymes. The Escherichia coli KDPGal aldolase gene, cloned into a T7 expression vector and overexpressed in E. coli, catalyzes retro-aldol cleavage of the natural substrate, KDPGal, with values of k(cat)/K(M) and k(cat) of 1.9x10(4)M(-1)s(-1) and 4s(-1), respectively. In the synthetic direction, KDPGal aldolase efficiently catalyzes an aldol addition using a limited number of aldehyde substrates, including d-glyceraldehyde-3-phosphate (natural substrate), d-glyceraldehyde, glycolaldehyde, and 2-pyridinecarboxaldehyde. A preparative scale reaction between 2-pyridinecarboxaldehyde and pyruvate catalyzed by KDPGal aldolase produced the aldol adduct of the R stereochemistry in >99.7% ee, a result complementary to that observed using the related KDPG aldolase. The native crystal structure has been solved to a resolution of 2.4A and displays the same (alpha/beta)(8) topology, as KDPG aldolase. We have also determined a 2.1A structure of a Schiff base complex between the enzyme and its substrate. This model predicts that a single amino acid change, T161 in KDPG aldolase to V154 in KDPGal aldolase, plays an important role in determining the stereochemical course of enzyme catalysis and this prediction was borne out by site-directed mutagenesis studies. However, additional changes in the enzyme sequence are required to prepare an enzyme with both high catalytic efficiency and altered stereochemistry.

Mechanism of enzymatic fluorination in Streptomyces cattleya.

Recently a fluorination enzyme was identified and isolated from Streptomyces cattleya, as the first committed step on the metabolic pathway to the fluorinated metabolites, fluoroacetate and 4-fluorothreonine. This enzyme, 5'-fluoro-5'-deoxy adenosine synthetase (FDAS), has been shown to catalyze C-F bond formation by nucleophilic attack of fluoride ion to S-adenosyl-l-methionine (SAM) with the concomitant displacement of l-methionine to generate 5'-fluoro-5'-deoxy adenosine (5'-FDA). Although the structures of FDAS bound to both SAM and products have been solved, the molecular mechanism remained to be elucidated. We now report site-directed mutagenesis studies, structural analyses, and isothermal calorimetry (ITC) experiments. The data establish the key residues required for catalysis and the order of substrate binding. Fluoride ion is not readily distinguished from water by protein X-ray crystallography; however, using chloride ion (also a substrate) with a mutant of low activity has enabled the halide ion to be located in nonproductive co-complexes with SAH and SAM. The kinetic data suggest the positively charged sulfur of SAM is a key requirement in stabilizing the transition state. We propose a molecular mechanism for FDAS in which fluoride weakly associates with the enzyme exchanging two water molecules for protein ligation. The binding of SAM expels remaining water associated with fluoride ion and traps the ion in a pocket positioned to react with SAM, generating l-methionine and 5'-FDA. l-methionine then dissociates from the enzyme followed by 5'-FDA.