Molecular cold-adaptation of protein function and gene regulation: The case for comparative genomic analyses in marine ciliated protozoa

Euplotes focardii is a marine ciliated protozoan discovered in the Ross Sea near Terra Nova Bay, Antarctica. This organism is strictly psychrophilic, survives and reproduces optimally at 4–5 °C, and has a genome rich in A/T base pairs. Like other ciliated protozoans, Euplotes spp. are characterized by nuclear dimorphism: 1) the germline micronucleus contains the entire genome as large chromosomes; and 2) the somatic macronucleus ( 50 megabases, or 5% of the micronuclear genome) contains small linear DNA nanochromosomes [1–12 kilobases], each of which constitutes a single genetic unit. These characteristics make E. focardii an ideal model for genome-level analysis to understand the evolutionary mechanisms that determine the adaptation of organisms to cold environments. Here we describe two examples that are controlled by phylogenetically appropriate comparison with mesophilic and psychrotolerant Euplotes species: 1) the genes and encoded proteins of the E. focardii tubulin superfamily, including α-, β-, and γ-tubulins; and 2) the genes of the heat-shock protein (Hsp) 70 family. The tubulins provide particular insight into protein-level structural changes that are likely to facilitate microtubule nucleation and polymerization in an energy poor environment. By contrast, the hsp70 genes of E. focardii and of its psychrotolerant relative E. nobilii reveal adaptive alterations in the regulation of gene expression in the cold. The unique characteristics of the E. focardii genome and the results that we present here argue strongly for a concerted effort to characterize the relatively low complexity macronuclear genome of this psychrophilic organism.     

Citrate synthase from Antarctic ciliates: adaptation to low temperatures and comparison with temperate ciliates

The activity of citrate synthase (CS), a mitochondrial enzyme in the energy metabolism pathway, was studied in Euplotes focardii (Hypotrichia) and Uronema marinum (Scuticociliatia), isolated from samples of Antarctic seawater and, for comparison in E. vannus and U. nigricans, related ciliates from the Adriatic. The temperature-dependent characteristics of CS were investigated in the range 0–55°C, to evaluate mechanisms of thermal regulation, such as optimal temperature levels, activation energy, and kinetic parameters. CS activity was up to double that recorded in temperate organisms at reaction temperatures between 0 and 10°C. The optimal temperature for enzymatic activity was independent of Tamb. Activation energy for CS was significantly lower in Antarctic ciliates than in temperate ones, indicating a significant increase in the reaction rate. KM at various temperatures of the reaction substrates indicated the higher affinity of CS for acetyl-CoA in both Antarctic organisms at lower temperatures. The data for thermal profiles for KM, showing greater differences between E. focardii and E. vannus than between U. marinum and U. nigricans, support the distinctive physiological characteristics of each species. E. focardii is an endemic and psychrophilic species, whereas U. marinum is a typically ubiquitous species, more adaptable to environmental changes.     

Divergences in the Response to Ultraviolet Radiation Between Polar and Non-Polar Ciliated Protozoa

Ultraviolet (UV) radiation has detrimental effects on marine ecosystems, in particular in the polar regions where stratospheric ozone reduction causes higher levels of solar radiation. We analyzed two polar species of Euplotes, Euplotes focardii and Euplotes nobilii, for the sensitivity to UV radiation in comparison with two akin species from mid-latitude and tropical waters. Results showed that they face UV radiation much more efficiently than the non-polar species by adopting alternative strategies that most likely reflect different times of colonization of the polar waters. While E. focardii, which is endemic to the Antarctic, survives for longer exposed to UV radiation, E. nobilii, which inhabits both the Antarctic and Arctic, recovers faster from UV-induced damage.     

Improved description of the bipolar ciliate,Euplotes petzi,and definition of its basal position in the Euplotes phylogenetic tree

Data improving the characterization of the marine Euplotes species, E. petzi Wilbert and Song, 2008, were obtained from morphological, ecological and genetic analyses of Antarctic and Arctic wild-type strains. This species is identified by a minute (mean size, 46 μm × 32 μm) and ellipsoidal cell body which is dorsally decorated with an argyrome of the double-patella type, five dorsal kineties (of which the median one contains 8–10 dikinetids), five sharp-edged longitudinal ridges, and a right anterior spur. Ventrally, it bears 10 fronto-ventral, five transverse, two caudal and two marginal cirri, 30–35 adoral membranelles, and three inconspicuous ridges. Euplotes petzi grows well at 4 °C on green algae, does not produce cysts, undergoes mating under the genetic control of a multiple mating-type system, constitutively secretes water-borne pheromones, and behaves as a psychrophilic microorganism unable to survive at >15 °C. While the α-tubulin gene sequence determination did not provide useful information on the E. petzi molecular phylogeny, the small subunit rRNA (SSU rRNA) gene sequence determination provided solid evidence that E. petzi clusters with E. sinicus Jiang et al., 2010a, into a clade which represents the deepest branch at the base of the Euplotes phylogentic tree.     

Crystal structure of non-redox regulated SSADH from Escherichia coli

SSADH is involved in the final step of GABA degradation, converting SSA to succinic acid in the human mitochondrial matrix, and its activity is known to be regulated via ‘redox-switch modulation’ of the catalytic loop. We present the crystal structure of EcSSADH, revealing that the catalytic loop of EcSSADH, unlike that of human SSADH, does not undergo disulfide bond-mediated structural changes upon changes of environmental redox status. Subsequent redox change experiments using recombinant proteins confirm the non-redox regulation of this protein. Detailed structural analysis shows that a difference in the conformation of the connecting loop (β15-β16) causes the formation of a water molecule-mediated hydrogen bond network between the connecting loop and the catalytic loop in EcSSADH, making the catalytic loop of EcSSADH more rigid compared to that of human SSADH. The cytosolic localization of EcSSADH and the cellular function of the GABA shunt in E. coli might result in the non-redox mediated regulatory mechanisms of the protein.     

Key roles of the Escherichia coli AhpC C-terminus in assembly and catalysis of alkylhydroperoxide reductase,an enzyme essential for the alleviation of oxidative stress

2-Cys peroxiredoxins (Prxs) are a large family of peroxidases, responsible for antioxidant function and regulation in cell signaling, apoptosis and differentiation. The Escherichia coli alkylhydroperoxide reductase (AhpR) is a prototype of the Prxs-family, and is composed of an NADH-dependent AhpF reductase (57 kDa) and AhpC (21 kDa), catalyzing the reduction of H₂O₂. We show that the E. coli AhpC (EcAhpC, 187 residues) forms a decameric ring structure under reduced and close to physiological conditions, composed of five catalytic dimers. Single particle analysis of cryo-electron micrographs of C-terminal truncated (EcAhpC_1 -172 and EcAhpC_1 -182) and mutated forms of EcAhpC reveals the loss of decamer formation, indicating the importance of the very C-terminus of AhpC in dimer to decamer transition. The crystallographic structures of the truncated EcAhpC_1 -172 and EcAhpC_1 -182 demonstrate for the first time that, in contrast to the reduced form, the very C-terminus of the oxidized EcAhpC is oriented away from the AhpC dimer interface and away from the catalytic redox-center, reflecting structural rearrangements during redox-modulation and -oligomerization. Furthermore, using an ensemble of different truncated and mutated EcAhpC protein constructs the importance of the very C-terminus in AhpC activity and in AhpC–AhpF assembly has been demonstrated.     

八肋游仆虫大核基因组中组蛋白基因 H4A和H4B的克隆及分析

组蛋白作为核小体的基本组分,是染色质的结构和功能必需的。组蛋白的变体和修饰共同参与染色质修饰及基因的表达调控。真核生物细胞中的5种组蛋白在进化中高度保守,然而纤毛虫的组蛋白H4与其他真核生物相比有较大的差异。本实验应用PCR技术从八肋游仆虫(Euplotes octocarinatus)中获得了2种组蛋白H4基因,分别为H4A和H4B,GenBank登录号为:JN715068和JN715069。序列分析表明,H4A基因开放阅读框324 bp,预测编码107个氨基酸,分子量为11.6 ku,等电点为10.99。而H4B基因编码框384 bp,编码127个氨基酸,分子量为14.4 ku,等电点为9.93。Blast结果显示,H4A序列与其他生物中H4的一致性相对较高,达81%～94%,而H4B的一致性为36%～70%。H4A和H4B的一致性仅为44.7%。实时荧光定量PCR表明,H4A的转录本高于H4B。结果提示:在进化过程中八肋游仆虫可能进化出特殊的组蛋白H4基因,不同的组蛋白H4可能发挥不同的功能。     

Trigger Factor from the Psychrophilic Bacterium Psychrobacter frigidicola Is a Monomeric Chaperone

In eubacteria, trigger factor (TF) is the first chaperone to interact with newly synthesized polypeptides and assist their folding as they emerge from the ribosome. We report the first characterization of a TF from a psychrophilic organism. TF from Psychrobacter frigidicola (TF_Pf) was cloned, produced in Escherichia coli, and purified. Strikingly, cross-linking and fluorescence anisotropy analyses revealed it to exist in solution as a monomer, unlike the well-characterized, dimeric E. coli TF (TF_Ec). Moreover, TF_Pf did not exhibit the downturn in reactivation of unfolded GAPDH (glyceraldehyde-3-phosphate dehydrogenase) that is observed with its E. coli counterpart, even at high TF/GAPDH molar ratios and revealed dramatically reduced retardation of membrane translocation by a model recombinant protein compared to the E. coli chaperone. TF_Pf was also significantly more effective than TF_Ec at increasing the yield of soluble and functional recombinant protein in a cell-free protein synthesis system, indicating that it is not dependent on downstream systems for its chaperoning activity. We propose that TF_Pf differs from TF_Ec in its quaternary structure and chaperone activity, and we discuss the potential significance of these differences in its native environment.     

Secondary metabolites from Eurotium species, Aspergillus calidoustus and A. insuetus common in Canadian homes with a review of their chemistry and biological activities

As part of studies of metabolites from fungi common in the built environment in Canadian homes, we investigated metabolites from strains of three Eurotium species, namely E. herbariorum, E. amstelodami, and E. rubrum as well as a number of isolates provisionally identified as Aspergillus ustus. The latter have been recently assigned as the new species A. insuetus and A. calidoustus. E. amstelodami produced neoechinulin A and neoechinulin B, epiheveadride, flavoglaucin, auroglaucin, and isotetrahydroauroglaucin as major metabolites. Minor metabolites included echinulin, preechinulin and neoechinulin E. E. rubrum produced all of these metabolites, but epiheveadride was detected as a minor metabolite. E. herbariorum produced cladosporin as a major metabolite, in addition to those found in E. amstelodami. This species also produced questin and neoechinulin E as minor metabolites. This is the first report of epiheveadride occurring as a natural product, and the first nonadride isolated from Eurotium species. Unlike strains from mainly infection-related samples, largely from Europe, neither ophiobolins G and H nor austins were detected in the Canadian strains of A. insuetus and A. calidoustus tested, all of which had been reported from the latter species. TMC-120 A, B, C and a sesquiterpene drimane are reported with certainty for the first time from indoor isolates, as well as two novel related methyl isoquinoline alkaloids.     

Tubulin folding: the special case of a beta-tubulin isotype from the Antarctic psychrophilic ciliate Euplotes focardii

Folding assistance is a fundamental requirement of certain proteins, and it may be subjected to physicochemical constraints in case of organisms adapted to polar temperatures. Limited information is available about protein folding in the polar environment. Folding of tubulin provides one of the few studied cases. Here, we report a pilot folding analysis of a divergent beta-tubulin isotype, named EFBT3, from the Antarctic psychrophilic ciliate Euplotes focardii. To attain its native monomeric structure, beta-tubulin needs the assistance of the eukaryotic class II chaperonin CCT and cofactor A (CofA). The in vitro folding reaction of EFBT3 with CCT and CofA purified from rabbit did not generate any folded product. In contrast, the reaction performed with the rabbit reticulocyte lysate, that contains all the chaperones required for efficient tubulin folding, was productive, suggesting that additional factors besides purified CCT and CofA are required for EFBT3 to attain its monomeric structure. We also demonstrated that the rare Cys281 of EFBT3 is critical for the folding reaction. Model predictions indicate that EFBT3 binds to CofA differently from yeast beta-tubulin, suggesting a diverse folding mechanism that may be correlated with microtubule cold adaptation.     

Two new species of Electra (Bryozoa, Cheilostomata) from the Miocene of the Aquitaine Basin, France

Post-Cretaceous examples of Electridae, a primitive family of cheilostome bryozoans, are poorly represented in the fossil record, probably because of their thinly calcified zooids and preference for nearshore environments. Two new electrid species are here described from the Lower Miocene (Burdigalian) of Pontpourquey, Aquitaine, France: Electra triaurata nov. sp. and Electra aquitanica nov. sp. Both species belong to extant species groups, the E. indica and E. biscuta groups, respectively, that presently occur in the Indo-Pacific; both are the only fossil examples of these species groups. Whereas E. triaurata nov. sp. has uniserial colonies, zooids with porous gymnocysts, three flattened spines and basal windows allowing etching of the substrate to produce the trace fossil Leptichnus, E. aquitanica nov. sp. has multiserial colonies and zooids with a proximal gymnocyst bearing 2 to 5 spines.     

A barotolerant ciliate isolated from the abyssal deep sea of the North Atlantic: Euplotes dominicanus sp. n. (Ciliophora,Euplotia)

A new Euplotes species, isolated from abyssal depths (>4000 m) of the North Atlantic Ocean, was described based on morphology, ciliary pattern and molecular data. Euplotes dominicanus sp. n. is characterized by a small body size (29–40 × 17–27 μm in vivo), 18–22 adoral membranelles, 10 frontoventral, five transverse and two left marginal cirri and one caudal cirrus, five or six dorsolateral kineties with 7–9 dikinetids in mid-dorsolateral kinety (DK3), and dorsal silverline system of the double-eurystomus type. Phylogenetic analyses inferred from 18S rRNA sequences show that Euplotes dominicanus sp. n. is most closely related to E. curdsi, with a sequence similarity of 97.6 %. Euplotes dominicanus sp. n. was able to survive hydrostatic pressures up to 500 bar indicating its barotolerance. Metabarcoding data demonstrate the presence of E. dominicanus sp. n. in sediments of several deep-sea basins.     

Botryosphaeriaceae from tuart (Eucalyptus gomphocephala) woodland, including descriptions of four new species

Eucalyptus gomphocephala (tuart) is a tree native to the southwest coast of Western Australia, where, in some areas, there is a significant decline in the health of tuart. Botryosphaeriaceous taxa have been isolated as endophytes and canker pathogens from numerous hosts in many parts of the world and have been implicated in the decline of E. gomphocephala. In the present study, endophytic fungi were isolated from a wide variety of native woody plant species (Acacia cochlearis, A. rostellifera, Allocasuarina fraseriana, Agonis flexuosa, Banksia grandis, E. gomphocephala, E. marginata and Santalum acuminatum), at two locations in native E. gomphocephala woodland; a site in decline at Yalgorup National Park and a healthy site at Woodman Point Regional Park. Of the 226 isolates obtained, 154 were botryosphaeriaceous taxa, 80 % of which were found to be Neofusicoccum australe, isolated from all hosts at both collection sites. Four new species are described, Dothiorella moneti, Dothiorella santali, Neofusicoccum pennatisporum, and a species belonging to a genus only recently included in the Botryosphaeriaceae, Aplosporella yalgorensis. The other species isolated were Botryosphaeria dothidea on the new hosts A. rostellifera, A. cochlearis and E. marginata and Dichomera eucalypti, on the new host E. marginata. None of the new species formed lesions on excised stems of their host species, E. gomphocephala, or a common plantation species, E. globulus. However, Neofusicoccum australe formed lesions on excised stems of E. globulus and E. gomphocephala.     

Structural insights of the MenD from Escherichia coli reveal ThDP affinity

MenD (2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexadiene-1-carboxylate) synthase belongs to the superfamily of thiamin diphosphate-dependent decarboxylases, which converts isochorismate and 2-oxoglutarate to SHCHC, pyruvate, and carbon dioxide. Here, we report the first crystal structure of apo-MenD from Escherichia coli determined in tetragonal crystal form. The subunit displays the typical three-domain structure observed for ThDP-dependent enzymes. Analytical gel filtration shows that EcMenD behaves as a dimer as well as a tetramer. Circular dichroism and isothermal calorimetry results confirm EcMenD dependency on ThDP, which concomitantly helps to stabilize with better configuration.     

P NMR studies of O-acetylserine sulfhydrylase-B from Salmonella typhimurium

O-Acetylserine sulfhydrylase (OASS) is a pyridoxal 5′-phosphate (PLP)-dependent enzyme that catalyzes the conversion of O-acetylserine and bisulfide to l-cysteine and acetate in bacteria and higher plants. Enteric bacteria have two isozymes of OASS, A and B, produced under aerobic and anaerobic growth conditions, respectively, with different substrate specificities. The ³¹P chemical shift of the internal and external Schiff bases of PLP in OASS-B are further downfield compared to OASS-A, suggesting a tighter binding of the cofactor in the B-isozyme. The chemical shift of the internal Schiff base (ISB) of OASS-B is 6.2 ppm, the highest value reported for the ISB of a PLP-dependent enzyme. Considering the similarity in the binding sites of the PLP cofactor for both isozymes, torsional strain of the C5-C5′ bond (O4′-C5′-C5-C4) of the Schiff base is proposed to contribute to the further downfield shift. The chemical shift of the lanthionine external Schiff base (ESB) of OASS-B is 6.0 ppm, upfield from that of unliganded OASS-B, while that of serine ESB is 6.3 ppm. Changes in chemical shift suggest the torsional strain of PLP changes as the reaction proceeds.The apoenzyme of OASS-B was prepared using hydroxylamine as the resolving reagent. Apoenzyme was reconstituted to holoenzyme by addition of PLP. Reconstitution is pseudo-first order and exhibits a final maximum recovery of 81.4%. The apoenzyme shows no visible absorbance, while the reconstituted enzyme has a UV-visible spectrum that is nearly identical to that of the holoenzyme. Steady-state fluorescence spectra gave tryptophan emission of the apoenzyme that is 3.3-fold higher than the emission of either the native or reconstituted enzyme, suggesting that PLP is a potent quencher of tryptophan emission.     

Novel Structural Components Contribute to the High Thermal Stability of Acyl Carrier Protein from Enterococcus faecalis

Enterococcus faecalis is a Gram-positive, commensal bacterium that lives in the gastrointestinal tracts of humans and other mammals. It causes severe infections because of high antibiotic resistance. E. faecalis can endure extremes of temperature and pH. Acyl carrier protein (ACP) is a key element in the biosynthesis of fatty acids responsible for acyl group shuttling and delivery. In this study, to understand the origin of high thermal stabilities of E. faecalis ACP (Ef-ACP), its solution structure was investigated for the first time. CD experiments showed that the melting temperature of Ef-ACP is 78.8 °C, which is much higher than that of Escherichia coli ACP (67.2 °C). The overall structure of Ef-ACP shows the common ACP folding pattern consisting of four α-helices (helix I (residues 3–17), helix II (residues 39–53), helix III (residues 60–64), and helix IV (residues 68–78)) connected by three loops. Unique Ef-ACP structural features include a hydrophobic interaction between Phe⁴⁵ in helix II and Phe¹⁸ in the α₁α₂ loop and a hydrogen bonding between Ser¹⁵ in helix I and Ile²⁰ in the α₁α₂ loop, resulting in its high thermal stability. Phe⁴⁵-mediated hydrophobic packing may block acyl chain binding subpocket II entry. Furthermore, Ser⁵⁸ in the α₂α₃ loop in Ef-ACP, which usually constitutes a proline in other ACPs, exhibited slow conformational exchanges, resulting in the movement of the helix III outside the structure to accommodate a longer acyl chain in the acyl binding cavity. These results might provide insights into the development of antibiotics against pathogenic drug-resistant E. faecalis strains.     

Phenolic compounds in Hypericum species from the Trigynobrathys section

Phenolic compounds of thirteen Hypericum species growing in South Brazil were investigated aiming to evaluate the usefulness of their distribution as a taxonomic character. The HPLC analysis of the methanolic fractions displayed similar chemical profile and significant contents variation among the investigated taxa, being chlorogenic acid the main metabolite quantified in most of the species (ranging from traces to 16.65 mg% of extract), followed by hyperoside (between 0.27 and 11.48 mg%), quercitrin (0.09 and 13.34 mg%), guaijaverin (0.14 and 2.91 mg%) and isoquercitrin (0.14 and 6.97 mg%), whereas rutin and the xanthone mangiferin were not detected.     

Oxygen reactivity of PutA from Helicobacter species and proline-linked oxidative stress

Proline is converted to glutamate in two successive steps by the proline utilization A (PutA) flavoenzyme in gram-negative bacteria. PutA contains a proline dehydrogenase domain that catalyzes the flavin adenine dinucleotide (FAD)-dependent oxidation of proline to Δ¹-pyrroline-5-carboxylate (P5C) and a P5C dehydrogenase domain that catalyzes the NAD⁺-dependent oxidation of P5C to glutamate. Here, we characterize PutA from Helicobacter hepaticus (PutA_Hh) and Helicobacter pylori (PutA_Hp) to provide new insights into proline metabolism in these gastrointestinal pathogens. Both PutA_Hh and PutA_Hp lack DNA binding activity, in contrast to PutA from Escherichia coli (PutA_Ec), which both regulates and catalyzes proline utilization. PutA_Hh and PutA_Hp display catalytic activities similar to that of PutA_Ec but have higher oxygen reactivity. PutA_Hh and PutA_Hp exhibit 100-fold-higher turnover numbers (~30 min⁻¹) than PutA_Ec (<0. 3 min⁻¹) using oxygen as an electron acceptor during catalytic turnover with proline. Consistent with increased oxygen reactivity, PutA_Hh forms a reversible FAD-sulfite adduct. The significance of increased oxygen reactivity in PutA_Hh and PutA_Hp was probed by oxidative stress studies in E. coli. Expression of PutA_Ec and PutA from Bradyrhizobium japonicum, which exhibit low oxygen reactivity, does not diminish stress survival rates of E. coli cell cultures. In contrast, PutA_Hp and PutA_Hh expression dramatically reduces E. coli cell survival and is correlated with relatively lower proline levels and increased hydrogen peroxide formation. The discovery of reduced oxygen species formation by PutA suggests that proline catabolism may influence redox homeostasis in the ecological niches of these Helicobacter species.     

The hormone-sensitive lipase from Psychrobacter sp. TA144: New insight in the structural/functional characterization

Cold-adapted esterases and lipases have been found to be dominant activities throughout the cold marine environment, indicating their importance in bacterial degradation of the organic matter. lip2 Gene from Psychrobacter sp. TA144, a micro-organism isolated from the Antarctic sea water, was cloned and over-expressed in Escherichia coli. The recombinant protein (PsyHSL) accumulated in the insoluble fraction from which it was recovered in active form, purified to homogeneity and deeply characterised. Temperature dependence of PsyHSL activity was typical of psychrophilic enzymes, with an optimal temperature of 35 °C at pH 8.0. The enzyme resulted to be active on pNP-esters of fatty acids with acyl chain length from C₂ to C₁₂ and the preferred substrate was pNP-pentanoate showing a k_cat = 26.2 ± 0.1 s⁻¹, K_M = 0.122 ± 0.006 mM and a k_cat/K_M = 215 ± 11 mM⁻¹ s⁻¹. The enzyme was strongly inhibited by Hg²⁺, Zn²⁺, Cu²⁺, Fe³⁺, Mn²⁺ ions and it resulted to be activated in presence of methanol and acetonitrile, with calculated C₅₀ values of 1.98 M and 0.92 M, respectively.     

Crystal structure of Escherichia coli diaminopropionate ammonia-lyase reveals mechanism of enzyme activation and catalysis

Pyridoxal 5′-phosphate (PLP)-dependent enzymes utilize the unique chemistry of a pyridine ring to carry out diverse reactions involving amino acids. Diaminopropionate (DAP) ammonia-lyase (DAPAL) is a prokaryotic PLP-dependent enzyme that catalyzes the degradation of d- and l-forms of DAP to pyruvate and ammonia. Here, we report the first crystal structure of DAPAL from Escherichia coli (EcDAPAL) in tetragonal and monoclinic forms at 2.0 and 2.2 Å resolutions, respectively. Structures of EcDAPAL soaked with substrates were also determined. EcDAPAL has a typical fold type II PLP-dependent enzyme topology consisting of a large and a small domain with the active site at the interface of the two domains. The enzyme is a homodimer with a unique biological interface not observed earlier. Structure of the enzyme in the tetragonal form had PLP bound at the active site, whereas the monoclinic structure was in the apo-form. Analysis of the apo and holo structures revealed that the region around the active site undergoes transition from a disordered to ordered state and assumes a conformation suitable for catalysis only upon PLP binding. A novel disulfide was found to occur near a channel that is likely to regulate entry of ligands to the active site. EcDAPAL soaked with dl-DAP revealed density at the active site appropriate for the reaction intermediate aminoacrylate, which is consistent with the observation that EcDAPAL has low activity under crystallization conditions. Based on the analysis of the structure and results of site-directed mutagenesis, a two-base mechanism of catalysis involving Asp¹²⁰ and Lys⁷⁷ is suggested.