M-ammonium Sulphate Research Articles

HicAB toxin-antitoxin complex from Escherichia coli: expression and crystallization.

Toxin-antitoxin (TA) systems are widespread in both bacteria and archaea, where they enable cells to adapt to environmental cues. TA systems play crucial roles in various cellular processes, such as programmed cell death, cell growth, persistence and virulence. Here, two distinct forms of the type II toxin-antitoxin complex HicAB were identified and characterized in Escherichia coli K-12, and both were successfully overexpressed and purified. The two proposed forms, HicABL and HicABS, differed in the presence or absence of a seven-amino-acid segment at the N-terminus in the antitoxin HicB. The short form HicABS readily crystallized under the conditions 0.1 M Tris-HCl pH 8.0, 20%(w/v) PEG 6000, 0.2 M ammonium sulfate. The HicABS crystal diffracted and data were collected to 2.5 Å resolution. The crystal belonged to space group I222 or I212121, with unit-cell parameters a = 67.04, b = 66.31, c = 120.78 Å. Matthews coefficient calculation suggested the presence of two molecules each of HicA and HicBS in the asymmetric unit, with a solvent content of 55.28% and a Matthews coefficient (VM) of 2.75 Å3 Da-1.

Crystallization and X-ray analysis of the extracellular adhesion domain of Helicobacter pylori adhesin A: the significance of the cation composition in the crystallization precipitant.

Adherence to host cells is a crucial step in the process of bacterial infection, which is usually mediated by a number of outer membrane proteins identified as adhesins. Helicobacter pylori adhesin A (HpaA) is a member of the adhesin family that mediates the adherence of Helicobacter pylori to gastric epithelial cells, and consequently assists the bacteria in becoming a life-long colonizer of the human stomach. In this study, two constructs were made for the production of truncated HpaA proteins comprising residues 31-260 and 53-260, respectively. The products of both constructs were crystallized, but only the protein from the shorter construct (residues 53-260) formed crystals that were capable of diffraction. In the subsequent optimization trials, crystals in different forms were unexpectedly obtained by using lithium sulfate and ammonium sulfate as the precipitant. An X-ray data set was collected to 1.95 Å resolution on beamline BL18U1 at SSRF using a crystal grown with 1.92 M lithium sulfate, which belonged to space group P65 with unit-cell parameters a = b = 95.42, c=54.72 Å, γ = 120°, while another crystal grown with 1.9 M ammonium sulfate diffracted to 2.60 Å resolution and the collected data set was indexed in space group P21212, with unit-cell parameters a = 121.01, b = 190.56, c = 106.31 Å. The collection of diffraction data has established a solid basis for structure determination.

Crystallographic analysis of the Arabidopsis thaliana BAG5-calmodulin protein complex.

Arabidopsis thaliana BAG5 (AtBAG5) belongs to the plant BAG (Bcl-2-associated athanogene) family that performs diverse functions ranging from growth and development to abiotic stress and senescence. BAG family members can act as nucleotide-exchange factors for heat-shock protein 70 (Hsp70) through binding of their evolutionarily conserved BAG domains to the Hsp70 ATPase domain, and thus may be involved in the regulation of chaperone-mediated protein folding in plants. AtBAG5 is distinguished from other family members by the presence of a unique IQ motif adjacent to the BAG domain; this motif is specific for calmodulin (CaM) binding, indicating a potential role in the plant calcium signalling pathway. To provide a better understanding of the IQ motif-mediated interaction between AtBAG5 and CaM, the two proteins were expressed and purified separately and then co-crystallized together. Diffraction-quality crystals of the complex were grown using the sitting-drop vapour-diffusion technique from a condition consisting of 0.1 M Tris-HCl pH 8.5, 2.5 M ammonium sulfate. The crystals belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 64.56, b = 74.89, c = 117.09 Å. X-ray diffraction data were recorded to a resolution of 2.5 Å from a single crystal using synchrotron radiation. Assuming the presence of two molecules in the asymmetric unit, a Matthews coefficient of 2.44 Å(3) Da(-1) was calculated, corresponding to a solvent content of approximately 50%.

Purification, crystallization and preliminary X-ray analysis of the periplasmic haem-binding protein HutB from Vibrio cholerae.

The mechanism of haem transport across the inner membrane of pathogenic bacteria is currently insufficiently understood at the molecular level and no information is available for this process in Vibrio cholerae. To obtain structural insights into the periplasmic haem-binding protein HutB from V. cholerae (VcHutB), which is involved in haem transport through the HutBCD haem-transport system, at the atomic level, VcHutB was cloned, overexpressed and crystallized using 1.6 M ammonium sulfate as a precipitant at pH 7.0. X-ray diffraction data were collected to 2.4 Å resolution on the RRCAT PX-BL-21 beamline at the Indus-2 synchrotron, Indore, India. The crystals belonged to space group P4₃2₁2, with unit-cell parameters a = b = 62.88, c = 135.8 Å. Matthews coefficient calculations indicated the presence of one monomer in the asymmetric unit, with an approximate solvent content of 45.02%. Molecular-replacement calculations with Phaser confirmed the presence of a monomer in the asymmetric unit.

Crystallization and preliminary X-ray crystallographic analysis of a putative acetylxylan esterase from Talaromyces cellulolyticus.

Acetylxylan esterase (AXE) catalyzes the hydrolytic cleavage of the ester bond between acetic acid and hemicellulose in plant cell walls. A putative AXE gene exhibiting high homology to carbohydrate esterase family 3 was found in the genome database of the fungus Talaromyces cellulolyticus (formerly known as Acremonium cellulolyticus). A truncated form of the protein, the catalytic domain of the enzyme, was prepared and crystallized. The best crystal was obtained at 293 K using 0.17 M ammonium sulfate, 28% PEG 4000, 5%(v/v) glycerol, 0.5%(w/v) n-octyl-β-D-glucoside. X-ray diffraction data were collected to 1.50 Å resolution. The crystal belonged to space group P41212 or P43212, with unit-cell parameters a = 70.90, b = 70.90, c = 87.09 Å. One enzyme molecule per asymmetric unit gave a crystal volume per protein mass (VM) of 2.62 Å(3) Da(-1) and a solvent content of 53.0%(v/v).

Expression, purification, crystallization and preliminary X-ray analysis of glucose-1-phosphate uridylyltransferase (GalU) from Erwinia amylovora.

Glucose-1-phosphate uridylyltransferase from Erwinia amylovora CFPB1430 was expressed as a His-tag fusion protein in Escherichia coli. After tag removal, the purified protein was crystallized from 100 mM Tris pH 8.5, 2 M ammonium sulfate, 5% ethylene glycol. Diffraction data sets were collected to a maximum resolution of 2.46 Å using synchrotron radiation. The crystals belonged to the hexagonal space group P62, with unit-cell parameters a = 80.67, b = 80.67, c=169.18. The structure was solved by molecular replacement using the structure of the E. coli enzyme as a search model.

Cloning, expression, purification, crystallization and X-ray crystallographic analysis of the (S)-3-hydroxybutyryl-CoA dehydrogenase PaaH1 from Ralstonia eutropha H16.

The (S)-3-hydroxybutyryl-CoA dehydrogenase PaaH1 from Ralstonia eutropha (RePaaH1) is an enzyme used in the biosynthesis of n-butanol from acetyl-CoA by the reduction of acetoacetyl-CoA to (S)-3-hydroxybutyryl-CoA. The RePaaH1 protein was crystallized using the hanging-drop vapour-diffusion method in the presence of 1.4 M ammonium sulfate, 0.1 M sodium cacodylate pH 6.0, 0.2 M sodium chloride at 295 K. X-ray diffraction data were collected to a maximum resolution of 2.6 Å on a synchrotron beamline. The crystal belonged to space group P3221, with unit-cell parameters a=b=135.4, c=97.2 Å. With three molecules per asymmetric unit, the crystal volume per unit protein weight (VM) is 2.68 Å3 Da(-1), which corresponds to a solvent content of approximately 54.1%. The structure was solved by the single-wavelength anomalous dispersion method and refinement of the structure is in progress.

Purification, crystallization and preliminary X-ray diffraction analysis of effector protein MoHrip2 fromMagnaporthe oryzae

MoHrip2, a novel effector protein from the pathogenic fungus Magnaporthe oryzae, was purified and crystallized using the sitting-drop vapour-diffusion method. Native crystals and selenomethionine-labelled crystals were obtained using 2.2 M ammonium sulfate as a precipitant. A native data set was collected to 2.0 Å resolution at 100 K using an in-house X-ray source and a selenomethionine-labelled data set containing anomalous signal was collected to 1.8 Å resolution at 100 K using a synchrotron source. Based on the anomalous signal generated from the Se atom, the MoHrip2 structure was successfully solved using the single-wavelength anomalous dispersion (SAD) method.

A new crystal form of MshB fromMycobacterium tuberculosiswith glycerol and acetate in the active site suggests the catalytic mechanism

MshB, a zinc-based deacetylase, catalyses a step in the mycothiol biosynthetic pathway that involves the deacetylation of 1-O-(2-acetamido-2-deoxy-α-D-glucopyranosyl)-D-myo-inositol (GlcNAc-Ins), via cleavage of an amide bond, to 1-O-(2-amino-2-deoxy-α-D-glucopyranosyl)-D-myo-inositol (GlcN-Ins) and acetate. In this study, MshB was expressed, purified and crystallized. A new crystal form was encountered in 0.1 M sodium acetate, 0.2 M ammonium sulfate, 25% PEG 4000 pH 4.6. The crystals diffracted to 1.95 Å resolution and the resulting electron-density map revealed glycerol and the reaction product, acetate, in the active site. These ligands enabled the natural substrate GlcNAc-Ins to be modelled in the active site with some certainty. One acetate O atom is hydrogen bonded to Tyr142 and is located 2.5 Å from the catalytic zinc. The other acetate O atom is located 2.7 Å from a carboxylate O atom of Asp15. This configuration strongly suggests that Asp15 acts both as a general base catalyst in the nucleophilic attack of water on the amide carbonyl C atom and in its protonated form acts as a general acid to protonate the amide N atom. The configuration of Tyr142 differs from that observed previously in crystal structures of MshB (PDB entries 1q74 and 1q7t) and its location provides direct structural support for recently published biochemical and mutational studies suggesting that this residue is involved in a conformational change on substrate binding and contributes to the oxyanion hole that stabilizes the tetrahedral intermediate.

Interaction of the Bacteriophage T4 Gene 59 Helicase Loading Protein and Gene 41 Helicase with Each Other and with Fork, Flap, and Cruciform DNA

Interaction of the Bacteriophage T4 Gene 59 Helicase Loading Protein and Gene 41 Helicase with Each Other and with Fork, Flap, and Cruciform DNA

Solution and Crystal Structures of a Sperm Whale Myoglobin Triple Mutant That Mimics the Sulfide-binding Hemoglobin fromLucina pectinata

The bivalve mollusc Lucina pectinata harbors sulfide-oxidizing chemoautotrophic bacteria and expresses a monomeric hemoglobin I, HbI, with normal O2, but extraordinarily high sulfide affinity. The crystal structure of aquomet Lucina HbI has revealed an active site with three residues not commonly found in vertebrate globins: Phe(B10), Gln(E7), and Phe(E11) (Rizzi, M., Wittenberg, J. B., Coda, A., Fasano, M., Ascenzi, P., and Bolognesi, M. (1994) J. Mol. Biol. 244, 86-89). Engineering these three residues into sperm whale myoglobin results in a triple mutant with approximately 700-fold higher sulfide affinity than for wild-type. The single crystal x-ray structure of the aquomet derivative of the myoglobin triple mutant and the solution 1H NMR active site structures of the cyanomet derivatives of both the myoglobin mutant and Lucina HbI have been determined to examine further the structural origin of their unusually high sulfide affinities. The major differences in the distal pocket is that in the aquomet form the carbonyl of Gln64(E7) serves as a H-bond acceptor, whereas in the cyanomet form the amido group acts as H-bond donor to the bound ligand. Phe68(E11) is rotated approximately 90 degrees about chi2 and located approximately 1-2 A closer to the iron atom in the myoglobin triple mutant relative to its conformation in Lucina HbI. The change in orientation potentially eliminates the stabilizing interaction with sulfide and, together with the decrease in size of the distal pocket, accounts for the 7-fold lower sulfide affinity of the myoglobin mutant compared with that of Lucina HbI.

Crystallization, structure determination and least-squares refinement to 1.75 Å resolution of the fatty-acid-binding protein isolated from Manduca sexta L

The molecular structure of an insect fatty-acid-binding protein isolated from Manduca sexta L. has been determined and refined to a nominal resolution of 1.75 Å. Crystals used in the investigation were grown from 1.6 m-ammonium sulfate solutions buffered at pH 4.5 with 50 m m-sodium succinate, and belonged to space group P2 1 with unit cell dimensions of a = 27.5 A ̊ , b = 71.0 A ̊ , c = 28.7 A ̊ and β = 90.8 ° . An electron density map, phased with four heavy-atom derivatives and calculated to 2.5 Å resolution, allowed for complete tracing of the 131 amino acid residue polypeptide chain. Subsequent least-squares refinement of the model reduced the R-factor from 46.0% to 17.3% using all measured X-ray data from 30.0 Å to 1.75 Å. Approximately 92% of the amino acid residues fall into classical secondary structural elements including ten strands of anti-parallel β-pleated sheet, two α-helices, one type I turn, three type II turns, four type II′ turns and one type III turn. As in other fatty-acid-binding proteins, the overall molecular architecture of the insect molecule consists of ten strands of anti-parallel β-pleated sheet forming two layers that are nearly orthogonal to one another. A helix-turn-helix motif at the N-terminal portion of the protein flanks one side of the up-and-down β-barrel. The functional group of the fatty acid is within hydrogen-bonding distance of Gln39, Tyr129, Arg127 and a sulfate molecule, while the aliphatic portion of the ligand is surrounded by hydrophobic amino acid residues lining the β-barrel. The binding of the carboxylic acid portion of the ligand is very similar to that observed in P2 myelin protein and the murine adipocyte lipid-binding protein, but the positioning of the hydrocarbon tail after approximately C6 is completely different.

Crystallization and preliminary data for the ferric form of Lucina pectinata hemoglobin I

Cytoplasmic monomeric hemoglobin I from the bacteria-harboring gill of the bivalve mollusc Lucina pectinata has been crystallized in a form suitable for atomic resolution X-ray structural investigations. The crystals have been grown at pH 4.8, in 0.05 m-acetate buffer, using 2·6 m-ammonium sulfate as precipitating agent. The crystals belong to the monoclinic space group P2 1 with unit cell constants a = 500 A ̊ , b = 38.6 A ̊ , c = 42.1 A ̊ , β = 107.1∘ , and contain one molecule (14,000 M r) in the asymmetric unit. By means of single crystal microspectrophotometry it has been shown that the crystals contain the ferric form of L. pectinata “sulfide reactive” hemoglobin I. On the other hand, by careful control of the buffering medium composition, it has been possible to obtain stable crystals of the deoxy, oxy and sulfide forms of the protein.

Refined 1.83 Å structure of trypanosomal triosephosphate isomerase crystallized in the presence of 2.4 m-ammonium sulphate : A comparison with the structure of the trypanosomal triosephosphate isomerase-glycerol-3-phosphate complex

Triosephosphate isomerase (TIM) is a dimeric glycolytic enzyme. TIM from Trypanosoma brucei brucei has been crystallized at pH 7.0 in 2.4 m-ammonium sulphate. The well-diffracting crystals have one dimer per asymmetric unit. The structure has been refined at 1.83 Å resolution with an R-factor of 18.3% for all data between 6 Å and 1.83 Å (37,568 reflections). The model consists of 3778 protein atoms and 297 solvent atoms. Subunit 1 is involved in considerably more crystal contacts than subunit 2. Correlated with these differences in crystal packing is the observation that only in the active site of subunit 2 is a sulphate ion bound. Furthermore, significant differences with respect to structure and flexibility are observed in three loops near the active site. In particular, there is a 7 Å positional difference of the tip of the flexible loop (loop 6) when comparing subunit 1 and subunit 2. Also, the neighbouring loops (loop 5 and loop 7) have significantly different conformations and flexibility. In subunit 1, loop 6 is in an “open” conformation, in subunit 2, loop 6 is in an “almost closed” conformation. Only in the presence of a phosphate-containing ligand, such as glycerol-3-phosphate, does loop 6 take up the “closed” conformation. Loop 6 and loop 7 (and also to some extent loop 5) are rather flexible in the almost closed conformation, but well defined in the open and closed conformations. The closing of loop 6 (167 to 180), as observed in the almost closed conformation, slightly changes the main-chain conformation of the catalytic glutamate, Glu167, leading to a change of the χ 1 angle of this residue from ~−60 ° to ~60 ° and the weakening of the hydrogen bonds between its polar side-chain atoms and Ser96. In the closed conformation, in the presence of glycerol-3-phosphate, the main-chain atoms of Glu167 remain in the same position as in the almost closed conformation, but the side-chain has rotated around the CACB bond changing χ 1 from ∼60 ° to ∼−60 °. In this new position the hydrogen bonding to Ser96 is completely lost and also a water-mediated salt bridge between OE2(Glu167) and NE(Arg99) is lost. Comparison of the two independently refined subunits, showed that the root-mean-square deviation for all 249 CA atoms is 0.9 Å; for the CA atoms of the β-strands this is only 0.2 Å. The average B-factor for all subunit 1 and subunit 2 atoms is 20 Å 2 and 25 Å 2, respectively. This difference can be understood because subunit 1 is involved in many more crystal contacts than subunit 2. The correlation coefficient between the B-factor values of subunit 1 and subunit 2 is 0.7, despite the difference in flexibility of some loops. There are 279 water molecules; 25 water molecules are completely buried inside the protein. A subset of water molecules (approximately 100) consists of equivalent water molecules in subunit 1 and subunit 2: 50 water molecules contacting subunit 1 can be superimposed (within 0.5 Å) by the local dyad on water molecules near subunit 2. The interface residues in trypanosomal TIM are rather different from other TIMs. In addition, one of the interface loops, loop 3, is one residue shorter. In trypanosomal TIM there are two hydrophobic patches at the interface. In addition, there are polar interactions: salt bridges as well as non-charged hydrogen bonds. About 50% of the hydrogen bonds at the interface are water-mediated hydrogen bonds. At the interface there are 18 completely buried water molecules, nine of these water molecules are hydrogen-bonded to both subunits. These 18 buried water molecules are grouped in seven water clusters, consisting of one to five water molecules.

Comparison of the crystal structures of a flavodoxin in its three oxidation states at cryogenic temperatures

The focus of this study has been to determine the conformation of the holoprotein of recombinant flavodoxin from Desulfovibrio vulgaris with the FMN in each of its three oxidation states. The structure of the oxidized state of the wild-type flavodoxin at 2.0 Å from D. vulgaris was used as a starting model for refinement. Diffraction experiments were conducted at low temperature (−150 °C) in order to maintain the oxidation state of interest throughout the intensity data collection. Crystals of flavodoxin, in the oxidized state, form yellow bipyramids by the standard hanging-drop method from 3.2 m-ammonium sulfate in 0.1 m-Tris · HCl buffer at pH 7.0 with protein concentrations ranging from 0.7% to 0.9%. The reduced states of the crystals were achieved through the addition of sodium dithionite at pH 7.0 for the semiquinone (semi-reduced) and at pH 9.0 for the hydroquinone (fully reduced). Data sets consisting of one at room temperature (oxidized state) and three at low temperature (each oxidation state) were collected on a Nicolet P3F/Xentronics area detector X-ray diffractometer system. The four structures, hydroquinone at 2.25 Å resolution and all others at 1.9 Å resolution, were refined by the restrained parameter least-squares program PROLSQ. The final crystallographic R-values converged to 0.21 (hydroquinone), 0.20 (semiquinone), 0.20 (oxidized, low temperature), and 0.17 (oxidized, room temperature). The reduced states of flavodoxin show a different conformation of the protein polypeptide chain (Asp61-Gly62) in the vicinity of NH(5) of the isoalloxazine group relative to the oxidized state. However, there are only slight conformational differences between the semiquinone and hydroquinone states. In this report, structural comparisons of the three are made, with particular emphasis on the features that might be related to the difference in temperature of the diffraction data collections and differences in the oxidation state of the FMN.

Crystallization and preliminary X-ray diffraction studies of a deglycosylated glucose oxidase from Aspergillus niger

Deglycosylation was shown to be an important prerequisite step for the crystallization of glucose oxidase from Aspergillus niger. Whereas the glycosylated enzyme could not be crystallized, crystals of the deglycosylated enzyme suitable for X-ray diffraction analysis were reproducibly grown in the presence of 1.6 M-ammonium sulphate and octanetriol at pH 5.3 to 5.6. The crystals belong to the space group P3(1)21 or P3(2)21 with refined lattice constants of a = 66.5 A and c = 214.4 A, indicating a cell content of one monomer per asymmetric unit of the crystal. Crystals diffract to at least 2.5 A resolution. Cleavage of 95% of its carbohydrate moiety affected the kinetics of glucose oxidation, stability at low pH and some electrophoretic properties of glucose oxidase, such as molecular mass and the number of isoelectric forms. However, other properties, such as thermal stability, pH and temperature optima of activity were not affected.

Crystallization and preliminary crystallographic data for an antiprogesterone monoclonal antibody Fab′ and steroid-Fab′ complexes

Crystals of an Fab′ from an antiprogesterone monoclonal antibody (IgG1) have been grown from 1.5 m-ammonium sulfate, pH 5.5 to 8.5. The single crystals are hexagonal rods, space group P6 222 (or P6 422), with cell dimensions a = b = 135.2 A ̊ , and c = 124.2 A ̊ . Cocrystals of the Fab′ with progesterone, pregnanedione and other related steroids have been grown. The complex crystals have different morphology but are isomorphous with the native crystals. A hydroxy-progesterone derivative obtained by substituting an iodo-benzoyl group at the 11 α-hydroxyl position looks promising as a suitable heavy-atom candidate in addition to other potential conventional heavy-atom derivatives. All crystals diffract to at least 2.8 Å resolution and are suitable for high-resolution X-ray diffraction studies.

Association of RNA with the cytoskeleton and the nuclear matrix.

Heteronuclear RNA (hnRNA) is preferentially associated (76%) with the nuclear matrix in mammalian cells. Active mRNA, in the form of polyribosomes, is associated (greater than 97%) with the cytoskeletal framework. In this report, we present evidence that the association of both hnRNA and mRNA with structural networks of the cell may be essential features of gene expression. To study the association of polyribosomes with the cytoskeletal framework, cytochalasin D was used to release mRNA from the cytoskeletal framework. Protein synthesis was inhibited by cytochalasin D in direct proportion to the release of mRNA. The released mRNA is unaltered in its translatability as measured in vitro but is no longer translated in the cytochalasin-treated HeLa cells. The residual protein synthesis occurs on polyribosomes that are reduced in amount but display a normal sedimentation distribution. The results support the hypothesis that mRNA binding to the cytoskeletal framework is necessary, though not sufficient, for translation. Further fractionation of the cytoskeletal framework separates nuclear constituents into three distinct protein fractions. Chromatin proteins and 94% of the DNA are released by 0.25 M-ammonium sulphate after inter-nucleosomal DNA is cut with DNase I. The resulting structure retains 76% of the hnRNA in the form of ribonucleoprotein and is designated the RNP-containing nuclear matrix. The proteins of hnRNP complex are those associated with the nucleus only if RNA is intact. These proteins and 97% of the hnRNA are released after brief digestion with RNase A. Visualizing the nuclear matrix using resinless sections shows that nuclear RNA plays an important role in the organization of the nuclear matrix. Electron micrographs of resinless sections show the interior of the matrix to be a three-dimensional network of thick filaments bounded by the nuclear lamina. The filaments are densely covered with 20-30 nm electron-dense particles, which may contain the hnRNA. The RNP-depleted matrix is disordered and the interior fibres aggregated. These results suggest that hnRNA is involved in the spatial organization of the interior of the nuclear matrix.

Crystallization of phycoerythrocyanin from the cyanobacterium Mastigocladus laminosus and preliminary characterization of two crystal forms.

The light-harvesting protein phycoerythrocyanin from the cyanobacterium Mastigocladus laminosus Cohn has been crystallized in two different crystal forms by vapour diffusion. In 5% (w/v) polyethylene glycol at pH 8.5, hexagonal crystals of space group P63 with cell constants a = b = 158 A, c = 40.6 A were obtained, which turned out to be almost isomorphous with the hexagonal crystals of C-phycocyanin from the same organism. Consequently, the conformation of both phycobiliproteins must be very similar. From 1.5 M-ammonium sulfate (pH 8.5), orthorhombic crystals of space group P2221 with cell constants a = 60.5 A, b = 105 A, c = 188 A could be grown. Density measurements of these crystals indicate that the unit cell contains 18 (alpha beta)-units. A detailed packing scheme is proposed that is consistent with the observed pseudo-hexagonal X-ray intensity pattern and with the known size and shape of (alpha beta)3-trimers of C-phycocyanin. Accordingly, disc-like (alpha beta)3-trimers are associated face-to-face and stacked one upon another in rods with a period of 60.5 A, corresponding to the cell dimension a.

Preliminary X-ray diffraction studies on a ferredoxin from the thermophilic archaebacterium, Thermoplasma acidophilum

A ferredoxin from the thermophilic archaebacterium, Thermoplasma acidophilum, is supposed to contain two (4Fe-4S) active centers; one center could be linked by four cysteine residues to the protein and the other bonded with three cysteines and an unknown group. This ferredoxin has been crystallized by salting-out against 2.3 m-ammonium sulfate solution. The space group is P2 12 12 with cell dimensions of a = 59.20 A ̊ , b = 52.77 A ̊ and c = 41.28 A ̊ . Four molecules pack in the unit cell with V m = 2.03 A ̊ 3/dalton .