SelB is an elongation factor needed for the co-translational incorporation of selenocysteine. Selenocysteine is coded by a UGA stop codon in combination with a specific downstream mRNA hairpin. In bacteria, the C-terminal part of SelB recognizes this hairpin, while the N-terminal part binds GTP and tRNA in analogy with elongation factor Tu (EF-Tu). We present the crystal structure of a C-terminal fragment of SelB (SelB-C) from Moorella thermoacetica at 2.12 A

Arsenate reductase (AR) in B. subtilis is encoded by the chromosomal arsC gene. Together with arsB and arsR, arsC participates in detoxification processes for the arsenate and arsenite ions. Full-length arsenate reductase without any modification has been expressed in Escherichia coli and puri

Arsenate is an abundant oxyanion that, because of its ability to mimic the phosphate group, is toxic to cells. Arsenate reductase (EC 1.97.1.5; encoded by the arsC gene in bacteria) participates to achieve arsenate resistance in both prokaryotes and yeast by reducing arsenate to arsenite; the arsenite is then exported by a specific transporter. The crystal structure of Bacillus subtilis arsenate reductase in the reduced form with a bound sulfate ion in its active site is solved at 1.6-

The R2 protein of ribonucleotide reductase features a di-iron site deeply buried in the protein interior. The apo form of the R2 protein has an unusual clustering of carboxylate side chains at the empty metal-binding site. In a previous study, it was found that the loss of the four positive charge equivalents of the diferrous site in the apo protein appeared to be compensated for by the protonation of two histidine and two carboxylate side chains. We have studied the consequences of removing and introducing charged residues on the local hydrogen-bonding pattern in the region of the carboxylate cluster of Corynebacterium ammoniagenes and Escherichia coli protein R2 using site-directed mutagenesis and X-ray crystallography. The structures of the metal-free forms of wild-type C. ammoniagenes R2 and the mutant E. coli proteins D84N, S114D, E115A, H118A, and E238A have been determined and their hydrogen bonding and protonation states have been structurally assigned as far as possible. Significant alterations to the hydrogenbonding patterns, protonation states, and hydration is observed for all mutant E. coli apo proteins as compared to wild-type apo R2. Further structural variations are revealed by the wild-type apo C. ammoniagenes R2 structure. The protonation and hydration effects seen in the carboxylate cluster appear to be due to two major factors: conservation of the overall charge of the site and the requirement of electrostatic shielding of clustered carboxylate residues. Very short hydrogen-bonding distances between some protonated carboxylate pairs are indicative of low-barrier hydrogen bonding.

A novel calcium-independent serine protease from an alkaliphilic bacterium, Nesterenkonia sp. AL20, has been purified and crystallized at 296 Kusing sodium formate as the main precipitant. This enzyme is optimally active at pH 10, exhibits high stability towards autolytic digestion and its stability is not affected by the presence of EDTA or detergents. The triangular prism-shaped crystals diffracted X-rays to beyond 1.5 A