Structural basis for mRNA recognition by elongation factor SelB

In bacteria, incorporation of selenocysteine, the 21(st) amino acid, into proteins requires elongation factor SelB, which has the unusual property of binding to both transfer RNA (tRNA) and mRNA. SelB binds to an mRNA hairpin formed by the selenocysteine insertion sequence (SECIS) with extremely high specificity, the molecular basis of which has been unknown. We have determined the crystal structure of the mRNA-binding domain of SelB in complex with SECIS RNA at a resolution of 2.3 A. This is the first example of a complex between an RNA and a winged-helix (WH) domain, a motif found in many DNA-binding proteins and recently discovered in RNA-binding proteins. Notably, RNA binding does not induce a major conformational change in the WH motif. The structure reveals a new mode of RNA recognition with a geometry that allows the complex to wrap around the small ribosomal subunit.

Covalent immobilization of cyclodextrin glucosyltransferase (CGTase) in activated silica and Sepharose

Cyclodextrin glucanotransferase is a non-Leloir glycosyltransferase that directly employs the free energy of cleavage of starch to produce cyclodextrins. In presence of appropriate acceptors, this enzyme synthesizes oligosaccharides containing alpha(1-->4) bonds. We have investigated the covalent immobilization of CGTase onto different activated supports. Silica was aminated and further activated with glutaraldehyde. The maximum amount of bound protein was about 4 mg CGTase per gram of support; however, the catalytic efficiency of the immobilized enzyme was lower than 6%. Sepharose 4B activated with cyanogen bromide (CNBr-activated Sepharose) and Sepharose 4B with a spacer arm of 1,6-diaminohexane (EAH Sepharose) were also assayed. These gels react with the amino and carboxylic groups of CGTase, respectively. With CNBr-activated Sepharose, a low percentage of enzyme was bound to the support but with a significant catalytic efficiency (29%). A higher recovery of protein was obtained with EAH Sepharose (62%), but only 2.4% of the initial activity was present in the immobilized biocatalyst. The results were discussed in terms of CGTase structure and mechanism. In addition, the solvent accessibility of amino or carboxylic groups, calculated using the NACCESS software, was considered.