Assembly stoichiometry of bacterial selenocysteine synthase and SelC (tRNAsec).

In bacteria selenocysteyl-tRNA(sec) (SelC) is synthesized by selenocysteine synthase (SelA). Here we show by fluorescence anisotropy binding assays and electron microscopical symmetry analysis that the SelA-tRNA(sec) binding stoichiometry is of one tRNA(sec) molecule per SelA monomer (1:1) rather than the 1:2 value proposed previously. Negative stain transmission electron microscopy revealed a D5 pointgroup symmetry for the SelA-tRNA(sec) assembly both with and without tRNA(sec) bound. Furthermore, SelA can associate forming a supramolecular complex of stacked decamer rings, which does not occur in the presence of tRNA(sec). We discuss the structure-function relationships of these assemblies and their regulatory role in bacterial selenocysteyl-tRNA(sec) synthesis.