Assembly stoichiometry of bacterial selenocysteine synthase and SelC (tRNAsec)

Abstract

In bacteria selenocysteyl-tRNAsec (SelC) is synthesized by selenocysteine synthase (SelA). Here we show by fluorescence anisotropy binding assays and electron microscopical symmetry analysis that the SelA-tRNAsec binding stoichiometry is of one tRNAsec 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-tRNAsec assembly both with and without tRNAsec bound. Furthermore, SelA can associate forming a supramolecular complex of stacked decamer rings, which does not occur in the presence of tRNAsec. We discuss the structure-function relationships of these assemblies and their regulatory role in bacterial selenocysteyl-tRNAsec synthesis.