In all domains of life, selenocysteine (Sec) is delivered to the ribosome by selenocysteine-specific tRNA (tRNA Sec ) with the help of a specialized translation factor, SelB in bacteria. Sec-tRNA Sec recodes a UGA stop codon next to a downstream mRNA stem-loop. Here we present the structures of six intermediates on the pathway of UGA recoding in Escherichia coli by single-particle cryo-electron microscopy. The structures explain the specificity of Sec-tRNA Sec binding by SelB and show large-scale rearrangements of Sec-tRNA Sec . Upon initial binding of SelB-Sec-tRNA Sec to the ribosome and codon reading, the 30S subunit adopts an open conformation with Sec-tRNA Sec covering the sarcin-ricin loop (SRL) on the 50S subunit. Subsequent codon recognition results in a local closure of the decoding site, which moves Sec-tRNA Sec away from the SRL and triggers a global closure of the 30S subunit shoulder domain. As a consequence, SelB docks on the SRL, activating the GTPase of SelB. These results reveal how codon recognition triggers GTPase activation in translational GTPases.
Organizational Affiliation:
Department of Structural Dynamics, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany.
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