Crystal structure analysis of the translation factor RF3 (release factor 3).

The bacterial translational GTPases release factor RF3 promotes translation termination by recycling RF1 or RF2. Here, we present the crystal structures of RF3 complexed with GDP and guanosine 3',5'-(bis)diphosphate (ppGpp) at resolutions of 1.8 and 3.0Å, respectively. ppGpp is involved in the so-called "stringent response" of bacteria. ppGpp binds at the same site as GDP, suggesting that GDP and ppGpp are two alternative physiologically relevant ligands of RF3. We also found that ppGpp decelerates the recycling of RF1 by RF3. These lines of evidence suggest that RF3 functions both as a cellular metabolic sensor and as a regulator.