2013/07/29

Sensing low amino acids levels on the ribosome explored by cryoEM

In some bacteria the scarcity of amino acids is sensed on the ribosome, where RelA, the stringent factor, is activated by the presence of cognate and deacylated tRNA. Amino acid starvation and low levels of charged tRNAs stall translation, and this is the event that RelA detects while surveys the cellular population of ribosomes. After activation, RelA synthesizes the alarmone (p)ppGpp to modulate a wide range of genes via transcription regulation. Thus, the ribosome is used as a metabolic sensing platform.

To date, no structural framework has been established to understand how the metabolic sensor RelA exerts its action. Now, a research group from the Structural Biology Unit of CIC bioGUNE, in collaboration with Venki Ramakrishnan's (LMB, MRC, Cambridge) group, reports on this metabolic sensing pathway by solving the cryo-EM structure of RelA bound to stalled ribosomes bearing cognate, deacylated tRNA in the A-site. The data reveal that RelA on the ribosome stabilizes an unusual distorted form of the cognate, deacylated tRNA, with the 3' end making contact with RelA and far from its normal location in the peptidyl transferase center. This conformation, stabilized in a pseudo A/T state that evocates the one induced by EF-Tu with acylated-tRNAs during the delivery of cognate tRNAs in a standard elongation step, serves as the scaffold to nucleate the catalytic 3D conformation of the stringent factor RelA.

The findings have been published on EMBO Reports
http://www.nature.com/embor/journal/vaop/ncurrent/abs/embor2013106a.html

Caption

A) Three-dimensional cryo-EM structure of the 70S-RelA complex. Regions corresponding to the 30S subunit (yellow), the 50S subunit (blue) and tRNAs in the A (magenta), P (green) and E (orange) sites are displayed in transparent. RelA is shown in red. The atomic structures fitted to the density are shown as ribbon diagrams.

B) Cartoon-like schematic representation of RelA activation.