The ribosome is a fundamental molecular machine for protein synthesis in all organisms. It consists of a small and large subunit that are both large complexes assembled by ribosomal RNAs and plenty of proteins. In eukaryotic cells, ribosomes are assembled through a highly complicated and dynamic process, in which two ribosomal subunits associate with a large number of assembly factors and form a series of pre-ribosomal particles. The small ribosomal subunit forms two major intermediates during maturation: the early nucleolar 90S pre-ribosome and late cytoplasmic pre-40S ribosome. Their structures have been recently determined and show great differences. How the early intermediate transforms into the late intermediate remains unknown.

The YE Keqiong group at Institute of Biophysics, Chinese Academy of Sciences determined a series of structures for small subunit pre-ribosomes in the transition from the early to late intermediates by the single particle cryo-electron microscopy technique, revealing for the first time the dynamic transition at the structural level. These structural snapshots show that the 5' external transcribed spacer of rRNA precursor is degraded within 90S pre-ribosomes, driving stepwise dissociation of assembly factors and ribosome maturation. The investigators also observed the binding of the exosome, a ribonuclease complex degrading the spacer, at pre-ribosomes. This study sheds light on the fundamental process of ribosome assembly.

The paper "Cryo-EM structure of 90S small ribosomal subunit precursors in transition states" was published in Science on September 18, 2020. Dr. YE Keqiong is the corresponding author and Dr. DU Yifei and AN Weidong are the co-first authors. The Center of Biological Imaging at IBP provided cryo-EM facilities and technical support for the study. The research was funded by National Key R&D Program of China, Strategic Priority Research Program of Chinese Academy of Sciences and National Natural Science Foundation of China.

Figure. Dynamic steps in maturation of yeast small ribosomal subunit

(Image by Dr. YE Keqiong's group)

The web link for this research paper: https://science.sciencemag.org/content/369/6510/1477

Contact: YE Keqiong

Institute of Biophysics, Chinese Academy of Sciences

Beijing 100101, China

Email: yekeqiong@ibp.ac.cn

(Reported by Dr. YE Keqiong's group)