Cryo- EM structure of the mycobacterial 70S ribosome in complex with ribosome hibernation promotion factor RafH.

Abstract

Ribosome hibernation is a key survival strategy bacteria adopt under environmental stress, where a protein, hibernation promotion factor (HPF), transitorily inactivates the ribosome. Mycobacterium tuberculosis encounters hypoxia (low oxygen) as a major stress in the host macrophages, and upregulates the expression of RafH protein, which is crucial for its survival. The RafH, a dual domain HPF, an orthologue of bacterial long HPF (HPF^long), hibernates ribosome in 70S monosome form, whereas in other bacteria, the HPF^long induces 70S ribosome dimerization and hibernates its ribosome in 100S disome form. Here, we report the cryo- EM structure of M. smegmatis, a close homolog of M. tuberculosis, 70S ribosome in complex with the RafH factor at an overall 2.8 Å resolution. The N- terminus domain (NTD) of RafH binds to the decoding center, similarly to HPF^long NTD. In contrast, the C- terminus domain (CTD) of RafH, which is larger than the HPF^long CTD, binds to a distinct site at the platform binding center of the ribosomal small subunit. The two domain-connecting linker regions, which remain mostly disordered in earlier reported HPF^long structures, interact mainly with the anti-Shine Dalgarno sequence of the 16S rRNA.