Structure of the Bacillus subtilis hibernating 100S ribosome reveals the basis for 70S dimerization

Under stress conditions, such as nutrient deprivation, bacteria enter into a hibernation stage, which is characterized by the appearance of 100S ribosomal particles. In Escherichia coli , dimerization of 70S ribosomes into 100S requires the action of the ribosome modulation factor ( RMF ) and the hibernation‐promoting factor ( HPF ). Most other bacteria lack RMF and instead contain a long form HPF ( LHPF ), which is necessary and sufficient for 100S formation. While some structural information exists as to how RMF and HPF mediate formation of E. coli 100S ( Ec 100S), structural insight into 100S formation by LHPF has so far been lacking. Here we present a cryo‐ EM structure of the Bacillus subtilis hibernating 100S ( Bs 100S), revealing that the C‐terminal domain ( CTD ) of the LHPF occupies a site on the 30S platform distinct from RMF . Moreover, unlike RMF , the Bs HPF ‐ CTD is directly involved in forming the dimer interface, thereby illustrating the divergent mechanisms by which 100S formation is mediated in the majority of bacteria that contain LHPF , compared to some γ‐proteobacteria, such as E. coli .