2B-05 Two mechanisms for 100S ribosome formation in eubacteria

In the stationary growth phase of Escherichia coli, 70S ribosomes are dimerized by RMF (ribosome modulation factor) and HPF (hibernation promoting factor) to form 100S ribosomes. The 100S ribosomes have no translational activity. We named the process of 100S formation "ribosomal hibernation" (1). Other gamma proteobacteria that engage in 100S formation were also found to possess RMF and HPF homologs. In this study, we found long HPF homologs but no RMF homolog in Staphylococcus aureus (2), five Lactobacillaceae and Thermus thermophilus which engage in 100S formation. The long HPF homologs are approximately two-fold longer than E.coli HPF and are widely distributed in almost all eubacteria except for gamma proteobacteria. The results suggest that eubacteria have two mechanisms for 100S ribosome formation; one involving RMF and HPF, and the other involving only a long form of HPF. Furthermore, electron cryomicroscopy showed that the structure of S.aureus 100S is different from that of E.coli 100S in its 30S-30S binding mode. (1) Genes Cells 3: 203-208 (1998) (2) Genes Cells 15: 43-58 (2010)