Conservation of two distinct types of 100S ribosome in bacteria

In bacteria, 70S ribosomes (consisting of 30S and 50S subunits) dimerize to form 100S ribosomes, which were first discovered in Escherichia coli. Ribosome modulation factor (RMF) and hibernation promoting factor (HPF) mediate this dimerization in stationary phase. The 100S ribosome is translationally inactive, but it dissociates into two translationally active 70S ribosomes after transfer from starvation to fresh medium. Therefore, the 100S ribosome is called the hibernating ribosome'. The gene encoding RMF is found widely throughout the Gammaproteobacteria class, but is not present in any other bacteria. In this study, 100S ribosome formation in six species of Gammaproteobacteria and eight species belonging to other bacterial classes was compared. There were several marked differences between the two groups: (i) Formation of 100S ribosomes was mediated by RMF and short HPF in Gammaproteobacteria species, similar to E.coli, whereas it was mediated only by long HPF in the other bacterial species; (ii) RMF/short HPF-mediated 100S ribosome formation occurred specifically in stationary phase, whereas long HPF-mediated 100S ribosome formation occurred in all growth phases; and (iii) 100S ribosomes formed by long HPF were much more stable than those formed by RMF and short HPF.