The growth phase-dependent change in sucrose density gradient centrifugation patterns of ribosomes was analyzed for both laboratory strains of Escherichia coli and natural isolates from the ECOR collection. All of the natural isolates examined formed 100S ribosome dimers in the stationary phase, and ribosome modulation factor (RMF) was associated with the ribosome dimers in the ECOR strains as in the laboratory strain W3110. The ribosome profile (70S monomers versus 100S dimers) follows a defined pattern over time during lengthy culture in both the laboratory strains and natural isolates. There are four discrete stages: (i) formation of 100S dimers in the early stationary phase; (ii) transient decrease in the dimer level; (iii) return of dimers to the maximum level; and (iv) dissociation of 100S dimers into 70S ribosomes, which are quickly degraded into subassemblies. The total time for this cycle of ribosome profile change, however, varied from strain to strain, resulting in apparent differences in the ribosome profiles when observed at a fixed time point. A correlation was noted in all strains between the decay of 100S ribosomes and the subsequent loss of cell viability. Two types of E. coli mutants defective in ribosome dimerization were identified, both of which were unable to survive for a prolonged period in stationary phase. The W3110 mutant, with a disrupted rmf gene, has a defect in ribosome dimerization because of lack of RMF, while strain Q13 is unable to form ribosome dimers due to a ribosomal defect in binding RMF.During the growth transition of bacteria from exponential to stationary phase, the expression of growth-related genes is mostly turned off and instead a set of genes required for stationary-phase survival is switched on (19,23,24). For this drastic change in gene expression pattern, structural and functional modulations take place on both transcriptional and translational apparatuses (23,24). Previously, we identified a 100S form of ribosome dimers in stationary-phase Escherichia coli cells and the association of a small basic protein of 55 amino acid residues, ribosome modulation factor (RMF), with those ribosome dimers (47). RMF is one of the stationaryphase-specific gene products (47, 51), and the rmf mutant strain loses cell viability in stationary-phase culture more rapidly than the wild-type strain (51). The addition of RMF to 70S ribosomes promotes their conversion in vitro into 100S dimers and inhibits translation in vitro (46), supporting the idea that the 100S dimers are a storage form for ribosomes in stationary phase (46,51).In order to explore this hypothesis, we have examined the relationship between the formation of ribosome dimers and the fate of E. coli cells. Up to the present time, data concerning stationary-phase adaptability and its variation among E. coli strains have been obtained using a small number of E. coli laboratory strains. It is noted, however, that the laboratory strains often carry mutations in the genes which are expressed only in the stationary phase, pr...
