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Ribosomal proteins S10 and S2 were each fused with GFP to track the fates of these proteins in the stationary growth phase and the following decay period inEscherichia coli. The fused proteins localized mainly in the cytoplasm, and their amounts were proportional to the colony-forming unit. S10-GFP strains that lacked genes responsible for regulating 100S ribosomes and S2-GFP strain that was unable to form 100S both showed shortened stationary phases. This result indicates that these strains exhibit earlier death in the absence of 100S formation (S2-GFP, S10-GFPΔrmfand S10-GFPΔhpf) and breakdown (S10-GFPΔyfiA). Therefore, in addition to the mere presence of 100S, the correct timing of 100S formation and breakdown is required to maintain viability. We propose a model in which 100S acts as a tentative repository of ribosomes that are protected from degradation and provide a source of amino acids in later growth period.Microscopic images of ribosomes with chromosomal rpsJ fused with B-maggio gfp at 6 h (top) and at 60 h (bottom).