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The 100S ribosome: ribosomal hibernation induced by stress

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One of the most important cellular events in all organisms is protein synthesis (translation), which is catalyzed by ribosomes. The regulation of translational activity is dependent on the environmental situation of the cell. A decrease in overall translation under stress conditions is mainly accompanied by the formation of functionally inactive 100S ribosomes in bacteria. The 100S ribosome is a dimer of two 70S ribosomes that is formed through interactions between their 30S subunits. Two mechanisms of 100S ribosome formation are known: one involving ribosome modulation factor (RMF) and short hibernation promoting factor (HPF) in a part of Gammaproteobacteria including Escherichia coli, and the other involving only long HPF in the majority of bacteria. The expression of RMF is regulated by ppGpp and cyclic AMP‐cAMP receptor protein (cAMP‐CRP) induced by amino acid starvation and glucose depletion, respectively. When stress conditions are removed, the 100S ribosome immediately dissociates into the active 70S ribosomes by releasing RMF. The stage in the ribosome cycle at which the ribosome loses translational activity is referred to as ‘Hibernation’. The lifetime of cells that cannot form 100S ribosomes by deletion of the rmf gene is shorter than that of parental cells under stress conditions in E. coli. This fact indicates that the interconversion system between active 70S ribosomes and inactive 100S ribosomes is an important survival strategy for bacteria. WIREs RNA 2014, 5:723–732. doi: 10.1002/wrna.1242 This article is categorized under: Translation > Ribosome Structure/Function
(a) The ribosome cycle including the hibernation stage. The ribosomes proceed in a step‐by‐step manner in the canonical ribosome cycle ([1], [2], [3], [4], [5], [6]) for protein synthesis during the exponential growth phase. During the stationary phase, 100S ribosomes are formed by ribosome modulation factor (RMF) ([14]) and short hibernation promoting factor (HPF) ([15]) in Escherichia coli. In the majority of bacteria, except for a part of Gammaproteobacteria including E. coli, 100S ribosomes are only formed by long HPF ([19] in Figure (a)). (b) The expression pathways of factors relating to formation of 100S the ribosome in E. coli. RMF is expressed by the regulations of ppGpp and cyclic AMP‐cAMP receptor protein (cAMP‐CRP) induced by several stresses ([7], [9], [10], [11]). It is reported that the expressions of YfiA and short HPF are under controls of cAMP‐CRP ([12]) and autoinducer 2 (AI‐2; [13]), respectively.
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The distribution of ribosome modulation factor (RMF), short hibernation promoting factor (HPF), long HPF, and YfiA genes in bacteria. Although it is expected that bacteria having either rmf or long hpf are able to form the 100S ribosome, it has not been confirmed in all bacteria yet.
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(a) Structure of the 100S ribosome. The 50S subunits are colored cyan and pink, and the 30S subunits are colored yellow and green. The twofold symmetry axis is indicated in red in the right panel. (b) Contact point between the 30S subunits in the 100S ribosome. The slab of the part indicated by the box is viewed in stereo. The S2, S3, S4, and S5 proteins are shown by the orange, pink, purple, and cyan models, respectively. Domains and proteins are labeled as CP (central protuberance), St (L7/L12 stalk), hd (head), bk (beak), sp (spur), pt (platform), and L1 (L1 protein).
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Ribosome profiles of Escherichia coli. Cells were harvested 3 h, 5 h, 9 h, and 24 h after the inoculation. Cell extracts were subjected to 5 to 20% sucrose gradient centrifugation at 40,000 rpm [SW41Ti rotor (Beckman)] for 1.5 h at 4°C. After centrifugation, the absorbance of the sucrose gradient was measured at 260 nm.
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