RMF inactivates ribosomes by covering the peptidyl transferase centre and entrance of peptide exit tunnel

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Abstract
In gram‐negative bacteria such as Escherichia coli, protein synthesis is suppressed by the formation of 100S ribosomes under stress conditions. The 100S ribosome, a dimer of 70S ribosomes, is formed by ribosome modulation factor (RMF) binding to the 70S ribosomes. During the stationary phase, most of the 70S ribosomes turn to 100S ribosomes, which have lost translational activity. This 100S formation is called the hibernation process in the ribosome cycle of the stationary phase. If stationary phase cells are transferred to fresh medium, the 100S ribosomes immediately go back to active 70S ribosomes, showing that inactive 100S ↔ active 70S interconversion is a major system regulating translation activity in stationary phase cells. To elucidate the mechanisms of translational inactivation, the binding sites of RMF on 23S rRNA in 100S ribosome of E. coli were examined by a chemical probing method using dimethyl sulphate (DMS). As the results, the nine bases in 23S rRNA were protected from DMS modifications and the modification of one base was enhanced. Interestingly A2451 is included among the protected bases, which is thought to be directly involved in peptidyl transferase activity. We conclude that RMF inactivates ribosomes by covering the peptidyl transferase (PTase) centre and the entrance of peptide exit tunnel. It is surprising that the cell itself produces a protein that seems to inhibit protein synthesis in a similar manner to antibiotics and that it can reversibly bind to and release from the ribosome in response to environmental conditions.