Spermidine inhibits transient and stable ribosome subunit dissociation

Recent light-scattering experiments and sucrose density gradient centrifugational analyses suggested that the 70S ribosome undergoes RRF- and EF-G-triggered transient subunit dissociation that is followed by IF3-induced stable dissociation. However, the experimental conditions did not include the ubiquitous cellular polyamine spermidine, which is required for efficient translation. We found that when spermidine was present, the transient dissociation was inhibited. Moreover, the published experiments used ribosome concentrations that were far lower than the physiological concentration. We found that when spermidine and higher ribosome concentrations were included in the experimental conditions, only very limited stable subunit dissociation was observed. These results suggest that neither transient nor stable dissociation occurs under physiological conditions applied here.