Cyclic dissociation into stable subunits and re-formation of ribosomes during bacterial growth

Studies of the distribution of isotopic labels among ribosomes and ribosomal subunits following transfer of a growing bacterial culture from a heavy- to a light-isotopes medium show that: 1. (1) During growth ribosomes frequently undergo subunit exchange, presumably by dissociation into their 30 s and 50 s subunits and re-formation from a pool of free subunits. 2. (2) Bibosomal subunits are stable: they remain intact during growth and are continuously, recycled through ribosomes. On the basis of these results, it is suggested that ribosomes must dissociate into their subunits between successive rounds of translation.