Ribosomal protein L20 can replace the assembly-initiator protein L24 at low temperatures

The assembly of the 50S subunit from Escherichia coli ribosomes is initiated by two ribosomal proteins, L24 and L3. A mutant lacking the assembly-initiator protein L24 shows distinct phenotypic features (temperature sensitivity, growth rate reduced by a factor of 6 at permissive temperatures below 34 degrees C, underproduction of 50S subunits), which could be traced back to assembly effects caused by lack of L24 [Herold, M., Nowotny, V., Dabbs, E. R., & Nierhaus, K. H. (1986) Mol. Gen. Genet. 203, 281-287]. As expected, only one assembly protein was effective during in vitro assembly at nonpermissive temperatures, whereas surprisingly the restoration of active particle formation at permissive temperatures was paralleled by the reappearance of two initiator proteins. Here we analyze the initiation of assembly at permissive temperatures in the absence of L24. We demonstrate in a series of reconstitution experiments with purified proteins that the two initiator proteins are L20 and L3. Thus, L20 can replace L24 for the initiation of assembly at permissive temperatures.