NMR Structure of Bacterial Ribosomal Protein L20: Implications for Ribosome Assembly and Translational Control

L20 is a specific protein of the bacterial ribosome, which is involved in the early assembly steps of the 50 S subunit and in the feedback control of the expression of its own gene. This dual function involves specific interactions with either the 23 S rRNA or its messenger RNA. The solution structure of the free Aquifex aeolicus L20 has been solved. It is composed of an unstructured N-terminal domain comprising residues 1–58 and a C-terminal α-helical domain. This is in contrast with what is observed in the bacterial 50 S subunit, where the N-terminal region folds as an elongated α-helical region. The solution structure of the C-terminal domain shows that several solvent-accessible, conserved residues are clustered on the surface of the molecule and are probably involved in RNA recognition. In vivo studies show that this domain is sufficient to repress the expression of the cistrons encoding L35 and L20 in the IF3 operon. The ability of L20 C-terminal domain to specifically recognise RNA suggests an assembly mechanism for L20 into the ribosome. The pre-folded C-terminal domain would make a primary interaction with a specific site on the 23 S rRNA. The N-terminal domain would then fold within the ribosome, participating in its correct 3D assembly.