Double molecular mimicry in Escherichia coli:: binding of ribosomal protein L20 to its two sites in mRNA is similar to its binding to 23S rRNA

Escherichia coli ribosomal L20 is one of five proteins essential for the first reconstitution step of the 50S ribosomal subunit in vitro. It is purely an assembly protein, because it can be withdrawn from the mature subunit without effect on ribosome activity. In addition, L20 represses the translation of its own gene by binding to two sites in its mRNA. The first site is a pseudoknot formed by a base-pairing interaction between nucleotide sequences separated by more than 280 nucleotides, whereas the second site is an irregular helix formed by base-pairing between neighbouring nucleotide sequences. Despite these differences, the mRNA folds in such a way that both L20 binding sites share secondary structure similarity with the L20 binding site located at the junction between helices H40 and H41 in 23S rRNA. Using a set of genetic, biochemical, biophysical, and structural experiments, we show here that all three sites are recognized similarly by L20.