A ribosomal protein leader is a mechanism used in cells to control the cellular concentration of a protein that forms a part of the ribosome, and to make sure that the concentration is neither too high nor too low.
[1][2] Ribosomal protein leaders are RNA sequences that are a part of the 5' UTR of mRNAs encoding a ribosomal protein.
This binding event can lower gene expression via a number of mechanisms; for example, in the protein-bound state, the RNA could form an intrinsic transcription termination stem-loop.
When cellular concentrations of the ribosomal protein are not high, they are occupied in the ribosome, and are not available in significant quantities to bind the mRNA leader.
In many cases, the binding site within the leader structurally resembles the region of the ribosomal RNA to which the protein binds, in an example of molecular mimicry.