A ribosomal protein leader is a mechanism used in cells

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 to control the cellular concentraction of a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 that forms a part of the ribosome

Ribosome

A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....

, and to make sure that the concentration is neither too high nor too low. Ribosomal protein leaders are RNA

RNA

Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 sequences that are a part of the 5' UTR of mRNAs encoding a ribosomal protein. When cellular concentrations of the ribosomal protein are high, excess protein will bind to the mRNA leader. 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

Intrinsic termination

Intrinsic termination is a mechanism in prokaryotes that causes mRNA transcription to be stopped. In this mechanism, the mRNA contains a sequence that can base pair with itself to form a stem-loop structure 7-20 base pairs in length that is also rich in cytosine-guanine base pairs...

 stem-loop

Stem-loop

Stem-loop intramolecular base pairing is a pattern that can occur in single-stranded DNA or, more commonly, in RNA. The structure is also known as a hairpin or hairpin loop. It occurs when two regions of the same strand, usually complementary in nucleotide sequence when read in opposite directions,...

. 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. This leads to increased expression of the gene, which leads to the synthesis of more copies of the ribosomal protein. Many examples of ribosomal protein leaders are known in bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, including ribosomal protein L20 leader

Ribosomal protein L20 leader

This family is a putative ribosomal protein leader autoregulatory structure found in B. subtilis and other low-GC Gram-positive bacteria. It is located in the 5' untranslated regions of mRNAs encoding initiation factor 3 followed by ribosomal proteins L35 and L20...

 and ribosomal S15 leader

Ribosomal S15 leader

The E. coli ribosomal S15 leader is an RNA element that can form two alternative structures found in the ribosomal S15 protein. One of the two alternate structures is a series of three hairpins, the other includes a pseudoknot. This structure causes translational regulation of the S15 protein....

.