Five prime untranslated region

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The five prime untranslated region (5' UTR), also known as the leader sequence, is a particular section of messenger RNA (mRNA) and the DNA that codes for it. It starts at the +1 position (where transcription begins) and ends just before the start codon (usually AUG) of the coding region. It usually contains a ribosome binding site (RBS), in bacteria also known as the Shine Dalgarno sequence (AGGAGGU). The 5' UTR may be a hundred or more nucleotides long, and the 3' UTR may be even longer (up to several kilobases in length) (Molecular Cell Biology, 5th edition, Lodish et al. p113, chapter 4.2).

An mRNA molecule codes for a protein through translation. The mRNA also contains regions that are not translated: in eukaryotes this includes the 5' untranslated region, 3' untranslated region, 5' cap and poly-A tail.

In prokaryotic mRNA the 5' UTR is normally short. Some viruses and cellular genes have unusual long structured 5' UTRs which may have roles in gene expression.

mRNA structure, approximately to scale for a human mRNA, where the median length of the 5'UTR is 170 nucleotides

Several regulatory sequences may be found in the 5' UTR:

Binding sites for proteins, that may effect the mRNA's stability or translation, for example iron responsive elements, which occur in the 5' UTRs (and 3' UTRs) of a small number of eukaryotic mRNAs that regulate gene expression in response to iron.
Regulatory elements that do not depend on proteins, such as riboswitches.
Sequences that promote the initiation of translation.