The five prime untranslated region (5' UTR), can contain elements for controlling gene expression by way of regulatory elements. It begins at the transcription start site and ends one nucleotide (nt) before the start codon (usually AUG) of the coding region. In prokaryotes, the 5' UTR usually contains a ribosome binding site (RBS), also known as the Shine Dalgarno sequence (AGGAGGU).
The 5' UTR has a median length of ~150 nt in eukaryotes, but can be as long as several thousand bases. Some viruses and cellular genes have unusually long and structured 5' UTRs which may impact gene expression. On average, 3' UTR tend to be twice as long as the 5' UTR. In prokaryotic mRNAs the 5' UTR is normally shorter.
Several regulatory sequences may be found in the 5' UTR:
- Binding sites for proteins, that may affect the mRNA's stability or translation, for example iron responsive elements, that regulate gene expression in response to iron.
- Riboswitches.
- Sequences that promote or inhibit translation initiation.
- Introns within 5' UTRs have been linked to regulation of gene expression and mRNA export.
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