uORF

Upstream open reading frame (uORF) is a regulatory element in the 5' untranslated region of many eukaryotic mRNAs. A uORF begins at a start codon within the 5' leader and ends at a stop codon upstream of the main coding sequence. uORFs are typically translated by ribosomes that scan from the 5' cap and can modulate translation of the downstream primary open reading frame (ORF).

The impact of a uORF on downstream translation depends on several factors. If ribosomes initiate at the uORF, they may terminate before reaching the main ORF, reducing initiation at the downstream start codon. After translating a uORF, ribosomes may reinitiate, with efficiency influenced by intercistronic distance, uORF length, and the availability of initiation factors such as eIF2. The surrounding sequence context of the uORF start codon (Kozak consensus) also affects initiation. Under cellular stress or conditions that limit initiation factor activity, the balance between uORF and main-ORF translation can shift, sometimes derepressing or altering main-ORF expression.

Examples and significance: In yeast, GCN4 is regulated by multiple uORFs that tune translation in response to amino acid availability. In mammals, transcripts such as ATF4 use uORFs to achieve selective upregulation during stress. uORFs may also encode short regulatory peptides and are common across eukaryotes, including plants, animals, and fungi.

Clinical and research relevance: Mutations creating, removing, or modifying uORFs can change protein output and have been linked to disease and developmental disorders. Techniques such as ribosome profiling and reporter assays help identify functional uORFs and clarify their roles in gene regulation.