uORFs

Upstream open reading frames (uORFs) are short coding sequences located in the 5' untranslated region (5' UTR) of eukaryotic mRNAs. A uORF begins at an AUG codon (or a near-cognate start codon) and terminates before the main coding sequence, and its translation can influence the expression of the downstream primary ORF. uORFs are common across eukaryotes and vary in number, length, and sequence context.

Mechanism is typically through the scanning model of translation. Ribosomes bind the 5' cap and may initiate at a uAUG, producing a short peptide from the uORF. The ribosome may terminate and either reinitiate at the downstream main start codon or dissociate, with the outcome depending on factors such as the length of the intercistronic region, initiation factor availability, and the global state of translation. In many cases, uORFs repress downstream translation by sequestering ribosomes. Under cellular stress that reduces initiation availability, reinitiation downstream can be differentially regulated, sometimes increasing translation of certain main ORFs—an example being ATF4 in mammals and GCN4 in yeast.

Functions and diversity extend beyond simple repression. Some uORFs encode functional micropeptides that participate in regulatory pathways. Others primarily modulate translation efficiency of the main ORF without producing a distinct long protein. In some transcripts, translation of a uORF can influence mRNA stability or trigger decay pathways such as nonsense-mediated decay, shaping transcript abundance.

Detection and study commonly use sequence analysis, ribosome profiling to identify translating ribosomes in 5' leaders, and reporter assays with mutational analyses to quantify regulatory effects. The study of uORFs enhances understanding of gene expression control and cellular responses to stress.