reacquire translation factors before it can carry out translation of the main protein. For example, ATF4 regulation is performed by two uORFs further upstream, named uORF1 and uORF2, which contain three amino acids and fifty-nine amino acids, respectively. The location of uORF2 overlaps with the ATF4 ORF. During normal conditions, the uORF1 is translated, and then translation of uORF2 occurs only after eIF2-TC has been reacquired. Translation of the uORF2 requires that the ribosomes pass by the ATF4 ORF, whose start codon is located within uORF2. This leads to its repression. However, during stress conditions, the 40S ribosome will bypass uORF2 because of a decrease in concentration of eIF2-TC, which means the ribosome does not acquire one in time to translate uORF2. Instead, ATF4 is translated. Other mechanisms In addition to reinitiation, uORFs contribute to translation initiation based on: The nucleotides of an uORF may code for a codon that leads to a highly structured mRNA, causing the ribosome to stall. cis- and trans- regulation on translation of the main protein coding sequence. Interactions with IRES sites. Internal ribosome entry sites and viruses Viral (as well as some eukaryotic) 5′ UTRs contain internal ribosome entry sites, which is a cap-independent method of translational activation. Instead of building up a complex at the 5′ cap, the IRES allows for direct binding of the ribosomal complexes to the transcript to begin translation. The IRES enables the viral transcript to translate more efficiently due to the lack of needing a preinitation complex, allowing the virus to replicate quickly. Role in transcriptional regulation msl-2 transcript Transcription of the msl-2 transcript is regulated by multiple binding sites for fly Sxl at the 5′ UTR. In particular, these poly-uracil sites are located close to a small intron that is spliced in males, but kept in females through splicing inhibition. This splicing inhibition is maintained by Sxl. When present, Sxl will repress the translation of msl2 by increasing translation of a start codon located in a uORF in the 5′ UTR (see above for more information on uORFs). Also, Sxl outcompetes TIA-1 to a poly(U) region and prevents snRNP (a step in alternative splicing) recruitment to the 5′ splice site. See also Three prime untranslated region UORF Iron-responsive element-binding protein Iron response element Trans-splicing UTRdb References RNA Gene expression