The 5′ UTR begins at the transcription start site and ends one nucleotide (nt) before the initiation sequence (usually AUG) of the coding region. In prokaryotes, the length of the 5′ UTR tends to be 3–10 nucleotides long, while in eukaryotes it tends to be anywhere from 100 to several thousand nucleotides long. For example, the ste11 transcript in Schizosaccharomyces pombe has a 2273 nucleotide 5′ UTR while the lac operon in Escherichia coli only has seven nucleotides in its 5′ UTR. The differing sizes are likely due to the complexity of the eukaryotic regulation which the 5′ UTR holds as well as the larger pre-initiation complex that must form to begin translation.
The 5′ UTR can also be completely missing, in the case of leaderless mRNAs. Ribosomes of all three domains of life accept and translate such mRNAs. Such sequences are naturally found in all three domains of life. Humans have many pressure-related genes under a 2&ndash;3 nucleotide leader. Mammals also have other types of ultra-short leaders like the TISU sequence.