Drosophila melanogaster Ultrabithorax internal ribosome entry site
Internal ribosome entry site from the 5' leader of the Drosophila Ultrabithorax mRNA. Placed between two coding sequences it recruits ribosomes to start translation of the second sequence internally, independent of the 5' end of the transcript, so two proteins are made from one transcript.
Origin: Drosophila melanogaster Ultrabithorax mRNA leader
Characteristics
Non-coding RNA element of 594 nucleotides from the 5' leader of the Drosophila melanogaster Ultrabithorax mRNA, so a protein translation of it is not meaningful. An internal ribosome entry site lets ribosomes initiate translation in the middle of a transcript without scanning from the cap, so a downstream coding sequence is translated as a separate protein from the same transcript.
Applications: Bicistronic expression: placed between two coding sequences, the element lets the second be translated by internal entry instead of by 2A-peptide ribosome skipping, so the upstream protein is not left with a C-terminal 2A remnant and no cleavage step is involved.
Limitations: Translation from an internal entry site is generally less efficient than cap-dependent initiation of the first coding sequence, so the second protein is expected at lower levels. At 594 nucleotides it is far larger than a 2A peptide and uses payload budget.