Simian virus 40 late 16S/19S RNA intron
Intron sequence from the SV40 late gene region that enhances transgene expression 2-10 fold by promoting efficient mRNA splicing, nuclear export, and transcript stability. First characterized for mammalian expression vectors by Okayama and Berg in 1983 and now a standard element in plasmid and viral expression systems.
Origin: Simian virus 40 late gene region (16S/19S mRNA)
Characteristics
Small intron derived from the SV40 late transcription unit that is processed by the cellular splicing machinery. When placed between a promoter and coding sequence, the intron enhances steady-state mRNA levels and protein output through multiple mechanisms: increased nuclear export via recruitment of mRNA export factors, enhanced transcript stability through exon junction complex deposition, and potentially improved translation initiation. The splicing process itself is highly efficient in mammalian cells, with >95% of transcripts correctly processed. Functions independently of promoter context and works equally well with both viral (CMV, SV40) and cellular promoters.
Applications: Standard inclusion in mammalian expression vectors for enhanced cDNA expression when high protein output is needed. Particularly beneficial for cDNA constructs lacking native introns, where absence of splicing can result in poor nuclear export and low expression. Widely used in plasmid transfection systems, lentiviral vectors, and research-grade expression constructs. Especially valuable when expressing genes from organisms with different intron structures or when using synthetic genes. Commonly paired with strong viral promoters (CMV, SV40) in transient and stable transfection applications.
Limitations: Requires functional cellular splicing machinery and may not provide benefits in cell types with impaired splicing. Can occasionally cause aberrant splicing if cryptic splice sites exist in the flanking sequences. Provides diminishing returns when multiple introns are already present in the transcript.
Mechanism: The SV40 intron is recognized and excised by the spliceosome, recruiting the exon junction complex (EJC) that remains bound 20-24 nucleotides upstream of the exon-exon junction after splicing. This EJC enhances nuclear export by providing binding sites for mRNA export factors (TREX complex), increases translation efficiency through interactions with ribosomal subunits during the pioneer round of translation, and stabilizes the transcript by marking it as properly processed. The act of splicing itself remodels the mRNP structure in ways that favor nuclear export over nuclear retention. Net effect is 2-10 fold higher protein output without altering transcription rate.
Sequence
References
- Okayama et al. (1983). A cDNA cloning vector that permits expression of cDNA inserts in mammalian cells. Mol. Cell. Biol. - Okayama 1983 cDNA Vector