Human cytomegalovirus immediate-early enhancer/promoter with a chimeric β-globin/immunoglobulin intron
Human cytomegalovirus immediate-early enhancer/promoter followed by a chimeric intron (β-globin splice donor, immunoglobulin heavy-chain branch point and splice acceptor) placed upstream of the transgene. The intron is spliced out of the mature mRNA and increases expression over an intron-less CMV cassette, the design used in vectors such as pCI-neo.
Origin: Human CMV IE promoter + synthetic β-globin/IgG chimeric intron
Characteristics
The chimeric intron sits immediately downstream of the CMV transcription start site and is removed by splicing before translation, following the same hybrid-intron design (a 5' splice donor from one gene paired with a 3' branch point/acceptor from an immunoglobulin gene) that Choi et al. (1991) showed raises reporter expression 5- to 300-fold over an intron-deleted transgene in transgenic mice, with the magnitude varying by tissue and integration site. The construct retains the HCMV enhancer/promoter's broad, cell-type-spanning constitutive activity (Boshart et al. 1985).
Applications: Used in mammalian expression vectors (e.g., pCI-neo) to raise protein yield from an otherwise standard CMV cassette in transient transfection and stable cell-line generation in HEK293, CHO, and COS cells. Preferred over intron-less CMV constructs when higher per-cell output is needed without changing promoter identity.
Limitations: Adds roughly 650 bp of chimeric intron sequence to the cassette, which enlarges the construct compared with intron-less CMV promoters and can matter for size-constrained vectors such as AAV. Retains the parental CMV promoter's susceptibility to cell-type-dependent expression variability and epigenetic silencing in vivo.
Sequence
References
- Boshart et al. (1985). A very strong enhancer is located upstream of an immediate early gene of human cytomegalovirus. Cell 41:521-530 - Boshart 1985 CMV