Enhancer-trapping - an Overview

An enhancer-trap element is a mobile DNA element, usually a P element, that contains a 'reporter' gene. Significant levels of reporter expression require that the element become inserted into the Drosophila genome close to an endogenous enhancer of gene expression. Two basic types of enhancer-trap element are in common use (click on element name to see schematic representation):

P[lacZ]
The reporter of most P[lacZ] elements is a b-galactosidase fusion protein that is targeted to the cell nucleus (O'Kane and Gehring, 1987). Reporter expression is readily visualised using the chromogenic b-gal substrate, Xgal, or by immunohistochemistry. Nuclear-targeted b-gal does not reveal cell shape, however, making it difficult to establish which neuronal elements correspond to which staining patterns.

P[GAL4]
The reporter of this 'second generation' enhancer-trap element (Brand and Perrimon, 1993; Kaiser, 1993) is a yeast transcription factor that is functional in Drosophila. Rather than visualise GAL4 directly, we can use it to drive secondary reporters linked to a GAL4-dependent promoter (UASG). Expressed in this way, b-gal lacking a nuclear localisation determinant is an excellent marker for neuronal processes. A wide range of other transgenes can then substitute for lacZ, allowing in vivo manipulation of the GAL-expressing cells.

In addition to the reporter gene, both types of element include a visible marker that allows new insertions to be recognised. white+, for example, confers red eye colour in a white- genetic background. Many enhancer-trap elements also include an ampicillin resistance (ampR) determinant and an E. coli origin of replication (ori) to facilitate plasmid-rescue of flanking sequences.

References:

Brand, A.H. and Perrimon, N. Development 118, 401-415 (1993)

Kaiser, K. Current Biology 3, 560-562 (1993)

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