A novel mammalian expression screen exploiting green fluorescent protein-based transcription detection in single cells.

The accumulation of DNA sequence information from large-scale genomic and random library sequencing projects is leading to the rapid identification of many putative genes, virtual transcripts and ESTs of unknown function. There is therefore an increasing need for high throughput, sensitive and robust methods for identification and characterisation of genes, and/or their products, based on function. We describe a high throughput functional expression screen based on semi-quantitative analysis of enhanced green fluorescent protein expression in single cells by confocal microscopy. The assay was implemented in a micro-scale format, requiring around 10(4) cells/test. The system was validated by co-transfection of a series of cDNAs encoding pro-inflammatory cytokine intracellular signal mediators with a d2EGFP reporter containing a cytokine responsive promoter. The majority of the test plasmids gave a detectable signal above background at a pool size of 250-500. Replicate tests indicate that the assay is reproducible at this pool size. At this level we demonstrate that large (>10(6) transformants) libraries can be feasibly screened.