Regulatory elements (e.g. promoters) activated by a stimulus are isolated by a FACS-based method. Preferably, a library of random fragments representative of a target (e.g. bacterial) genome are cloned in front of a promoterless gfp (green fluorescent protein) sequence in a plasmid, and inserted into target cells. The resulting target cell mixture is sorted according to GFP levels in the presence and the absence of the stimulus. Suitable stimuli include compounds of interest (e.g. drugs), environmental factors (e.g. extracellular acidity), and complex stimuli such as in vivo environments of hosts infected by the target cells. The method allows identifying pathogen genes which are selectively expressed during infection.
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