Abstract: 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.

Abstract: Isolated mammalian nucleic acid molecules encoding receptor protein tyrosine kinases expressed in primitive hematopoietic cells and not expressed in mature hematopoietic cells are provided. Also included are the receptors encoded by such nucleic acid molecules; the nucleic acid molecules encoding receptor protein tyrosine kinases having the sequences shown in FIG. 1a (murine flk-2), FIG. 1b (human flk-2) and FIG. 2 (murine flk-1); the receptor protein tyrosine kinases having the amino acid sequences shown in FIG. 1a, FIG. 1b and FIG. 2; ligands for the receptors; nucleic acid sequences that encode the ligands; and methods of stimulating the proliferation and/or differentiation of primitive mammalian hematopoietic stem cells comprising contacting the stem cells with a ligand that binds to a receptor protein tyrosine kinase expressed in primitive mammalian hematopoietic cells and not expressed in mature hematopoietic cells.