GRADED TRANSCRIPTIONAL RESPONSE TO DIFFERENT CONCENTRATIONS OF A SINGLE TRANSACTIVATOR

Threshold mechanisms of transcriptional activation are thought to be c ritical for translating continuous gradients of extracellular signals into discrete all-or-none cellular responses, such as mitogenesis and differentiation. Indeed, unequivocal evidence for a graded transcripti onal response in which the concentration of inducer directly correlate s with the level of gene expression in individual eukaryotic cells is lacking. By using a novel binary tetracycline regulatable retroviral v ector system, we observed a graded rather than a threshold mechanism o f transcriptional activation in two different model systems,When polyc lonal populations of cells were analyzed at the single cell level, a d ose-dependent, stepwise increase in expression of the reporter gene, g reen fluorescent protein (GFP), was observed by fluorescence-activated cell sorting, These data provide evidence that, in addition to the ge nerally observed all-or-none switch, the basal transcription machinery also can respond proportionally to changes in concentration of extrac ellular inducers and transcriptional activators.