SINGLE CELL ASSAY OF A TRANSCRIPTION FACTOR REVEALS A THRESHOLD IN TRANSCRIPTION ACTIVATED BY SIGNALS EMANATING FROM THE T-CELL ANTIGEN RECEPTOR

Stimulation of T lymphocytes through their antigen receptor leads to the appearance of several transcription factors, including NF-AT and NF-κB, which are involved in regulating genes required for immunologic activation. To investigate the activity of a single transcription factor in individual viable cells, we have applied an assay that uses the fluorescence-activated cell sorter to quantitate β-galactosidase (β-gal). We have analyzed the distribution of NF-AT transcriptional activity among T cells undergoing activation by using a construct in which three tandem copies of the NF-AT-binding site directs transcription of the lacZ gene. Unexpectedly, stimulation of cloned stably transfected Jurkat T cells leads to a bimodal pattern of β-gal expression in which some cells express no β-gal and others express high levels. This expression pattern cannot be accounted for by cell-cycle position or heritable variation. Further results, in which β-gal activity is correlated with NF-AT-binding activity, indicate that the concentration of NF-AT must exceed a critical threshold before transcription initiates. This threshold likely reflects the NF-AT concentration-dependent assembly of transcription complexes at the promoter. Similar constructs controlled by NF-kB or the entire interleukin-2 enhancer show bimodal expression patterns during induction, suggesting that thresholds set by the concentration of transcription factors may be a common property of inducible genes.