Gradual and All-or-None

Abstract

For many genes, their expression can be regulated by several factors, such as the presence of enhancer or silencing cis-elements, and trans-acting repressors and activators. Biggar and Crabtree established a fluorescence-activated cell-sorting assay to detect between an all-or-none (binary) response and a graded, continuous response of a the GAL1 promoter to various regulatory factors in isolation or combined with activation of cell signaling pathways. If expression is driven through a two-hybrid-like interaction whereby the GAL4 DNA binding domain is fused to FKBP12 and the GAL4 activation domain is fused to calcineurin, then transcription can be activated by the addition of FK506. Under these conditions, expression shows a graded response to increasing concentrations of FK506. When expression is activated by FK506 and then the cells are incubated in glucose to repress transcription through the repressor Mig1, expression is repressed in a graded manner. However, when the promoter is used with its native regulators (that is, not through forced activation by FK506) to drive expression of a reporter gene, then decreasing glucose led to a binary switch from cells not expressing the reporter to cells expressing the reporter. This binary switch occurred in cells null for the repressor Mig1, but the response was graded in cells null for the inhibitor Gal80, which inactivates the transcriptional activator Gal4. A last set of experiments showed that the environmental conditions can dictate whether the response will be binary or graded. Cells grown in the repressing conditions with glucose exhibit the Gal80-dependent binary response to glucose deprivation; whereas cells grown without glucose exhibit a graded response exposed to varying concentrations of glucose. Thus, a single promoter can direct both binary and graded responses depending on the signaling pathways activated and the cell status.S. R. Biggar, G. R. Crabtree, Cell signaling can direct either binary or graded transcriptional responses. EMBO J. 20, 3167-3176 (2001). [Abstract] [Full Text]