Control of cell-type specific gene expression in Dictyostelium by the general transcription factor GBF

To understand how positional information within an organism specifies patterning during development, we are analyzing spatially regulated gene expression in Dictyostelium. CAR3 is a member of the cAMP, 7-span receptor family which directs the transition from unicellular to multicellular organism and regulates cellular differentiation and pattern formation. CAR3 mRNA is expressed maximally at 8-10 hours of development, as individual cells aggregate and differentiate, and is accumulated to equivalent levels in all cells. CAR3 is also induced in shaking cultures by response to extracellular cAMP. We now show, by extensive mutagenesis, that the maximum length of contiguous sequences required for accurate spatiotemporal regulation of CAR3 is approx. 350 bp. These sequences include three significant elements located in upstream and transcribed regions. Arrays of G-boxes (GBF regulatory sites) are centered near positions -165 and +50 and, although either is sufficient for induction by cAMP and expression in prespore cells, both are required for expression in prestalk cells. Another GC-rich element near position -80 is required for maximal expression of prespore-specific constructs, although full-length promoters carrying clustered mutations through the -80 region are still expressed in all cells, but with slightly reduced expression. Spatiotemporal expression of CAR3 during development, thus, requires cell-specific combinatorial interactions of multiple but redundant regulatory components. These essential elements are located in upstream and transcribed regions. However, most surprisingly, a primary control for spatial patterning of CAR3 expression appears to be mediated by GBF, a general transcription factor expressed ubiquitously during Dictyostelium development following early aggregation.