Characterisation of a DNA sequence element that directs Dictyostelium stalk cell-specific gene expression

The ecmB gene of Dictyostelium is expressed at culmination both in the pres talk cells that enter the stalk tube and in ancillary stalk cell structures such as the basal disc. Stalk tube-specific expression is regulated by seq uence elements within the cap-site proximal part of the promoter, the stalk tube (ST) promoter region. Dd-STATa, a member of the STAT transcription fa ctor family, binds to elements present in the ST promoter-region and repres ses transcription prior to entry into the stalk tube. We have characterised an activatory DNA sequence element, that lies distal to the repressor elem ents and that is both necessary and sufficient for expression within the st alk tube. We have mapped this activator to a 28 nucleotide region (the 28-m er) within which we have identified a GA-containing sequence element that i s required for efficient gene transcription. The Dd-STATa protein binds to the 28-mer in an in vitro binding assay, and binding is dependent upon the GA-containing sequence. However, the ecmB gene is expressed in a Dd-STATa n ull mutant, therefore Dd-STATa cannot be responsible for activating the 28- mer in vivo. Instead, we identified a distinct 28-mer binding activity in n uclear extracts from the Dd-STATa null mutant, the activity of this CA bind ing activity being largely masked in wild type extracts by the high affinit y binding of the Dd-STATa protein. We suggest, that in addition to the long range repression exerted by binding to the two known repressor sites, Dd-S TATa inhibits transcription by direct competition with this putative activa tor for binding to the GA sequence.