An orthologue of the Myelin-gene Regulatory Transcription Factor regulates Dictyostelium prestalk differentiation

The prestalk region of the Dictyostelium slug is comprised of an anterior population of pstA cells and a posterior population of pstO cells. They are distinguished by their ability to utilize different parts of the promoter of the ecmA gene. We identify, by mutational analysis and DNA transformation, CA-rich sequence elements within the ecmA promoter that are essential for pstA-specific expression and sufficient to direct pstA-specific expression when multimerised. The CA-rich region was used in affinity chromatography with nuclear extracts and bound proteins were identified by mass spectrometry. The CA-rich elements purify MrfA, a protein with extensive sequence similarity to animal Myelin-gene Regulatory Factor (MRF)-like proteins. The MRF-like proteins and MrfA also display more spatially limited but significant sequence similarity with the DNA binding domain of the yeast Ndt80 sporulation-specific transcription factor. Furthermore, the ecmA CA-rich elements show sequence similarity to the core consensus Ndt80 binding site (the MSE) and point mutation of highly conserved arginine residues in MrfA, that in Ndt80 make critical contacts with the MSE, ablate binding of MrfA to its sites within the ecmA promoter. MrfA null strains are delayed in multicellular development and highly defective in pstA-specific gene expression. These results provide a first insight into the intracellular signaling pathway that directs pstA differentiation and identify a non-metazoan orthologue of a family of molecularly uncharacterised transcription factors.