A SERUM RESPONSE FACTOR HOMOLOG IS REQUIRED FOR SPORE DIFFERENTIATIONIN DICTYOSTELIUM

A homolog of the Serum Response Factor (SRF) has been isolated from Di ctyostelium discoideum and its function studied by analyzing the conse quences of its gene disruption. The MADS-box region of Dictyostelium S RF (DdSRF) is highly conserved with those of the human, Drosophila and yeast homologs. srfA is a developmentally regulated gene expressed in prespore and spore cells. This gene plays an essential role in sporul ation as its disruption leads to abnormal spore morphology and loss of viability, The mutant spores were round and cellulose deposition seem ed to be partially affected. Initial prestalk and prespore cell differ entiation did not seem to be compromised in the mutant since the expre ssion of several cell-type-specific markers were found to be unaffecte d, However, the mRNA level of the spore marker spiA was greatly reduce d. Activation of the cAMP-dependent protein kinase (PKA) by 8-Br-cAMP was not able to fully bypass the morphological defects of srfA(-) muta nt spores, although this treatment induced spiA mRNA expression. Our r esults suggest that DdSRF is required for full maturation of spores an d participates in the regulation of the expression of the spore-coat m arker spiA and probably other maturation genes necessary for proper sp ore cell differentiation.