Regulation of expression of cell-type-specific genes of Dictyostelium discoideum

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After formation of multicellular tissue of Ddictyostelium discoideum, prestalk and prespore cells differentiate within it. To understand the molecular mechanisms of expression prespore specific genes, Dp87 and Sp96 (encoding spore coat protein Sp96) are good target genes because the 'run on' assays showed that they transcribed only in prespore cells. We identified at least one of the important cis-regulating regions for correct transcription of each of Dp87 and SP96 gene, by making chimeric genes including different lengths of 5'upstream regions conjugated with reporter genes and examining the expression of the reporter gene products in transformed cells. The region for Sp96 gene coincided well with one of the DNase-I hypersensitive sites that specifically appear at the slug stage. In the case of Dp87 gene, we also identified a possible transacting factor which specifically bind the cis-region and the factor was found only in slug cell nuclei. South-western analyses using this region indicated that there were three protein bands in the nuclear extract. We decided the binding site by DNase-I foot printing. Now we are looking for the cDNA of this binding protein. To understand whether the expression of Dp87 and Sp9G genes are regulated by cAMP which is known to control prespore cell differentiation, we examined the transcription of these genes in cells disaggregated from slugs. The results of northern analyses and 'run on' assays indicate that both Dp87 and Sp96 genes stop their transcription soon after disaggregation and their mRNAs already present are degraded rapidly. On the other hand, addition of exogenous cAMP to disaggregated cells induces immediate resumption of transcription of these genes through transduction of signals and synthesized mRNAs are stabilized by newly synthesized protein(s).