cAMP-dependent protein kinase (PKA) is an essential regulator of gene expression and cell differentiation during multicellular development of Dictyostelium discoideum. Here we show that PKA activity also regulates gene expression during the growth phase and at the transition from growth to development. Overexpression of PKA leads to overexpression of the discoidinIgamma promoter, while expression of the discoidinIgamma promoter is reduced when PKA activity is reduced, either by expression of a dominant negative mutant of the regulatory subunit or by disruption of the gene for the catalytic subunit (PKA-C). The discoidin phenotype of PKA-C null cells is cell autonomous. In particular, normal secretion of discoidin-inducing factors was demonstrated. In addition, PKA-C null cells are able to respond to media conditioned by PSF and CMF. We conclude that PKA is a major activator of discoidin expression. However, it is not required for production or transduction of the inducing extracellular signals. Therefore, PKA-dependent and PKA-independent pathways regulate the expression of the discoidin genes.
Multiple genes for thioredoxins (TRX) have been demonstrated in Dictyostelium discoideum. We expressed the cDNA for one of these genes (DdTrxl) in E. coli and purified the protein to homogeneity. The interaction with classic substrates as well as TRX reductases was analysed. It reacted with every tested substrate : insulin, NADP-dependent malate dehydrogenase and fructose-1,6-bisphosphatase. With a So.5 of 20 pM, the reactivity with the fructose-l,6-bisphosphatase is the highest ever found for a heterologous TRX. DdTRXl itself is accepted as a substrate by the chloroplast ferredoxindependent TRX reductase, as well as by the E. coli NADPH-dependent TRX reductase. Thus, the Dictyostelium TRX is functionally promiscuous. Its reactivity with insulin, chloroplast NADPdependent malate dehydrogenase and ferredoxin-dependent TRX reductase resemble those of other TRX. It is, however, clearly different in its good interaction with chloroplast fructose-l,6-bisphosphatase and in its poor interaction with E. cnli NADP-dependent TRX reductase.
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