REGULATION OF EXPRESSION OF THE CYCLIC-NUCLEOTIDE PHOSPHODIESTERASE GENE IN PHOSPHODIESTERASE INHIBITOR-NEGATIVE MUTANTS OF DICTYOSTELIUM-DISCOIDEUM

During early development of Dictyostelium discoideum, the enzyme cycli c nucleotide phosphodiesterase (PD) is produced at a low rate during t he period its specific inhibitor (PDI) is being synthesized. In additi on, PD gene expression is derepressed in the aggregation-deficient (Ag g(-)), Pdi(-) mutant HC35. These observations suggest that the PDI mig ht function to regulate PD gene expression, as well as modulate its ac tivity. To explore this idea further, five new Agg(-), Pdi(-) mutants were isolated and analyzed. All of the mutants produced high PD activi ty and overexpressed PD mRNA; four exhibited elevated levels of the 24 00-nucleotide aggregation transcript and one overproduced the 1900-nuc leotide vegetative transcript. In contrast, PD transcripts were not el evated in two Agg(-), Pdi(+) mutants. To determine if PDI production r egulates PD expression, HC35 cells were transformed with plasmids carr ying the PDI structural gene under the control of either the vegetativ e or aggregative PD promoter. Neither expression of PDI by the transfo rmants nor addition of partially purified PDI to HC35 cells affected P D transcription. These results suggest that PD overexpression in the P di(-) mutants is not a direct consequence of the inability of these ce lls to produce inhibitor.