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 cyclic nucleotide phosphodiesterase (PD) is produced at a low rate during the period its specific inhibitor (PDI) is being synthesized. In addition, PD gene expression is derepressed in the aggregation-deficient (Agg-), Pdi- mutant HC35. These observations suggest that the PDI might function to regulate PD gene expression, as well as modulate its activity. To explore this idea further, five new Agg-, Pdi- mutants were isolated and analyzed. All of the mutants produced high PD activity and overexpressed PD mRNA; four exhibited elevated levels of the 2400-nucleotide aggregation transcript and one overproduced the 1900-nucleotide vegetative transcript. In contrast, PD transcripts were not elevated in two Agg-, Pdi+ mutants. To determine if PDI production regulates PD expression, HC35 cells were transformed with plasmids carrying the PDI structural gene under the control of either the vegetative or aggregative PD promoter. Neither expression of PDI by the transformants nor addition of partially purified PDI to HC35 cells affected PD transcription. These results suggest that PD overexpression in the Pdi- mutants is not a direct consequence of the inability of these cells to produce inhibitor.