Developmentally regulated transcription of Dictyostelium discoideum plasmid Ddp1.

The presence of high copy number plasmid DNA has been described for several independent wild isolates of the lower eukaryote Dictyostelium discoideum, a model organism in developmental biology. The function of these plasmids has remained unclear so far. To understand the possible functions of the plasmids we have analyzed the transcription of one plasmid, Ddp1, in the wild strains NC4 and V12. The plasmid is transcribed in an identical fashion in both strains. Ddp1 codes for three RNA species in growth-phase cells, at least two of which are present in the poly(A) fraction. RNA isolated from cells of different developmental stages revealed the existence of five more transcripts, the majority of them being found in the poly(A) fraction. These transcripts are present at distinct time points during development. Both growth-phase and development-specific transcripts were found exclusively in Ddp1-containing strains. Mapping of the transcripts to the plasmid showed that some of them originated from the same area. These results indicated an overlapping of their coding regions.