DICTYOSTELIUM-DISCOIDEUM NUCLEAR PLASMID DDP6 IS A NEW MEMBER OF THE DDP2 PLASMID FAMILY

Ddp6 is a high-copy number, circular plasmid found in the nucleus of t he simple eukaryote Dictyostelium discoideum wild isolate NC47.2. The complete nucleotide sequence, 5257 bp, shows that Ddp6 has a structure similar to that of other members of the Ddp2 plasmid family: a single long 2.8-kb open reading frame (rep gene) and an inverted repeat cont aining a pair of 654-bp elements. A single constitutively expressed 3. 3-kb transcript of the rep gene was detected in RNA prepared from vege tative and developmental cells. Maintenance assays revealed that seque nces within the inverted repeat and the intact Ddp6 ORF are essential for maintenance of the plasmid. Mutation of the inverted repeat, or of the rep gene, lowered the plasmid copy number but did not affect auto nomous replication of shuttle-vector constructs. Comparisons of the pr edicted protein products of the rep genes of five members of the Ddp2 plasmid family identified a set of ten conserved features distributed throughout the peptides.