Organization of the ribosomal RNA genes of Dictyostelium discoideum: mapping of the nontranscribed spacer regions.

Most of the Dictyostelium ribosomal nontranscribed spacer DNA has been mapped using Eco R1, Hind III, and Sal I restriction digests and recombinant plasmids containing parts of the rDNA (ribosomal DNA). The repeat units are at least 42 kb (1 kilobase = 1000 base pairs = 1 kb) long; of this, 8 kb codes for the 36S rRNA precursor. The repeat units give a homogeneous pattern of Sal I, Eco R1, and Hind III restriction fragments. The repeats could be linked by a heterogeneous region in the nontranscribed spacer, not tandemly repeated, or linked in a more complicated structural arrangement. The total rDNA comprises at least 18% of the nuclear DNA.