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Extrachromosomal ribosomal DNA in the simple eukaryote Dictyostelium discoideum is readily separated from chromosomal DNA by orthogonal field electrophoresis (OFAGE), forming a prominent band in the 110-kb region of the gel. Here we show that mutations in at least two chromosomal genes give rise to a ladder of rDNA bands increasing in size up to about 300 kb. One of these mutations, the rrcA350 allele, which is recessive to wild type and maps to the centromere-proximal region of linkage group II, has an unstable phenotype; spontaneous revertants, which no longer exhibit the rDNA ladder, have been recovered. Another mutation rrc-351, provisionally mapped to linkage group IV, is dominant to wild type. The rDNA ladder is caused by concatamerization of a 34-kb fragment in the nontranscribed central spacer region of the 88-kb linear rDNA palindrome. Restriction enzyme analysis has revealed that each concatamer is generated by crossovers between two rDNA molecules.

Tandem repeats in extrachromosomal ribosomal DNA of Dictyostelium discoideum, resulting from chromosomal mutations.