Circular ribosomal DNA during ribosomal magnification in Drosophila melanogaster

Abstract

DNA from excised testes of Drosophila melanogaster males undergoing ribosomal DNA magnification shows two peaks on an ethidium bromide/CsCl density gradient: a minor peak that bands to a density of 1·570 g/cm3 and a main peak corresponding to 1·540 g/cm3 density. The minor peak is absent when DNA is extracted from testes of wild-type males or of males where rDNA magnification has gone to completion. Electron micrographs show that DNA at the minor peak is circular in shape. Ribosomal RNA/DNA hybridization experiments on the gradient and preparation of pure rRNA/DNA at minor peak hybrids on a Cs2SO4 density gradient indicate that most of these circular molecules are DNA complementary to rRNA. These results support the model that requires the formation of rDNA extra-copies for rDNA magnification (Ritossa, 1972) .