Circular DNA from the ribosomal gene cluster. (A) A schematic outline of the organization of ribosomal genes. Shown are the 28S and 18S genes (gray), the intergenic nontranscribed spacer (stippled), and the internal transcribed spacer (black). The lengths of the genes and the spacers are not drawn to scale. (B) Looping out between spacers or rDNA genes from different clusters yields large circles that include rDNA genes and spacer sequences. (C) Looping out within the short repeats in the spacer yields smaller circles of spacer repeats. (D) Hybridization of a 2D-gel blot of adult DNA with a 28S rDNA probe reveals large circles only (open arrowhead). (E) Short exposure following hybridization with a cloned 240-bp spacer probe reveals large circles (open arrowhead). (F) Longer exposure of the same blot in E reveals also an arc of smaller circles (solid arrowhead).
