Cytolocalization of Ribosomal Cistrons in Plant Polytene Chromosomes

In both pIants and animals the mechanism of ribo-somal R N A synthesis is thought to be the same, yet genetic and biochemical evidence points to a dispersed origin along chromosomal D N A in plants (1, 2, 3) and a strict localization of genes coding for ribosomal R N A in the nucleolar organizer regions of chromosomes in animals (4, 5). A precise means of locahzing ribosomal cistrons has been made available through cytologmal molecular hybridiza-tion of radioactive r R N A to its complementary D N A in sztu (6, 7, 8). This method has been used to show that in several species of insects r D N A is located in the nucleolar organizer regions of chromosomes and that the r D N A extends into the nucleolus proper. W e have now used this method to investigate whether ribosomal cistrons in two T~E JotraNAL OF CELL BIOLOGY • VoLtrME 53, 197~-pages 8~7-83¢ 827