Satellite DNA, localization of ribosomal cistrons and heterochromatin in Haplopappus gracilis

The total length of the two chromosomes (n = 2) which build the genome of Haplopappus gracilis is about 14.6 microns. A well-defined nucleolus organizer is found in chromosome No. 2. Heterochromatin is seen in interphase nuclei mainly associated with the nucleolus. This heterochromatin is late in replicating as shown by H3-thymidine incorporation. In the analytical ultracentrifuge Model E the DNA of Haplopappus shows a DNA satellite with a buoyant density of 1.699 g/ml corresponding to a guanine-cytosine content of 39.8 %, the corresponding values for the main band being 1.693 g/ml and 36.7 % GC. Hybridization between 28S hamster ribosomal H3-RNA and total Haplopappus DNA fractionated on CsCl gradients reveals that the ribosomal cistrons are present in the DNA satellite. When the fractions — of similar gradients — which contain the ribosomal DNA are collected and their DNA is centrifuged in the analytical ultracentrifuge, the DNA satellite increases appreciably in size. These results lead us to conclude that the ribosomal cistrons are localized in the DNA satellite. Moreover, there is a high level of redundancy of the genes for ribosomal RNA in Haplopappus. Since chromosome No. 2 contains the nucleolus it is suggested that the DNA sequences of this DNA satellite are located in this chromosome.