Ribosomal RNA cistrons in Euglena gracilis

Euglena gracilis chloroplasts contain about 12 fg DNA of average density 1.686 g cm^?3 and 1.7 pg RNA. The large (1.1 × 10⁶ mol. wt) and small (0.56 × 10⁶ mol. wt) ribosomal RNA components are coded for by separate cistrons, both of which band at a density of 1.696 g cm^?3 in a CsCl gradient. About 6% of the chloroplast DNA codes for rRNA indicating that there are 240 cistrons for rRNA in each chloroplast or about three to six cistrons per chloroplast genome. Similar studies with rRNA from cytoplasmic ribosomes indicate that the cistrons for cytoplasmic rRNA band at a density of 1.716 g cm^?3, denser than that of the main-band DNA, and that there are 1000 cistrons for cytoplasmic rRNA per cell. Fractionation of E. gracilis DNA on CsCl gradients and subsequent hybridization experiments, as well as melting curves of DNA-RNA hybrids, show that chloroplast rRNA does not anneal specifically with either the cistrons for cytoplasmic rRNA or any DNA in the dark-grown cell, in contrast to those results found in some higher plants.