Ribosomal RNA genes in the nucleus and chloroplast of Euglena

Centrifugation of DNA from Euglena gracilis in Hg2+-CS2SO4 equilibrium gradients allows the chloroplast DNA to be separated clearly from the nuclear DNA. Cytoplasmic ribosomal RNA isolated from a heat-bleached strain hybridizes only to the nuclear DNA. The ribosomal RNA cistrons in the chloroplast DNA are not related to those in the nuclear DNA. A possible origin for the chloroplast genome is suggested by the observation that roughly one-fourth of the ribosomal RNA complementary sequences in chloroplast DNA anneal with RNA from the blue-green alga Anacystis nidulans.