Expression of the chloroplast ribosomal RNA genes of Euglena gracilis during chloroplast development

The cellular content and transcription program of the chloroplast ribosomal RNA genes of Euglena gracilis Z have been determined during the light-induced development of chloroplasts by hybridization of total cell DNA or RNA to purified 3H-labeled chloroplast ribosomal DNA ([3H]ctrDNA). Pancreatic DNase activated, partially purified chloroplast rDNA was enzymatically labeled in vitro by E. coli DNA polymerase I with [3H]TTP as a substrate. The [3H] DNA was denatured and hybridized with a vast excess of purified chloroplast 16 and 23S rRNA. The rRNA-[3H]ct rCNA hybrid was isolated by chromatography on hydroxylapatite. The [3H]ct rDNA was purified and characterized by the kinetics of its renaturation with chloroplast DNA and rRNA, and by the thermal stability of [3H]DNA-DNA and [3H]DNA-RNA hybrids. [3H]ct rDNA was hybridized in trace amounts to cellular RNA or DNA isolated from Euglena cells 0,4,8,12,24,48, and 72 h after the onset of chloroplast development. From a comparison of the kinetics of hybridization with hybridization of standards of known kinetic complexity quantitative estimates of the cellular rRNA and rDNA gene content were made. Chloroplast rRNA increases from 2 to 26% of the cellular RNA during development, while the percentage of cellular DNA represented by ct rDNA increases two- to threefold. Correcting for the change in cellular RNA and DNA content during development, the number of copies of the rRNA gene increases less than twofold, while the number of copies of rRNA per cell increases sixfold. The results are consistent with either a transcriptional activation of the ribosomal genes or an increased rRNA stability during developmental.