Synthesis and maturation of chloroplast and cytoplasmic ribosomal RNA in Chlamydomonas reinhardi

Abstract

The work of a large number of people has shown that the ribosomes of the chloroplast differ from those of the cytoplasm in the spectrum of their sensitivity to inhibitors of protein synthesis, the size of their rRNAs, and in the sensitivity of chloroplast rRNA synthesis to rifamycin SV. The genes for chloroplast rRNAs reside on the chloroplast DNA while those for cytoplasmic rRNAs are found on the nuclear DNA. We have characterized the synthesis and processing of cytoplasmic and chloroplast rRNAs of the alga Chlamydomonas reinhardi in order to further elucidate the difference between chloroplast and nuclear genetic systems. Cytoplasmic rRNAs of C. reinhardi are cleaved from a single precursor of molecular weights 2.4 · 10 6 to a mature rRNA (0.69 · 10 6 mol. wt) and a 1.4 · 10 6 -mol. wt precursor of the mature 1.3 · 10 6 -mol. wt rRNA. The kinetics of incorporation of radioactive label into the rRNAs suggest that the 0.69 · 10 6 -mol. wt rRNA gene is located closer to the promotor than is the gene for the 1.4 · 10 6 -mol. wt rRNA. The synthesis of cytoplasmic rRNAs is extremely sensitive to camptothecin, an inhibitor of nuclear rRNA synthesis, but synthesis of chloroplast rRNA is quite resistant to the inhibitor. This has allowed us to demonstrate that chloroplast rRNAs are processed from precursors which resemble those of blue-green algae. A 1.14 · 10 6 -mol. wt precursor is processed to the 1.07 · 10 6 -mol. wt mature chloroplast rRNA, and a 0.64 · 10 6 -mol. wt precursor is cleaved to a 0.56 · 10 6 -mol. wt species and then to the mature 0.54 · 10 6 -mol. wt rRNA. This study demonstrates two new ways in which the function of the chloroplast genome resemble those of prokaryotes more than those of the nucleus.