Maturation of chloroplast and cytoplasmic ribosomal RNAs from single large precursors in two algal species, Cyanidium and Chlorella.

In the present study the synthesis and the processing of cytoplasmic and chloroplast rRNAs of the algae Cyanidium caldarium (Rhodophyceae), Chlorella pyrenoidosa and Chlorella vulgaris (Chlorophyceae) are characterized. The results of pulse-labeling of the cells under autotrophic and heterotrophic conditions, of kinetic analyses of labeling and of pulse-chase experiments demonstrate that the mature chloroplast ribosomal RNAs arise from a single large precursor with an apparent molecular weight of 1.8×10 daltons. For detection of this primary transcript in Chlorella lowering of the temperature to 15°C during pulse-labeling was mandatory. The findings also show that it is processed to form the mature species via immediate precursors of about 1.2×10 and 0.59×10 daltons. These results are reinforced by those of additional experiments which were performed in the presence of rifampicin or in which colorless mutant cells of Chlorella vulgaris were studied. Cytoplasmic ribosomal RNAs are apparently cleaved from a single precursor of about 2.8×10 daltons molecular weight in Cyanidium, and of 2.4×10 daltons in Chlorella. In Cyanidium the immediate precursors to the mature species (1.4×10 and 0.69×10 daltons) have molecular weights of about 1.6×10 and 0.76×10 daltons, respectively. In Chlorella RNA of 1.4×10 daltons serves as immediate precursor to the large cytoplasmic rRNA species (1.3×10 daltons), and one of 0.8×10 daltons as precursor to the small species (0.68×10 daltons). From the data of labeling kinetics performed with cells of Cyanidium it is evident that the relative positions of the sequences of mature cytoplasmic ribosomal RNAs in the molecule of the primary transcript (2.8×10 daltons) agree very well with those found in other plant organisms so far. - On the basis of these results schemes for the two pathways of ribosomal RNA synthesis in the two algal species are proposed. [ABSTRACT FROM AUTHOR]