The Synthesis and Origin of Chloroplast Low-Molecular-Weight Ribosomal Ribonucleic Acid in Spinach

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Abstract
Chloroplasts isolated from young spinach leaves incorporate [3H]uridine into RNA species which co‐electrophorese with 5‐S rRNA and tRNA, but show very little incorporation into 4.5‐S rRNA. Chloroplast 4.5‐S rRNA is labelled in vivo after a distinct lag period relative to 5‐S rRNA and tRNA. The kinetics of labelling in vivo of chloroplast 5‐S rRNA are similar to those of the immediate precursors to the 1.05 × 106‐Mr and 0.56 × 106‐Mr rRNAs, whereas the kinetics of labelling of the 4.5‐S rRNAare similar to those of mature 1.05 × 106‐Mr and 0.56 × 106‐Mr rRNAs. Chloramphenicol inhibits the labelling of chloroplast 4.5‐S rRNA in vivo, and concomitantly inhibits the processing of the immediate precursors to the 1.05 × 106‐Mr and 0.56 × 106‐Mr rRNAs, but has little effect on the appearance of label in chloroplast 5‐S rRNA. DNA/RNA hybridization using 125I‐labelled RNAs suggests that chloroplast DNA contains a 2–3‐fold excess of 4.5‐S and 5‐S rRNA genes relative to the high‐molecular‐weight rRNA genes. Competition hybridization experiments show that the immediate precursor to the 1.05 × 106‐Mr rRNA effectively competes with 125I‐labelled 4.5‐S rRNA for hybridization with chloroplast DNA, and is therefore a likely candidate for a common precursor to both the 1.05 × 106‐Mr and 4.5‐S rRNAs.

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