RNA Synthesis in Carrot Chromoplasts and Amyloplasts: Plastid Genome Transcription
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Chloroplast rRNA and tRNA are encoded by the plastid genome and synthesised inside the organelles themselves. The mRNAs for the polypeptides made within chloroplasts are also encoded by the plastome, and there is no evidence that any RNA enters the plastid stroma from the cytoplasm.
On this basis it could be assumed that the RNA found in the chromoplasts and amyloplasts of carrot taproots is encoded by the plastid genome and synthesised inside the organelles. To test this assumption, the fractional composition of ³H-RNA synthesised in a system of isolated plastids was studied.
Labelled RNA from amyloplasts and chromoplasts was separated by electrophoresis in the presence of unlabelled total RNA from carrot leaves, which served as both a marker and a carrier. 
What RNA do chromoplasts of forming carrot taproots synthesise?
Chromoplasts of forming red carrot taproots incorporated most of the labelled precursor into RNA migrating in the region of the 23S and 16S ribosomal RNAs. Analysis of the radioactivity profiles of labelled chromoplast RNA made this the dominant feature.
Noticeable levels of radioactivity also appeared in the pre-ribosomal zone and in the region of 4–5S RNA. As incubation time increased, the height of the radioactivity peaks rose, but the pattern of label distribution stayed the same.
Similar radioactivity profiles were observed for newly synthesised RNA from chromoplasts isolated from taproots in the middle and at the end of the growing season (Fig. 1).
How does RNA synthesis in amyloplasts change through the growing season?
Amyloplasts of forming white carrot taproots shift their RNA synthesis pattern as the taproot develops. In the earliest stage their behaviour resembles that of chromoplasts; later it changes markedly.
In young taproots (3–5 mm in diameter), amyloplasts also incorporated most of the label into RNA migrating in the 23S and 16S ribosomal regions. Peaks of radioactivity appeared in the pre-ribosomal zone and the 4–5S region as well, but they were considerably lower than the ribosomal 23S and 16S peaks.
By mid-season (taproots 8–10 mm in diameter), amyloplasts directed most of the ³H-UTP into low-molecular-weight RNA, while only a small amount of label entered the ribosomal and pre-ribosomal zones.
At the end of the season (taproots 16–20 mm in diameter), amyloplasts lost the ability to incorporate the labelled precursor into RNA intensively, which makes identifying the components of newly synthesised RNA at this developmental stage difficult.
Are the newly synthesised ribosomal RNAs precursors?
The newly synthesised ribosomal RNA components appear to be precursors of the mature 23S and 16S molecules. Comparing the positions of the radioactivity peaks with the 23S and 16S peaks of the total carrot leaf RNA preparation shows that the freshly made ribosomal RNA components migrate more slowly during electrophoresis than mature 23S and 16S rRNA.
This slower migration suggests that the newly synthesised molecules are precursors of the mature 23S and 16S ribosomal RNAs of chromoplasts and of amyloplasts (in the first stages of white carrot taproot formation).
The same pattern is known for chloroplasts: their newly synthesised 23S and 16S rRNA components also appear as precursors slightly larger than the mature 23S and 16S molecules. Moreover, many authors studying the fractional composition of newly synthesised chloroplast RNA in algae and higher plants have reported a common precursor for the 23S and 16S RNAs that migrates more slowly in polyacrylamide gel than the high-molecular-weight RNAs of plastid ribosomes.
The peak detected in the pre-ribosomal zone is therefore most likely a common precursor for the 23S and 16S RNAs of red carrot chromoplasts and of the forming amyloplasts of white carrot taproots.
How does RNA content compare between chromoplasts and amyloplasts?
Chromoplasts contain RNA whose amount varies with the plant's developmental phase, as established by these studies on carrot taproot plastids. The RNA content of red carrot chromoplasts is 2–3 times higher than their DNA content, and it is also noticeably higher than the RNA content of white carrot amyloplasts.
Chromoplasts show a strong capacity to synthesise their own ribonucleic acids in a system of isolated organelles, and this capacity depends on the developmental phase of the carrot plant. As in the chloroplasts of higher plants and algae, the main chromoplast RNA fractions are the ribosomal 23S and 16S species together with the low-molecular-weight 4–5S RNA.
The fractional composition of chromoplast RNA differs substantially from that of mature amyloplasts, whose total RNA is represented by a low-molecular-weight component migrating in the 4–5S region during electrophoresis in polyacrylamide gel — previously designated as 4.2S RNA.
The main components of newly synthesised chromoplast RNA are likewise those migrating in the 23S, 16S and 4–5S regions. These results give grounds to conclude that the ribosomal 23S and 16S RNAs of chromoplasts, as in plant chloroplasts, are synthesised as precursors somewhat larger than the mature 23S and 16S rRNA molecules. They also point to the possibility that the 23S and 16S rRNAs are produced as a common precursor migrating in the region between 23S rRNA and DNA.
