Transcriptional and post-transcriptional processes regulate expression of RNA encoding the small subunit of ribulose-1,5-biphosphate carboxylase differently in petunia and in soybean.

The effects of white light, far-red light and darkness on the in vitro transcription and RNA levels of the small subunit of ribulose-1,5-bisphosphate carboxylase (rbcS) were investigated in petunia and in soybean. In petunia plants treated with 48 hours of darkness the in vitro transcription rate of two of the rbcS subfamilies of petunia, rbcS A and rbcS C, declined 32- and 8-fold respectively, whereas treatment of dark-adapted plants with light caused the in vitro transcription rate of these subfamilies to return to their light-grown levels. Relative RNA levels of rbcS A and rbcS C declined in parallel with in vitro transcription rate changes upon treatment of petunia plants with darkness. However, while relative RNA levels of rbcS C changed in parallel with in vitro transcription rate under all conditions of far-red light and white light tested, there were differences between the changes in rbcS A in vitro transcription rate and RNA levels which were consistent with post-transcriptional regulation of rbcS A RNA. In addition we observed that nuclei isolated from the leaves of plants which were exposed to darkness for periods of 72 hours or longer were transcriptionally inactive. Similar experiments on the in vitro transcription and relative levels of the rbcS RNA in soybean seedlings have lead to the hypothesis that rbcS RNA is less stable in light than in darkness. In contrast, small decreases in rbcS in vitro transcription rate in mature soybean plants treated with darkness were accompanied by large decreases in rbcS RNA, suggesting that rbcS RNA was degraded more rapidly in darkness than in light in these plants. We have shown that differences in the modulation of rbcS RNA levels by post-transcriptional mechanisms exist between plants which belong to different orders, and between different developmental states of the same plant species.