Diurnal oscillations in steady-state mRNA levels and transcription rates were measured for seven transcripts (five of which encode chloroplast-localized proteins) in tomato seedlings: photosystem I and photosystem II chlorophyll a/b binding proteins (CAB/I and CAB/II), small subunit of RuBisCO (RBCS), actin, subunit II of the photosystem I reaction center (PSAD), subunit I of the photosystem II oxygen-evolving enzyme (OEE1), and a biotin-binding protein of unknown function. CAB/II mRNA levels were found to oscillate greater than 20-fold, showing a peak at noon, while only marginal diurnal oscillations are seen in RBCS transcripts. The oscillations are at least partially controlled at the transcriptional level. Transcription rates of both CAB/II and RBCS, measured by nuclear run-on experiments, were found to oscillate, with a peak around 8 a.m. Transcription rates of the 'biotin' clone also oscillated, with a peak around noon. Transfer of plants to constant darkness or constant light conditions alters the amplitude of the transcriptional oscillation, but does not abolish it, suggesting that it is at least partially controlled by a circadian clock. The oscillations are still visible after three days in complete darkness, and have a period very close to 24 h. The oscillator phase can be reset by out-of-phase light treatment.

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