CIRCADIAN OSCILLATIONS OF NUCLEAR-ENCODED CHLOROPLAST PROTEINS IN PEA (PISUM-SATIVUM)

The diurnal and circadian expression of light-inducible chloroplast proteins, i.e. light-harvesting chlorophyll a/b protein (LHCP), early light-inducible protein (ELIP) and Fe-S Rieske, has been studied in young pea plantlets at the level of mRNA integrated into polysomal complexes and at the level of proteins. Under light-dark as well as constant light conditions the levels of the three nuclear-encoded chloroplast proteins oscillate while the investigated plastid-encoded proteins, large subunit of ribulose-1,5-bisphosphate carboxylase (LSU), reaction center protein D1 and cytochrome f, do not show oscillations at the protein level. The levels of the nuclear-encoded polysome-bound mRNAs fluctuate in parallel with the changes in the levels of poly(A) RNA which were described previously. Under constant light conditions the oscillation at the level of polysomal bound mRNA is readily dampened while the steady-state levels of the investigated nuclear-encoded proteins still fluctuate. We conclude that the extent of expression of the genes for the nuclear-encoded chloroplast proteins studied is controlled by a circadian oscillator primarily, but not exclusively, at the level of transcription.