Light-Induced Biogenesis of Light-Harvesting Complex I (LHC I) during Chloroplast Development in Barley (Hordeum vulgare) (Studies Using cDNA Clones of the 21- and 20-Kilodalton LHC I Apoproteins)

The light-harvesting complex (LHC) Ib pigment-proteins form the major component of the LHC I complex in higher plants. They comprise chlorophylls a and b, xanthophylls, and at least two polypeptide subunits of 21 and 20 kD in barley (Hordeum vulgare). We have identified two cDNA clones, LHC Ib-21 and LHC Ib-20, encoding the 21- and 20-kD LHC Ib apoproteins, respectively. N-terminal protein sequences of the purified LHC Ib polypeptides were used for the unequivocal correlation of these clones to their respective apoproteins. The cDNA clones encode two proteins that have strong sequence similarity to other LHC I and LHC II pigment-binding polypeptides of photosystems I and II. The 21-kD polypeptide contains 201 amino acid residues (22.14 kD), and the 20-kD polypeptide contains 200 amino acid residues (22.18 kD). The biogenesis of the LHC Ib apoproteins and the pigmented LHC I during the light-induced development of the chloroplast was studied. Accumulation of the two LHC Ib mRNAs is induced by light, and their amount is regulated by phytochrome. LHC Ib polypeptide accumulation in the thylakoid membrane temporally lags behind transcript accumulation. The rates of accumulation of LHC Ib transcripts and of their apoproteins lag behind those of the major LHC II component, LHC IIb. Complete assembly of the LHC Ib pigment-protein, as observed by low-temperature fluorescence spectroscopy, requires exposure of dark-grown seedlings to 72 h or more of light.