Structure and expression of a nuclear gene for the PSI-D subunit of photosystem I in Nicotiana sylvestris.

The PSI-D subunit is the ferredoxin-binding site of photosystem I, and is encoded by the nuclear gene psaD. We isolated a psaD genomic clone from Nicotiana sylvestris, by screening a genomic library with a psaD cDNA which we previously cloned from N. sylvestris (Yamamoto et al., Plant Mol Biol 17: 1251, 1991). Nucleotide sequence analysis revealed that this genomic clone contains a psaD gene, which does not correspond to the psaD cDNA, so we designated these genes psaDb and psaDa, respectively. The psaDb clone encodes a protein of 214 amino acids uninterrupted by introns. The N-terminal sequence determined for the N. sylvestris PSI-D protein encoded by psaDb begins at the 49th residue. The products of psaDa and psaDb share 82.7% and 79.5% identity at the amino acid and nucleotide levels, respectively. Genomic Southern analysis showed that two copies of psaD are present in the N. sylvestris genome. Ribonuclease protection assays and immunoblot analysis in N. sylvestris indicate that both genes are expressed in leaves, stems and flower buds, but neither is expressed in roots. During leaf development, the ratio of psaDb to psaDa mRNA increases from 0.12 in leaf buds to 0.36 in mature leaves. The relative abundance of the corresponding proteins decreased over the same developmental period. These results indicate that differential regulation mechanisms control psaDa and psaDb expression at both the mRNA and protein levels during leaf development.