Knock-out of the chloroplast-encoded PSI-J subunit of photosystem I in Nicotiana tabacum.

The plastid-encoded psaJ gene encodes a hydrophobic low-molecular-mass subunit of photosystem I (PSI) containing one transmembrane helix. Homoplastomic transformants with an inactivated psaJ gene were devoid of PSI-J protein. The mutant plants were slightly smaller and paler than wild-type because of a 13% reduction in chlorophyll content per leaf area caused by an approximately 20% reduction in PSI.

ACCUMULATION OF PHOTOSYSTEM ONE1, a member of a novel gene family, is required for accumulation of [4Fe-4S] cluster-containing chloroplast complexes and antenna proteins.

To investigate the nuclear-controlled mechanisms of [4Fe-4S] cluster assembly in chloroplasts, we selected Arabidopsis thaliana mutants with a decreased content of photosystem I (PSI) containing three [4Fe-4S] clusters. One identified gene, ACCUMULATION OF PHOTOSYSTEM ONE1 (APO1), belongs to a previously unknown gene family with four defined groups (APO1 to APO4) only found in nuclear genomes of vascular plants. All homologs contain two related motifs of approximately 100 amino acid residues that could potentially provide ligands for [4Fe-4S] clusters.

The universally conserved HCF101 protein is involved in assembly of [4Fe-4S]-cluster-containing complexes in Arabidopsis thaliana chloroplasts.

The seedling-lethal nuclear Arabidopsis hcf101 (high chlorophyll fluorescence) mutant is impaired in photosynthesis and complemented by the wild-type HCF101 cDNA. Photosystem I (PSI) activity is abolished, and PSI core complexes fail to accumulate in hcf101, whereas levels of other thylakoid membrane proteins are unaffected. Northern and in vivo labelling analyses as well as studies on polysome loading show that PSI transcript levels and translation rates of proteins, which belong to PSI, are normal in hcf101.

A peptide chain release factor 2 affects the stability of UGA-containing transcripts in Arabidopsis chloroplasts.

Positional cloning of the hcf109 (high chlorophyll fluorescence) mutation in Arabidopsis has identified a nucleus-encoded, plastid-localized release factor 2-like protein, AtprfB, indicating that the processes of translational termination in chloroplasts resemble those of eubacteria. Control of atprfB expression by light and tissues is connected to chloroplast development. A point mutation at the last nucleotide of the second intron causes a new splice site farther downstream, resulting in a deletion of seven amino acid residues in the N-terminal region of the Hcf109 protein.