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

Abstract

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 ;100 amino acid residues that could potentially provide ligands for [4Fe-4S] clusters. APO1 is essentially required for photoautotrophic growth, and levels of PSI core subunits are below the limit of detection in the apo1 mutant. Unlike other Arabidopsis PSI mutants, apo1 fails to accumulate significant amounts of the outer antenna subunits of PSI and PSII and to form grana stacks. In particular, APO1 is essentially required for stable accumulation of other plastid-encoded and nuclear-encoded [4Fe-4S] cluster complexes within the chloroplast, whereas [2Fe-2S] cluster-containing complexes appear to be unaffected. In vivo labeling experiments and analyses of polysome association suggest that translational elongation of the PSI transcripts psaA and psaB is specifically arrested in the mutant. Taken together, our findings suggest that APO1 is involved in the stable assembly of several [4Fe-4S] cluster-containing complexes of chloroplasts and interferes with translational events probably in association with plastid nucleoids.

Regulatory factors for the assembly of thylakoid membrane protein complexes

“…By contrast, APO1 is conserved only among higher plants. In the apo1 Arabidopsis mutant, levels of the PSI core subunits as well as the intrinsic and peripheral PSI subunits are reduced [82]. APO1 had been proposed to be required for the accumulation of [4Fe-4S]-cluster-containing chloroplast complexes and antenna proteins.…”

“…HCF101 (high chlorophyll fluorescence 101) and APO1 (accumulation of PSI 1), two chloroplast proteins, have been genetically identified as essential and specific factors for the assembly of [4Fe -4S]-clustercontaining protein complexes, including PSI complexes [46,82]. HCF101 was identified from the seeding-lethal Arabidopsis mutant hcf101 in which PSI activity is abolished [46].…”

APO1 Promotes the Splicing of Chloroplast Group II Introns and Harbors a Plant-Specific Zinc-Dependent RNA Binding Domain

“…The apo1-1 allele was described originally by Amann et al (2004) and has a T-DNA insertion at +32 bp with respect to the start codon. This allele is in a Wassilewskija ecotype background.…”

“…This sequence was aligned with all members of the rice (Oryza sativa) and Arabidopsis thaliana APO family (as defined in Amann et al, 2004) using Muscle Introns are designated as Subgroup IIA or Subgroup IIB according to Michel et al (1989). Introns found in Arabidopsis but not in maize are marked with an asterisk.…”

Chloroplastic iron-sulfur scaffold protein NFU3 is essential to overall plant fitness

“…4 Over-accumulation of reduced Q A and increases in the redox state of the PSII acceptor side could be caused by a defect in intersystem electron transport or PSI. [11][12][13] The near-complete loss of PSI activity in the nfu3 mutants strongly suggests that these observations are initially caused by the defects in PSI. In summary, these data provide additional support that, in the nfu3 mutants, PSI suffers greatly from the lack of the 4Fe-4S cluster and the reduction in PSII activity is likely a secondary effect of the near-complete loss of PSI activity.…”