To understand the regulatory mechanisms underlying the biogenesis of photosystem II (PSII) we have characterized the nuclear mutant hcf136 of Arabidopsis thaliana and isolated the affected gene. The mutant is devoid of any photosystem II activity, and none of the nuclear‐ and plastome‐encoded subunits of this photosystem accumulate to significant levels. Protein labelling studies in the presence of cycloheximide showed that the plastome‐encoded PSII subunits are synthesized but are not stable. The HCF136 gene was isolated by virtue of its T‐DNA tag, and its identity was confirmed by complementation of homozygous hcf136 seedlings. Immunoblot analysis of fractionated chloroplasts showed that the HCF136 protein is a lumenal protein, found only in stromal thylakoid lamellae. The HCF136 protein is produced already in dark‐grown seedlings and its levels do not increase dramatically during light‐induced greening. This accumulation profile confirms the mutational data by showing that the HCF136 protein must be present when PSII complexes are made. HCF136 homologues are found in the cyanobacterium Synechocystis species PCC6803 (slr2034) and the cyanelle genome of Cyanophora paradoxa (ORF333), but are lacking in the plastomes of chlorophytes and metaphytes as well as from those of rhodo‐ and chromophytes. We conclude that HCF136 encodes a stability and/or assembly factor of PSII which dates back to the cyanobacterial‐like endosymbiont that led to the plastids of the present photosynthetic eukaryotes.

Thirty-four recessive photosynthetic mutants of the high-chlorophyll-fluorescence (hcf) phenotype have been isolated by screening 7700 M2 progenies of ethyl methane sulfonate-treated seeds ofArabidopsis thaliana. Most of the mutants isolated were found to be seedlinglethal, but could be grown on sucrose-supplemented media. Chlorophyll (Chl) fluorescence induction, absorption changes in the reaction-centre chlorophyll of PS I (P700) at 830 nm and Chla/Chlb ratios were recorded in order to probe the photosynthetic functions and to define the mutational lesion. These studies were complemented by immunoblot and Northern analyses which finally led to the classification of the mutants into six different groups. Four classes of mutants were affected in PS I, PS II (two different classes) or the intersystem electron-transport chain, respectively. A fifth mutant class was of pleiotropic nature and the sixth class comprised a Chlb-deficient mutant. Several of the mutants showed severe deficiencies in the levels of subunits of PS I, PS II or the cytochromeb ₆/f complex. Thus the mutational lesions could be located precisely. Only one mutant was defective in the transcript patterns of some plastid-encoded photosynthesis genes. Hence most of the mutants isolated appear to be affected in translational and post-translational regulatory processes of thylakoid membrane biogenesis or in structural genes encoding constituent subunits of the thylakoid protein complexes.

Phylloquinone is a compound present in all photosynthetic plants serving as cofactor for Photosystem I-mediated electron transport. Newly identified seedling-lethal Arabidopsis thaliana mutants impaired in the biosynthesis of phylloquinone possess reduced Photosystem I activity. The affected gene, called PHYLLO, consists of a fusion of four previously individual eubacterial genes, menF, menD, menC, and menH, required for the biosynthesis of phylloquinone in photosynthetic cyanobacteria and the respiratory menaquinone in eubacteria. The fact that homologous men genes reside as polycistronic units in eubacterial chromosomes and in plastomes of red algae strongly suggests that PHYLLO derived from a plastid operon during endosymbiosis. The principle architecture of the fused PHYLLO locus is conserved in the nuclear genomes of plants, green algae, and the diatom alga Thalassiosira pseudonana. The latter arose from secondary endosymbiosis of a red algae and a eukaryotic host indicating selective driving forces for maintenance and/or independent generation of the composite gene cluster within the nuclear genomes. Besides, individual menF genes, encoding active isochorismate synthases (ICS), have been established followed by splitting of the essential 3′ region of the menF module of PHYLLO only in genomes of higher plants. This resulted in inactivation of the ICS activity encoded by PHYLLO and enabled a metabolic branch from the phylloquinone biosynthetic route to independently regulate the synthesis of salicylic acid required for plant defense. Therefore, gene fusion, duplication, and fission events adapted a eubacterial multienzymatic system to the metabolic requirements of plants.

Summary: 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. PSI‐specific fluorescence at 77 K is shifted from 735 to 728 nm in hcf101, indicating that exitons cannot efficiently be transferred to the PSI reaction centre, whereby the PSI antenna is almost unaffected. Mutant plants not only fail to accumulate mature PSI, which contains three [4Fe‐4S]clusters (FSCs), but also are characterized by reduced levels of the soluble FSC‐containing complex ferredoxin‐thioredoxin reductase (FTR) in the stroma. Inhibited FTR maturation is not a secondary effect stemming from lack of PSI because the mutant hcf145, which also lacks PSI, accumulates FTR at normal levels. Levels of the [2Fe‐2S] cluster‐containing soluble and membrane proteins, ferredoxin and PetC, respectively, were unchanged in hcf101 plants. These data suggest a specific role of HCF101 in FSC biogenesis. HCF101 is plastid localized and belongs to an ancient and universally conserved family of P‐loop ATPases previously designated as the ‘MRP’ (metGrelated protein) family. The function identified for HCF101 suggests a new designation, FSC, for this family.