Structure and expression of cytochrome f in an Oenothera plastome mutant

Coordination of nuclear and chloroplast gene expression in plant cells

Plastid proteins are encoded in two genomes, one in the nucleus and the other in the organelle. The expression of genes in these two compartments in coordinated during development and in response to environmental parameters such as light. Two converging approaches reveal features of this coordination: the biochemical analysis of proteins involved in gene expression, and the genetic analysis of mutants affected in plastid function or development. Because the majority of proteins implicated in plastid gene expression are encoded in the nucleus, regulatory processes in the nucleus and in the cytoplasm control plastid gene expression, in particular during development. Many nucleus-encoded factors involved in transcriptional and posttranscriptional steps of plastid gene expression have been characterized. We are also beginning to understand whether and how certain developmental or environmental signals perceived in one compartment may be transduced to the other.

Detection of point mutations in chloroplast genes of Antirrhinum majus L. I. Identification of a point mutation in the psaB gene of a photosystem I plastome mutant

A point mutation in the plastome-encoded psaB gene of the mutant en:alba-1 of Antirrhinum majus L. was identified by an analysis of chloroplast DNA with a modified PCR-SSCP technique. Application of this technique is indicated when a gene or a group of genes is known in which the point mutation is located. Analysis of primary photosynthetic reactions in the yellowish white plastome mutant indicated a dysfunction of photosystem (PS) I. The peak wavelength of PS I-dependent chlorophyll (Chl) fluorescence emission at 77 K was shifted by 4 nm to 730 nm, as compared to fluorescence from wild-type. There were no redox transients of the reaction center Chl P700 upon illumination of leaves with continuous far-red light or with rate-saturating flashes of white light. The PS I reaction center proteins PsaA and PsaB are not detectable by SDS-PAGE in mutant plastids. Hence, plastome encoded PS I genes were regarded as putative sites of mutation. In order to identify plastome mutations we developed a modified SSCP (single-strand conformation polymorphism) procedure using a large PCR fragment which can be cleaved with various restriction enzymes. When DNA from wild-type and en:alba-1 was submitted to SSCP analysis, a single stranded HinfI fragment of a PCR product of the psaB gene showed differences in electrophoretic mobility. Sequence analysis revealed that the observed SSCP was caused by a single base substitution at codon 136 (TAT-->TAG) of the psaB gene. The point mutation produces a new stop codon that leads to a truncated PsaB protein. The results presented indicate that the mutation prevents the assembly of a functional PS I complex. The applicability to other plastome mutants of the new method for detection of point mutations is discussed.

Maturation of pre-apocytochrome f in vivo. A site-directed mutagenesis study in Chlamydomonas reinhardtii

The biosynthesis of cytochrome f is a multistep process which requires processing of the precursor protein and covalent ligation of a c-heme upon membrane insertion of the protein. The crystal structure of a soluble form of cytochrome f has revealed that one axial ligand of the c-heme is provided by the alpha-amino group of Tyr1 generated upon cleavage of the signal sequence from the precursor protein (Martinez S. E., Huang D., Szczepaniak A., Cramer W.A., and Smith J. L. (1994) Structure 2, 95-105). We therefore investigated, by site-directed mutagenesis, the possible interplay between protein processing and heme attachment to cytochrome f in Chlamydomonas reinhardtii. These modifications were performed by chloroplast transformation using a petA gene encoding the full-length precursor protein and also a truncated version lacking the C-terminal membrane anchor. We first substituted the two cysteinyl residues responsible for covalent ligation of the c-heme, by a valine and a leucine, and showed that heme binding is not a prerequisite for cytochrome f processing. In another series of experiments, we replaced the consensus cleavage site for the thylakoid processing peptidase, AQA, by an LQL sequence. The resulting transformants were nonphototrophic and displayed delayed processing of the precursor form of cytochrome f, but nonetheless both the precursor and processed forms showed heme binding and assembled in cytochrome b6f complexes. Thus, pre-apocytochrome f adopts a suitable conformation for the cysteinyl residues to be substrates of the heme lyase and pre-holocytochrome f folds in an assembly-competent conformation. In the last series of experiments, we compared the rates of synthesis and degradation of the various forms of cytochrome f in the four types of transformants under study: (i) the C terminus membrane anchor apparently down-regulates the rate of synthesis of cytochrome f and (ii) degradation of misfolded forms of cytochrome f occurs by a proteolytic system intimately associated with the thylakoid membranes.

Cytochrome f: Structure, function and biosynthesis

Cytochrome f is an intrinsic membrane component of the cytochrome bf complex, transferring electrons from the Rieske FeS protein to plastocyanin in the thylakoid lumen. The protein is held in the thylakoid membrane by a single transmembrane span located near its C-terminus with a globular hydrophilic domain extending into the lumen. The globular domain of the turnip protein has recently been crystallised, offering the prospect of a detailed three-dimensional structure. Reaction with plastocyanin involves localised positive charges on cytochrome f interacting with the acidic patch on plastocyanin and electron transfer via the surface-exposed tyrosine residue (Tyr83) of plastocyanin. Apocytochrome f is encoded in the chloroplast genome and is synthesised with an N-terminal presequence which targets the protein to the thylakoid membrane. The synthesis of cytochrome f is coordinated with the synthesis of the other subunits of the cytochrome bf complex.

A plastome mutation affects processing of both chloroplast and nuclear DNA-encoded plastid proteins

Immunoblotting of a chloroplast mutant (pm7) of Oenothera showed that three proteins, cytochrome f and the 23 kDa and 16 kDa subunits of the oxygen-evolving subcomplex of photosystem II, were larger than the corresponding mature proteins of the wild type and, thus, appear to be improperly processed in pm7. The mutant is also chlorotic and has little or no internal membrane development in the plastids. The improperly processed proteins, and other proteins that are completely missing, represent products of both the plastid and nuclear genomes. To test for linkage of these defects, a green revertant of pm7 was isolated from cultures in which the mutant plastids were maintained in a nuclear background homozygous for the plastome mutator (pm) gene. In this revertant, all proteins analyzed co-reverted to the wild-type condition, indicating that a single mutation in a plastome gene is responsible for the complex phenotype of pm7. These results suggest that the defect in pm7 lies in a gene that affects a processing protease encoded in the chloroplast genome.