Genetic engineering of thylakoid protein complexes by chloroplast transformation in Chlamydomonas reinhardtii

Targeted random mutagenesis to identify functionally important residues in the D2 protein of photosystem II in Synechocystis sp. strain PCC 6803

To identify important residues in the D2 protein of photosystem II (PSII) in the cyanobacterium Synechocystis sp. strain PCC 6803, we randomly mutagenized a region of psbDI (coding for a 96-residue-long C-terminal part of D2) with sodium bisulfite. Mutagenized plasmids were introduced into a Synechocystis sp. strain PCC 6803 mutant that lacks both psbD genes, and mutants with impaired PSII function were selected. Nine D2 residues were identified that are important for PSII stability and/or function, as their mutation led to impairment of photoautotrophic growth. Five of these residues are likely to be involved in the formation of the Q(A)-binding niche; these are Ala249, Ser254, Gly258, Ala260, and His268. Three others (Gly278, Ser283, and Gly288) are in transmembrane alpha-helix E, and their alteration leads to destabilization of PSII but not to major functional alterations of the remaining centers, indicating that they are unlikely to interact directly with cofactors. In the C-terminal lumenal tail of D2, only one residue (Arg294) was identified as functionally important for PSII. However, from the number of mutants generated it is likely that most or all of the 70 residues that are susceptible to bisulfite mutagenesis have been altered at least once. The fact that mutations in most of these residues have not been picked up by our screening method suggests that these mutations led to a normal photoautotrophic phenotype. A novel method of intragenic complementation in Synechocystis sp. strain PCC 6803 was developed to facilitate genetic analysis of psbDI mutants containing several amino acid changes in the targeted domain. Recombination between genome copies in the same cell appears to be much more prevalent in Synechocystis sp. strain PCC 6803 than was generally assumed.

Site-directed mutagenesis and analysis of second-site revertants indicates a requirement for C-terminal amino acids of PsaB for stable assembly of the photosystem I reaction center complex in Chlamydomonas reinhardtii

The PsaA and PsaB polypeptides form the reaction center core heterodimer of photosystem I (PSI). Both PsaA and PsaB are predicted to have 11 hydrophobic domains, although it is unclear how both polypeptides fold within the thylakoid membrane. If all 11 hydrophobic regions form membrane-spanning domains, the N- and C-terminus must be located on opposite sides of the membrane. The C-terminus of PsaB is very conserved in a wide range of organisms and may be important for PSI assembly or function. Using chloroplast transformation in Chlamydomonas reinhardtii we have generated a series of C-terminal extension and deletion mutants of the PsaB polypeptide. Analysis of these mutants and spontaneous revertants indicates that the C-terminus may be extended by at least 14 amino acids without impairing PSI assembly. Deletion of amino acids 732-736 also has no impact on PSI, whereas deletion of amino acids 727-736 results in no accumulation of the complex. The site of truncation in the 727-736 deletion coincides with the end of the hydrophobic domain XI supporting a location of the C-terminus of PsaB on the lumenal side of PSI.

THE CHLOROPHYLL-CAROTENOID PROTEINS OF OXYGENIC PHOTOSYNTHESIS

The chlorophyll-carotenoid binding proteins responsible for absorption and conversion of light energy in oxygen-evolving photosynthetic organisms belong to two extended families: the Chl a binding core complexes common to cyanobacteria and all chloroplasts, and the nuclear-encoded light-harvesting antenna complexes of eukaryotic photosynthesizers (Chl a/b, Chl a/c, and Chl a proteins). There is a general consensus on polypeptide and pigment composition for higher plant pigment proteins. These are reviewed and compared with pigment proteins of chlorophyte, rhodophyte, and chromophyte algae. Major advances have been the determination of the structures of LHCII (major Chl a/b complex of higher plants), cyanobacterial Photosystem I, and the peridinen-Chl a protein of dinoflagellates to atomic resolution. Better isolation methods, improved transformation procedures, and the availability of molecular structure models are starting to provide insights into the pathways of energy transfer and the macromolecular organization of thylakoid membranes.

Oxygenic photosynthesis. Electron transfer in photosystem I and photosystem II

Photosystems I and II drive oxygenic photosynthesis. This requires biochemical systems with remarkable properties, allowing these membrane-bound pigment-protein complexes to oxidise water and produce NAD(P)H. The protein environment provides a scaffold in the membrane on which cofactors are placed at optimum distance and orientation, ensuring a rapid, efficient trapping and conversion of light energy. The polypeptide core also tunes the redox potentials of cofactors and provides for unidirectional progress of various reaction steps. The electron transfer pathways use a variety of inorganic and organic cofactors, including amino acids. This review sets out some of the current ideas and data on the cofactors and polypeptides of photosystems I and II.