With the recent development of techniques for analyzing transmembrane thylakoid proteins by two-dimensional gel electrophoresis, systematic approaches for proteomic analyses of membrane proteins became feasible. In this study, we established detailed two-dimensional protein maps of Chlamydomonas reinhardtii light-harvesting proteins (Lhca and Lhcb) by extensive tandem mass spectrometric analysis. We predicted eight distinct Lhcb proteins. Although the major Lhcb proteins were highly similar, we identified peptides which were unique for specific lhcbm gene products. Interestingly, lhcbm6 gene products were resolved as multiple spots with different masses and isoelectric points. Gene tagging experiments confirmed the presence of differentially N-terminally processed Lhcbm6 proteins. The mass spectrometric data also revealed differentially N-terminally processed forms of Lhcbm3 and phosphorylation of a threonine residue in the N terminus. The N-terminal processing of Lhcbm3 leads to the removal of the phosphorylation site, indicating a potential novel regulatory mechanism. At least nine different lhca-related gene products were predicted by comparison of the mass spectrometric data against Chlamydomonas expressed sequence tag and genomic databases, demonstrating the extensive variability of the C. reinhardtii Lhca antenna system. Out of these nine, three were identified for the first time at the protein level. This proteomic study demonstrates the complexity of the light-harvesting proteins at the protein level in C. reinhardtii and will be an important basis of future functional studies addressing this diversity.In all eukaryotic oxygenic photosynthetic organisms, lightharvesting chlorophyll a-or b-binding proteins (LHC proteins) function in the collection and transfer of light energy to the reaction centers of photosystem II (PSII) (Lhcb proteins) and photosystem I (PSI) (Lhca proteins). Additionally these proteins are also involved in light dissipation and energy quenching. Therefore, light-harvesting proteins are important components of the photosynthetic machinery that optimize photosynthetic function and minimize photooxidative damage in response to light quantity and quality. It has been known for several years that light-harvesting proteins are products of many genes. This concept is illustrated by a recent analysis of the Arabidopsis genome which revealed that the lhc gene family is composed of more than 20 genes (24). Besides the large number of lhc gene products, posttranslational modifications, such as phophorylation, contribute to even more complexity at the protein level (31, 45). Phosphorylation of the major Lhcb proteins of PSII is important in the process of state transitions. This process leads to a redistribution of excitation energy between PSII and PSI by reorganization of the antennae and thereby regulates energy flow between the photosystems. The importance of phosphorylation for state transitions is shown by the phenotype of the Chlamydomonas reinhardtii Stt7 mutant. This mutant is markedly ...

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Active oxygen (AO) species generated in plants under stress conditions trigger degradation of Rubisco (EC 4.1.1.39). To ®nd out whether AO species activate proteases or make the protein susceptible to proteolysis, puri®ed and 14 C-labelled Rubisco protein was incubated with stromal preparations obtained from barley (Hordeum vulgare L.) leaves. The protein was degraded into distinct fragments only after a treatment with AO. This result shows that AO-treated Rubisco has been modi®ed to become a substrate for stromal protease(s) and dismisses the possibility of protease activation. Upon degradation, distinct fragments accumulated with time. The fragmentation pattern was indistinguishable from that obtained with intact chloroplasts subjected to oxidative conditions (cf. M. Desimone et al., 1996, Plant Physiol 111: 789±796). Degradation required ATP-hydrolysis, since AMP, ADP or non-hydrolysable ATPanalogs did not support proteolysis. The ClpP-de®cient stromal preparations degraded AO-modi®ed Rubisco, making the involvement of the ClpC/P protease unlikely.

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The D1-D2 heterodimer in the reaction center core of phototrophs binds the redox plastoquinone cofactors, Q A and Q B , the terminal acceptors of the photosynthetic electron transfer chain in the photosystem II (PSII). This complex is the target of the herbicide atrazine, an environmental pollutant competitive inhibitor of Q B binding, and consequently it represents an excellent biomediator to develop biosensors for pollutant monitoring in ecosystems. In this context, we have undertaken a study of the Chlamydomonas reinhardtii D1-D2 proteins aimed at designing site directed mutants with increased affinity for atrazine. The three-dimensional structure of the D1 and D2 proteins from C. reinhardtii has been homology modeled using the crystal structure of the highly homologous Thermosynechococcus elongatus proteins as templates. Mutants of D1 and D2 were then generated in silico and the atrazine binding affinity of the mutant proteins has been calculated to predict mutations able to increase PSII affinity for atrazine. The computational approach has been validated through comparison with available experimental data and production and characterization of one of the predicted mutants. The latter analyses indicated an increase of one order of magnitude of the mutant sensitivity and affinity for atrazine as compared to the control strain. Finally, D1-D2 heterodimer mutants were designed and selected which, according to our model, increase atrazine binding affinity by up to 20 kcal/mol, representing useful starting points for the development of high affinity biosensors for atrazine.

Sequence analysis of herbicide-resistant Chlamydomonas mutants revealed a new mutation in the psbA gene coding for the herbicide-binding (Dl) protein. A point mutation at codon 251 leading to an amino acid substitution from alanine in wild-type cells to valine in the mutant was identified by sequencing total cellular RNA. The folding pattern of Dl predicts Ala 251 to be part of the QB-binding niche.

Efficient chloroplast transformation systems now available allow the manipulation of the evolutionarily highly conserved psbA gene in the eucaryotic organism Chlamydomonas reinhardtii. Two copies of this gene in the inverted repeat region of the chloroplast genome contain four large group I introns. To analyse possible functions of these introns and to generate a mutant for simplified psbA gene manipulations, a psbA cDNA fragment was introduced into a psbA deletion mutant using the biolistic transformation method. A transformant with no introns in the psbA gene has been obtained and represents the first example of the removal of a complete set of introns from a chloroplast gene. The newly generated strain is photosynthetically competent and contains no detectable recipient genome copies. The loss of all four introns appears to be phenotypically silent.

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