Rapid isolation and purification of photosystem I chlorophyll-binding protein from Chlamydomonas reinhardtii.

The available procedures for isolation and purification of photosystem I (PSI) from Chlamydomonas reinhardtii are time consuming and usually require several hours of sucrose gradient ultracentrifugation steps. This may lead to structural and functional impairment, including release of pigments and/or dissociation of protein subunits. Moreover, it is difficult to isolate intact complexes from thylakoids containing mutated PSI that accumulate to lower levels. Hence, isolation of intact PSI core complex depends on the speed of the procedure and the mildness of the extraction and purification. We have, therefore, modified the procedure for PSI isolation to both increase the yield of PSI and to reduce contamination by other pigment protein complexes. The modified procedure involves dodecyl maltoside solubilization of crude-thylakoid membranes followed by single-step column chromatography using a weak anion-exchanger. PSI eluted from the column between 13 mM and 15 mM Mg S04. This new rapid purification procedure yielded pure PSI preparations with a Chl/P700 ratio of approx 90 and showing typical absorption difference spectra with a maximum bleaching occurring at 696 nm. Femtosecond transient absorption spectroscopy of purified PSI complex revealed a high degree of similarity in terms of excitation energy transfer within the PSI core to observations in cyanobacterial PSI.

A simple method for chloroplast transformation in Chlamydomonas reinhardtii.

Photosystem (PS)I is a multi-subunit pigment-protein complex that uses light energy to transfer electrons from plastocyanin to ferredoxin. Application of genetic engineering to photo-synthetic reaction center proteins has led to a significant advancement in our understanding of primary electron transfer events and the role of the protein environment in modulating these processes. Chlamydomonas reinhardtii provides a system particularly amenable to analyze the structure-function relationship of PSI. Chlamydomonas reinhardtii is also a favorable organism for chloroplast transformation because it contains a single chloroplast and grows heterotrophically when supplemented with acetate. Chlamydomonas has served as a model organism for the development of chloroplast transformation procedures and the study of photosynthetic mutants generated using this method. Exogenous cloned cpDNA can be introduced into the chloroplast by using this biolistic gene gun method. DNA-coated tungsten or gold particles are bombarded onto cells. Upon its entry into chloroplasts, the transforming DNA is released from the particles and integrated into the chloroplast genome through homologous recombination. The most versatile chloroplast selectable marker is aminoglycoside adenyl transferase (aadA), which can be expressed in the chloroplast to confer resistance to spectinomycin or streptomycin. This chapter describes the procedures for chloroplast transformation.