Characterization of chlorophyll a/b proteins of photosystem I from Chlamydomonas reinhardtii.

In this study we have isolated the chlorophyll a/b-binding proteins from a photosystem I preparation of the green alga Chlamydomonas reinhardtii and characterized them by N-terminal sequencing, fluorescence, and absorption spectroscopy and by immunochemical means. The results indicate that in this organism, the light-harvesting complex of photosystem I (LHCI) is composed of at least seven distinct polypeptides of which a minimum number of three are shown to bind chlorophyll a and b. Both sequence homology and immunological cross-reactivity with other chlorophyll-binding proteins suggest that all of the LHCI polypeptides bind pigments. Fractionation of LHCI by mildly denaturing methods showed that, in contrast to higher plants, the long wavelength fluorescence emission typical of LHCI (705 nm in C. reinhardtii) cannot be correlated with the presence of specific polypeptides, but rather with changes in the aggregation state of the LHCI components. Reconstitution of both high aggregation state and long wavelength fluorescence emission from components that do not show these characteristics confirm this hypothesis.

Directed chloroplast transformation in Chlamydomonas reinhardtii: insertional inactivation of the psaC gene encoding the iron sulfur protein destabilizes photosystem I.

The chloroplast gene psaC encoding the iron sulfur protein of photosystem I (PSI) from the green alga Chlamydomonas reinhardtii has been cloned and characterized. The deduced amino acid sequence is highly related to that of higher plants and cyanobacteria. Using a particle gun, wild type C. reinhardtii cells have been transformed with a plasmid carrying the psaC gene disrupted by an aadA gene cassette designed to express spectinomycin/streptomycin resistance in the chloroplast. Transformants selected on plates containing acetate as a reduced carbon source and spectinomycin are unable to grow on minimal medium lacking acetate and are deficient in PSI activity. Southern blot analysis of total cell DNA of the transformants shows that the wild type psaC gene has been replaced by the interrupted psaC gene through homologous recombination. While authentic transcripts of the psaC gene are no longer detected, aadA gives rise to a few transcripts in the transformants. Biochemical analysis indicates that neither PSI reaction center subunits nor the seven small subunits belonging to PSI accumulate stably in the thylakoid membranes of the transformants. Pulse-chase labeling of cell proteins shows that the PSI reaction center subunits are synthesized normally but turn over rapidly in the transformants. We conclude that the iron sulfur binding protein encoded by the psaC gene is an essential component, both for photochemical activity and for stable assembly of PSI. The present study suggests that any chloroplast gene encoding a component of the photosynthetic apparatus can be disrupted in C. reinhardtii using the strategy described.