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Title: ACTIVE PHOTOSYNTHESIS IN SITE-DIRECTED MUTANTS OF ANABAENA VARIABILIS ATCC 29413 WITH MODIFICATIONS IN THE LIGANDS FOR THE IRON-SULFUR CLUSTERS IN THE PSAC PROTEIN OF PHOTOSYSTEM I

Author
item MANNAN, R - WASHINGTON UNIV, ST LOUIS
item HE, WEI - U OF CALIFORNIA, BERKELY
item METZGER, SABINE - UNIV OF ILLINOIS URBANA
item WHITMARSH, CLIFFORD
item MALKIN, RICHARD - U OF CALIFORNIA, BERKELEY
item PAKRASI, HIMADRI - WASHINGTON UNIV, ST LOUIS

Submitted to: European Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/1995
Publication Date: N/A
Citation: N/A

Interpretive Summary: During oxygenic photosynthesis in green plants, algae, and cyanobacteria, two pigment-protein complexes, photosystem I and photosystem II, convert light energy into chemical energy and mediate the transfer of electrons from water to carbon dioxide. Within the photosystem I complex, a small protein binds the two iron-sulfur clusters that act as electron carriers. Recent progress in the development of a gene replacement system in this organism has allowed us to create site-directed mutant that is lacking one of the electron carriers. We show that photosynthesis is active in the absence of the carrier. This work will help scientists establish rate-limiting steps in photosynthetic energy conversion. Understanding regulatory steps in photosynthesis is essential for developing genetically engineered plants designed to perform well under a wide range of environmental conditions.

Technical Abstract: PsaC is an extrinsic protein associated with the Photosystem I membrane protein complex of cyanobacteria, algae and plants. PsaC coordinates two 4Fe-4S clusters and is essential for photosynthetic growth. To elucidate the role of the Fe-S clusters in electron transfer and in the assembly of photosystem I we used site-directed mutagenesis to create two mutant strains of the cynaobacterium Anabaena variabilis in which a cysteine ligand was changed to aspartate. Both mutants grow photoautotrophically and the maximum rate of light-induced electron transport is comparable to wild-type. However, comparison of electron transfer rates at low light levels indicate that the relative quantum yield of one of the mutants is lower than that of the wild-type strains. The data one of the clusters is absent in and therefore is not essential for the assembly of the PsaC protein within the photosystem I complex. The fact that the mutant can grow photoautotrophically and maintain high rates of electron transfer indicate that the cluster is not required for electron transfer to ferredoxin.