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Insertion of the N-terminal Part of PsaF from Chlamydomonas reinhardtii into Photosystem I from Synechococcus elongatus Enables Efficient Binding of Algal Plastocyanin and Cytochrome c 6 *

M. Hippler,F. Drepper,J. Rochaix,U. Mühlenhoff

Published 1999 in Journal of Biological Chemistry

ABSTRACT

A strain of the cyanobacteriumSynechococcus elongatus was generated that expresses a hybrid version of the photosystem I subunit PsaF consisting of the first 83 amino acids of PsaF from the green alga Chlamydomonas reinhardtii fused to the C-terminal portion of PsaF from S. elongatus. The corresponding modified gene was introduced into the genome of the psaF-deletion strain FK2 by cointegration with an antibiotic resistance gene. The transformants express a new PsaF subunit similar in size to PsaF from C. reinhardtiithat is assembled into photosystem I (PSI). Hybrid PSI complexes isolated from these strains show an increase by 2 or 3 orders of magnitude in the rate of P700+ reduction by C. reinhardtii cytochrome c 6 or plastocyanin in 30% of the complexes as compared with wild type cyanobacterial PSI. The corresponding optimum second-order rate constants (k 2 = 4.0 and 1.7 × 107 m 1 s1 for cytochromec 6 and plastocyanin) are similar to those of PSI from C. reinhardtii. The remaining complexes are reduced at a slow rate similar to that observed with wild type PSI fromS. elongatus and the algal donors. At high concentrations of C. reinhardtii cytochrome c 6, a fast first-order kinetic component (t 1 2 = 4 μs) is revealed, indicative of intramolecular electron transfer within a complex between the hybrid PSI and cytochromec 6. This first-order phase is characteristic for P700+ reduction by cytochromec 6 or plastocyanin in algae and higher plants. However, a similar fast phase is not detected for plastocyanin. Cross-linking studies show that, in contrast to PSI from wild typeS. elongatus, the chimeric PsaF of PSI from the transformed strain cross-links to cytochrome c 6 or plastocyanin with a similar efficiency as PsaF from C. reinhardtii PSI. Our data indicate that development of a eukaryotic type of reaction mechanism for binding and electron transfer between PSI and its electron donors required structural changes in both PSI and cytochrome c 6 or plastocyanin.

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