N-terminal mutants of chloroplast cytochrome f. Effect on redox reactions and growth in Chlamydomonas reinhardtII.

The N-terminal tyrosine of cytochrome f, which provides the sixth ligand to the heme group, has been changed by site-directed mutagenesis in Chlamydomonas reinhardtii to evaluate the role of this amino acid in assembly and function. The second and third residues, proline and valine, respectively, have also been mutated. Y1P is the only strain that did not grow photoautotrophically. The other strains show cytochrome b6f complex/photosystem I reaction center chlorophyll, photosystem I unit size and chlorophyll a+b/cell ratios comparable with wild-type cells. Rates of cytochrome f photooxidation in all strains were similar (t1/2 approximately = 300 microsec), whereas the rate of re-reduction sensitive to stigmatellin (at Eh = 0 mV, (where Eh is the ambient redox potential) for wild-type, Y1W, Y1F, Y1S, P2V, and V3P had a tl/2 of 3, 4, 5, 9, 40, and 2 ms, respectively. Rates of oxygen evolution by whole cells of P2V, Y1F, and Y1S were 67, 80, and 80% of wild-type rates, respectively. At low light intensity, all competent strains had the same growth rate whereas at saturating intensities, only P2V showed a significant inhibition. These results are considered in relation to structure-function relationships in the cytochrome f molecule.