On the cover: PSI catalyzes oxidation of plastocyanin or cytochrome c6 and reduction of ferredoxin in the electron transfer system of oxygenic photosynthesis using light energy. PSI forms a large complex consisting of a number of subunits and cofactors. The function and architecture of the PSI complex have been extensively characterized, but the molecular mechanism of PSI complex assembly remains elusive. It was reported in 1997 that the chloroplast-encoded Ycf4 is essential for PSI complex assembly, whereas it remains elusive how Ycf4 functions in PSI complex assembly. In this issue, Onishi and Takahashi (pp. 1750-1760) introduced site-directed mutations in the chloroplast-encoded ycf4 gene of the green alga Chlamydomonas reinhardtii to study the effect of amino acid substitutions on PSI complex assembly. The mutants, R120A and R120Q, assembled PSI complexes at the wild-type level but accumulated Ycf4 at 20% of the wild-type level, suggesting that wild-type cells accumulate a super. uous amount of Ycf4. The double mutant E179/181Q accumulated Ycf4 at wild-type levels but did not assemble any mature PSI complex. However, the double mutant accumulated a small amount of PSI subcomplex consisting of a PsaA-PsaB heterodimer. Pulse-chase protein labeling experiments demonstrated that the subcomplex was an unstable assembly intermediate and the subsequent assembly steps were blocked by the E179/181Q mutation. The cover is designed from green spots of Chlamydomonas cells grown on agar plates to test if the mutants grow photoautotrophically.
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