Cytochromeb6fcomplex inhibition by antimycin-A requires Stt7 kinase activation but not PGR5

Ferredoxin-plastoquinone reductase (FQR) activity during cyclic electron flow (CEF) was first ascribed to the cytochrome b 6 f complex ( b 6 f ). However, this was later dismissed since b 6 f inhibition by antimycin-A (AA) could not be reproduced. AA presumably fails to ligate with haem b h , at variance with cytochrome bc 1 complex, owing to a specific Qi-site occupation in b 6 f . Currently, PROTON GRADIENT REGULATION5 (PGR5) and the associated PGR5-Like1 are considered as FQR in the AA-sensitive CEF pathway. Here, we show that the b 6 f is conditionally inhibited by AA in a PGR5-independent manner when CEF is promoted. AA inhibition, demonstrated by single b 6 f turnover and electron transfer measurements, coincided with an altered Qi-site function which required Stt7 kinase activation by a strongly reduced plastoquinone pool. Thus, PGR5 and Stt7 were necessary for b 6 f activity and AA-sensitive electron transfer in CEF-favouring conditions. Extending previous findings, a new FQR activity model of the b 6 f is discussed.