Abstract
Antimycin A-sensitive cyclic electron flow (AA-sensitive CEF) was discovered by Arnon and co-workers more than 50 years ago and serves to recycle electrons from ferredoxin (Fd) to plastoquinone (PQ). A role in AA-sensitive CEF has been attributed to the two thylakoid proteins PGR5 and PGRL1 ever since their identification, but this assignment remains controversial. While current technical limitations have prevented unequivocal clarification of their precise function in CEF in vivo, recent biochemical experiments have implied that PGRL1/PGR5 complexes possess Fd-PQ reductase (FQR) activity in vitro. Consequently, PGRL1-PGR5 complexes in flowering plants appear to shuttle between photosystem I (PSI) and the cytochrome (Cyt) b6f complex, whereas in the green alga Chlamydomonas PGRL1 (but not PGR5) has been detected in a PSI-Cyt b6f supercomplex that has intrinsic CEF activity.
Highlights
THE GENETIC EVIDENCE A breakthrough in CEF research was achieved with the discovery that a multiprotein complex, which resembles Complex I in the mitochondrial respiratory chain, mediates CEF in cyanobacteria
(B) In the green alga Chlamydomonas, CEF can be mediated by a photosystem I (PSI)-Cyt b6f -PGRL1-ANR1-CAS supercomplex (Terashima et al, 2012). (C) Model for the FdPQ reductase (FQR) activity of PGRL1/PGR5 according to Hertle et al (2013)
It is worth pointing out here that, irrespective of the status of the CEF measurements, the following major conclusions derived from the analyses of the pgr5 mutant remain valid: (1) PGR5 and PGRL1 are necessary for non-photochemical quenching (NPQ) induction and protection of PSI from photoinhibition (Munekage et al, 2002; DalCorso et al, 2008)
Summary
THE GENETIC EVIDENCE A breakthrough in CEF research was achieved with the discovery that a multiprotein complex, which resembles Complex I in the mitochondrial respiratory chain, mediates CEF in cyanobacteria (reviewed in: Ogawa and Mi, 2007). To demonstrate that the pgr5 and crr mutants are defective in CEF, an assay was employed in which Fddependent PQ reduction activity was measured in ruptured chloroplasts.
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