Photosynthesis: A new function for an old cytochrome?

Abstract

In many cyanobacteria and algae, cytochrome c6 transports electrons between the cytochrome bf complex and photosystem I, replacing plastocyanin when copper is deficient. Higher plants, however, were thought to lack cytochrome c6 until the existence of a modified form in several species was inferred from genomic evidence. By measuring oxygen evolution with inside-out thylakoids, Gupta et al. inferred that heterologously expressed Arabidopsis cytochrome c6 can replace plastocyanin from Synechocystis or Arabidopsis in reconstitution experiments in vitro. From structural and kinetic evidence, however, we find that Arabidopsis cytochrome c6 cannot carry out the same function as Arabidopsis plastocyanin or as cytochrome c6 from the alga Monoraphidium braunii. This suggests that cytochrome c6 in higher plants may have lost its original primitive function in photosynthesis.