Abstract
Ferredoxin (Fd) either transfers reducing equivalents from PSI to oxidoreductase enzymes, or returns them to the thylakoid electron transfer system. Fd forms transient complexes with enzymes during electron transfer. In the crystal structure of the maize leaf mesophyll cell Fd (FdI) and Fd:NADP+ reductase (FNR) complex, intermolecular salt bridges ensure optimal topology. A different Fd isoform (FdII) is present in bundle sheath cells (BSC) and lacks an aspartate necessary for one such salt bridge. FdII also has weaker affinity for FNR, implying electron partitioning around PSI differs according to Fd type. This hypothesis was tested by separately transforming cyanobacterial cells to express FdI or FdII. Tranformant cell contents differed in ratios of NADPH/NADP+ and ATP/ADP, and FdII transformed cells showed a marked increase in cyclic electron flow. We will discuss the possibility that cyclic electron transfer in BSC depends on the distinctive protein-protein interactions of Fd with its counterparts.
