Structure-based control of the rate limitation of photosynthetic electron transport

The 2.5 Å structure of the cytochrome (cyt) b₆ f complex provides a basis for control of the rate-limiting electron transfer step of oxygenic photosynthesis associated with the plastoquinol/quinone exchange pathway. Here, a structural change was made at a site containing two proline residues which border the intra-cyt pathway for plastoquinol/quinone exchange. The proline side chains confer a larger aperture for passage of plastoquinol/quinone. Change of these prolines to alanine in the cyanobacterium Synechococcus sp. PCC 7002 results in attenuation of this rate-limiting step, observed by a two-fold reduction in the rate of cell growth, O₂ evolution, and plastoquinol-mediated reduction of cyt f. This study demonstrates modification by site-directed mutagenesis of photosynthetic energy transduction based on rational application of information in the atomic structure.