Photosynthetic electron transport in cyanobacteria

Oxygenic photosynthesis converts light-energy into chemical free energy by reducing NADP{dollar}sp+{dollar} to NADPH and producing ATP. Eukaryotic plants and algae as well as prokaryotic cyanobacteria and prochlorophytes have a similar photosynthetic apparatus. The light-driven reactions take place in the thylakoid membranes and three major membrane-intrinsic protein complexes carry out photosynthetic electron transfer, photosystem II, the cytochrome bf complex and photosystem I. To elucidate photosynthetic electron transport in oxygenic photosynthesis site-directed mutagenesis and biophysical techniques were employed in cyanobacteria which have the advantage over plants that they are easily genetically manipulated. This work focused on electron transfer linking the cytochrome bf complex and photosystem I, characterization of the plastoquinol oxidation site of the cytochrome bf complex and electron transfer within the PsaC polypeptide of the photosystem I complex. Major findings include (1) in vivo electron transfer between the cytochrome bf complex and photosystem I is similarly efficient in the presence of either plastocyanin or cytochrome {dollar}csb6{dollar} in the cyanobacterium Synechocystis PCC 6803; (2) mutant strains that lack both these electron carriers show 5-8 fold slowed down kinetics, and the results indicate the presence of an alternate carrier; (3) characterization of a cyt M/ cyt {dollar}csb6{dollar} double deletion mutant indicates a role of cytochrome M in photosynthetic electron transfer, and gives evidence for respiration pathways that exclude the participation of cytochrome {dollar}csb6,{dollar} cytochrome M and plastocyanin in Synechocystis PCC 6803; (4) studies of inhibitor binding specificity and sensitivity at the plastoquinol oxidation site in Synechococcus PCC 7002 revealed that the residues D148, A154, S159 in the cytochrome {dollar}bsb6{dollar} of Synechococcus PCC 7002 participate in plastoquinol and inhibitor binding at the plastoquinol oxidation site of the cytochrome bf complex; (5) elucidation of the electron pathway within the PsaC polypetide of photosystem I in Anabaena vaviabilis ATCC 29413 revealed that forward electron transfer within photosystem I can occur via the {dollar}rm Fsb{lcub}A{rcub}{dollar} cluster in the absence of a functional F{dollar}rmsb{lcub}B{rcub}{dollar} cluster; (6) a nalysis of the extinction coefficient of plant (turnip) cytochrome f exhibited a value of 28 mM{dollar}sp{lcub}-1{rcub}{dollar} cm{dollar}sp{lcub}-1{rcub},{dollar} which is about 30% higher than previously published values.