Crystal Structure of Synechocystis Mutants and Plant Photosystem I

Abstract

Plant Photosystem I (PSI) is one of the most intricate membrane complexes in Nature. It comprises two loosely bound reaction center and light-harvesting (LHC) complexes containing 18 subunits 174 chlorophylls, 32 carotenoids 2 quinines and 3 iron-sulfur clusters. The crystal structure at 3.1 A resolution reported her add several new features that were not detected in the previous structures. The location of an additional ten β-carotenes as well as five chlorophylls and several loop regions are now modeled. This represents the most complete plant Photosystem I structure obtained thus far, revealing the locations of and interactions among 18 protein subunits and 211 non-covalently bound photochemical cofactors. We also detected 12 additional carotenoids especially in the LHC complex as well as structural lipids. The latter suggest evolutionary scenario where Lhca1 and Lhca2 coevolved with LHCII and Lhca4 coevolved with CP29.Recently an operon encoding PSI was identified in cyanobacterial marine viruses. A PSI that mimics the most important feature was generated in Synechocystis 6803. This PSI is promiscuous for its electron donor and can accept electrons from respiratory cytochromes. The wild type and promiscuous PSI complexes were isolated, crystallized and their structure was solved to 4 and 3.8 A resolution, respectively. In addition we solved the virus-like structure of PsaL and PsaI minus PSI mutant at 2.8 A resolution. The novel structure of PSI from mesophilic cyanobacterium is different in several aspects in comparison with the previously published structure of PSI from Thermosynechococcus elongatus. The common properties of mesophilic cyanobacterial and plant PSI will be discussed.