The Tll0287 protein is a hemoprotein associated with the PsbA2-Photosystem II complex in Thermosynechococcus elongatus

Abstract

Cyanobacteria have multiple psbA genes encoding PsbA, the D1 reaction center protein of the Photosystem II (PSII) complex. The thermophilic cyanobacterium Thermosynechococcus elongatus has three psbA genes differently expressed depending on the environmental conditions. Among the 344 residues of PsbA, there are 21 substitutions between PsbA1 and PsbA3, 31 between PsbA1 and PsbA2 and 27 between PsbA2 and PsbA3. In this study, we found a new hemoprotein that is expressed when the T. elongatus genome has only the psbA2 gene for D1. This hemoprotein was found in both the non-membrane proteins and associated to the purified PsbA2-PSII core complex. This protein could be removed by the washing of PSII with Tris-washing or CaCl2-washing. From MALDI-TOF/TOF spectrometry, N-terminal sequencing and MALDI-MS/MS analysis upon tryptic digestion, the new hemoprotein was identified to be the tll0287 gene product with a molecular mass close to 19kDa. Until now, tll0287 was registered as a gene encoding a hypothetical protein with an unknown function. From the amino acid sequence and the EPR spectrum the 5th and 6th axial ligands of the heme iron are the His145 and likely either the Tyr93, Tyr159 or Tyr165, respectively. From EPR, the heme containing Tll0287 protein associated to PsbA2-PSII corresponds to approximately 25% of the Cytc550 content whereas, from SDS page analysis, the total amount of Tll0287 with and without the heme seems almost in a stoichiometric amount with PsbA2-PSII. Homologous genes to tll0287 are found in several cyanobacteria. Possible roles for Tll0287 are suggested.