Modeling Photosystem I with the alternative reaction center protein PsaB2 in the nitrogen fixing cyanobacterium Nostoc punctiforme

• Published in: Biochimica et biophysica acta 2011-09, Vol.1807 (9), p.1152-1161
• Main Authors: Magnuson, Ann, Krassen, Henning, Stensjö, Karin, Ho, Felix M., Styring, Stenbjörn
• Format: Article
• Language: English
• Subjects: amino acid composition; Amino Acid Sequence; crystal structure; Electron transfer; electron transport chain; glutamine; Homology model; Models, Molecular; Molecular Sequence Data; Multiple sequence alignment; NATURAL SCIENCES; NATURVETENSKAP; Nitrogen Fixation; Nostoc - metabolism; Nostoc - physiology; Nostoc punctiforme; operon; Photosystem I Protein Complex - chemistry; Photosystem I Protein Complex - genetics; Photosystem I Protein Complex - metabolism; Photosystem-I; phylloquinone; protein structure; sequence alignment; Sequence Homology, Amino Acid
• ISSN: 0005-2728; 0006-3002; 1879-2650; 1879-2650
• DOI: 10.1016/j.bbabio.2011.05.004

Five nitrogen fixing cyanobacterial strains have been found to contain PsaB2, an additional and divergent gene copy for the Photosystem I reaction center protein PsaB. In all five species the divergent gene, psaB2, is located separately from the normal psaAB operon in the genome. The protein, PsaB2, was recently identified in heterocysts of Nostoc punctiforme sp. strain PCC 73102. 12 conserved amino acid replacements and one insertion, were identified by a multiple sequence alignment of several PsaB2 and PsaB1 sequences. Several, including an inserted glutamine, are located close to the iron-sulfur cluster F X in the electron transfer chain. By homology modeling, using the Photosystem I crystal structure as template, we have found that the amino acid composition in PsaB2 will introduce changes in critical parts of the Photosystem I protein structure. The changes are close to F X and the phylloquinone (PhQ) in the B-branch, indicating that the electron transfer properties most likely will be affected. We suggest that the divergent PsaB2 protein produces an alternative Photosystem I reaction center with different structural and electron transfer properties. Some interesting physiologcial consequences that this can have for the function of Photosystem I in heterocysts, are discussed. ► Five cyanobacterial species contain PsaB2, a divergent gene copy of the Photosystem I protein PsaB. ► PsaB2 is found in heterocysts of the Nostoc genus and may be involved in nitrogen fixation. ► Conserved amino acid replacements are found in PsaB2, in highly conserved regions close to F X. ► The sequence of PsaB2 is likely to affect electron transfer on the acceptor side of Photosystem I.