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Xu, Qian-Zhao; Han, Jia-Xin; Tang, Qi-Ying; Ding, Wen-Long; Miao, Dan; Zhou, Ming; Scheer, Hugo and Zhao, Kai-Hong (2016): Far-red light photoacclimation: Chromophorylation of FR induced α- and β-subunits of allophycocyanin from Chroococcidiopsis thermalis sp. PCC7203. In: Biochimica et Biophysica Acta-Bioenergetics, Vol. 1857, No. 9: pp. 1607-1616

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Cyanobacterial light-harvesting complexes, phycobilisomes, can undergo extensive remodeling under varying light conditions. Acclimation to far-red light involves not only generation of red-shifted chlorophylls in the photosystems, but also induction of additional copies of core biliproteins that have been related to red-shifted components of the phycobilisome (Gan et al., Life 5, 4, 2015). We are studying the molecular basis for these acclimations in Chroococcidiopsis thermalis sp. PCC7203. Five far-red induced allophycocyanin subunits (ApcA2, ApcA3, ApcB2, ApcB3 and ApcF2) were expressed in Escherichia coli, together with S-type chromophoreprotein lyases and in situ generated chromophore, phycocyanobilin. Only one subunit, ApcF2, shows an unusual red-shift (lambda(Amax) similar to 675 nm, lambda(Fmax) similar to 698 nm): it binds the chromophore non-covalently, thereby preserving its full conjugation length. This mechanism operates also in two Cys-variants of the induced subunits of bulky APC. All other wild-type subunits bind phycocyanobilin covalently to the conventional Cys-81 under catalysis of the lyase, CpcS1. Although three of them also show binding to additional cysteines, all absorb and fluoresce similar to conventional APC subunits (lambda(Amax) similar to 610 nm, lambda(Fmax) similar to 640 nm). Another origin of red shifted complexes was identified, however, when different wild-type alpha- and beta-subunits of the far-red induced bulky APC were combined in a combinatorial fashion. Strongly red-shifted complexes (lambda(Fmax) <= 722 nm) were formed when the alpha-subunit, PCB-ApcA2, and the beta-subunit, PCB-ApcB2, were generated together in E. coli. This extreme aggregation-induced red shift of similar to 90 nm of covalently bound chromophores is reminiscent, but much larger, than the similar to 30 nm observed with conventional APC. (C) 2016 Elsevier B.V. All rights reserved.

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