Fluorescence Spectroscopy of the Longwave Chlorophylls in Trimeric and Monomeric Photosystem I Core Complexes from the Cyanobacterium Spirulina platensis

Abstract

The organization and interaction of chlorophylls (Chl) and the kinetics of the energy transfer in the core antenna of photosystem I (PSI) trimeric and monomeric complexes, isolated from Spirulina platensis with Triton X-100 have been studied by stationary and time-resolved fluorescence. At 295 K both complexes show an unusually intense long-wavelength emission band with prominent peaks at 730 nm (trimers) or 715 nm (monomers), whose intensity is independent of the redox state of P700. A broad band extending from 710 to 740 nm in the absorption and fluorescence excitation spectra of trimers also indicates the existence of the longwave Chls at 295 K. The 77 K fluorescence emission of PSI trimers frozen after addition of dithionite under illumination (P700 and the PSI acceptor side reduced) shows an intense band at 760 (F760) and a smaller one at 725 nm (F725); when P700 is oxidized, the intensity of F760 decreases about 15 times. In the 77 K spectrum of monomers only F725 is present in the longwave region, and its intensity does not depend on the redox state of P700. Bands of Chls with maxima near 680, 710, and 738 nm were found in the 77 K excitation spectrum of trimers, and bands near 680 and 710 nm were seen in the spectrum of monomers. Five spectrally different red Chl forms in PSI trimers and three red Chl in monomers have been resolved by deconvolution of their 77 K absorption spectra. The difference absorption spectrum, trimers-minus-monomers, shows that the appearance of the 735 nm band in trimers is accompanied by a decrease of 708, 698, and 688 nm bands present in monomers. The reversible changes of F760 intensity of Spirulina membranes as a result of their salt treatment confirm the idea that the most longwave Chl form originates from an interaction of Chls bound to different monomeric PSI subunits forming the trimer. The time-resolved fluorescence spectra of PSI trimers and monomers, measured at 287 K in the region 680-770 nm, are substantially different, although a set of similar lifetimes (9, approximately 30, approximately 66, and 1400-2200 ps) was necessary for a good fit. No effect of P700 redox state was observed on the fluorescence kinetics of both complexes at 287 K.

Characterization and Evolution of Tetrameric Photosystem I from the Thermophilic Cyanobacterium Chroococcidiopsis sp TS-821

“…This species is lost when the complex is denatured, dried, or exposed to high levels of detergent (Kiley et al, 2005). This shift in emission wavelength between the PSI monomer and trimer forms has been observed in various cyanobacteria (Karapetyan et al, 1997;Gobets et al, 2001;El-Mohsnawy et al, 2010;Schlodder et al, 2011) with the PSI trimer containing more far-red chlorophylls. It has been observed that in T. elongatus, this far-red species is lost or significantly shifted to the blue in the monomeric form (El-Mohsnawy et al, 2010).…”

“…Although we do not know the chemical composition of this central cavity, it is clearly different from what was observed in trimeric PSI preps and most likely is filled with lipids and/or detergent molecules. This may provide a similar environment to the far-red chlorophyll molecules as observed in monomeric PSI isolated from native or saltand heat-treated thylakoid membrane (Karapetyan et al, 1997;El-Mohsnawy et al, 2010). To confirm that the emission characteristics of the isolated TS-821 PSI was not an artifact, we investigated the fluorescence properties of PSI in its native form in the membrane by taking the whole cell emission spectrum at 77K ( Figure 6B).…”

“…The Spirulina platensis and T. elongatus PSI trimer have an emission peak at ;730 nm with the monomer emission blue-shifted to ;715 to 725 nm (Karapetyan et al, 1997;ElMohsnawy et al, 2010). The 77K fluorescence showed that TS-821 PSI monomer, dimer, and tetramer share a common emission peak at 715 nm ( Figure 6A); however, similar analysis of the isolated monomers and trimers from T. elongatus display different peak emissions, in agreement with previously published values of <725 and ;730 nm, respectively ( Figure 6A).…”

In Vitro Oligomerization of a Membrane Protein Complex

“…Third, higher plants contain only monomeric PS I, whereas PS I complexes of cyanobacteria are organized as monomers and trimers (5, 8 -10). It could be shown that PsaL is responsible for this specific trimerization of cyanobacterial PS I (11) and that the ratio between trimers and monomers within the detergent-free cyanobacterial thylakoid membrane can be regulated through the ionic strength of the medium (5,12). Presumably, electrostatic interactions between the complexes are altered, causing the transformation of trimers into monomers at high salt and the reverse reaction at low salt conditions.…”

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“…The trimerisation of PS I in vivo has been suggested to mediate state transitions in cyanobacteria (Kruip et al 1994). It has been proposed in the case of Spirulina platensis (called Spirulina from here on) that the absence of the far red fluorescence emission at 77 K goes along with an inability to form PS I trimers (Karapetyan et al 1997). This hypothesis, however, cannot be generalised for other PS I trimer forming species as they do not show this very long wavelength emission (Gobets et al 2001).…”