Photosynthetic Pigment Localization and Thylakoid Membrane Morphology Are Altered in Synechocystis 6803 Phycobilisome Mutants    

Abstract

(M.L., H.B.P.) Cyanobacteria are oxygenic photosynthetic prokaryotes that are the progenitors of the chloroplasts of algae and plants. These organisms harvest light using large membrane-extrinsic phycobilisome antenna in addition to membrane-bound chlorophyllcontaining proteins. Similar to eukaryotic photosynthetic organisms, cyanobacteria possess thylakoid membranes that house photosystem (PS) I and PSII, which drive the oxidation of water and the reduction of NADP + , respectively. While thylakoid morphology has been studied in some strains of cyanobacteria, the global distribution of PSI and PSII within the thylakoid membrane and the corresponding location of the light-harvesting phycobilisomes are not known in detail, and such information is required to understand the functioning of cyanobacterial photosynthesis on a larger scale. Here, we have addressed this question using a combination of electron microscopy and hyperspectral confocal fluorescence microscopy in wild-type Synechocystis species PCC 6803 and a series of mutants in which phycobilisomes are progressively truncated. We show that as the phycobilisome antenna is diminished, large-scale changes in thylakoid morphology are observed, accompanied by increased physical segregation of the two photosystems. Finally, we quantified the emission intensities originating from the two photosystems in vivo on a per cell basis to show that the PSI:PSII ratio is progressively decreased in the mutants. This results from both an increase in the amount of photosystem II and a decrease in the photosystem I concentration. We propose that these changes are an adaptive strategy that allows cells to balance the light absorption capabilities of photosystems I and II under light-limiting conditions.

Lateral Segregation of Photosystem I in Cyanobacterial Thylakoids

“…HCFM (Vermaas et al, 2008) was used to image the in vivo distribution of PSI-YFP complexes; this form of microscopy combines spatial and spectral information within every pixel in the image, and the overlapping spectra for exogenous fluorescent protein and endogenous phycobilisome and photosystem components are resolved computationally by multivariate curve resolution (MCR). This approach has already been used to image the spatial distribution of pigments in Synechocystis (Vermaas et al, 2008;Collins et al, 2012;Liberton et al, 2013a). Here, we used spectra for phycobilisome and photosystem and YFP components and employed controls in which non-thylakoid cell compartments in Synechocystis were labeled, namely, "free" YFP in the cytoplasm and YFP exported to the periplasm using the Twin-arginine translocase (Spence et al, 2003).…”

Organization and Flexibility of Cyanobacterial Thylakoid Membranes Examined by Neutron Scattering

“…In the PAL mutant (Fig. 1G), thylakoid membranes consistently formed straight bands with little curvature (10). The interthylakoidal distances in PAL averaged 190 Ϯ 30 Å, with center-to-center membrane distances of 340 Å (Fig.…”

“…Cyanobacteria such as Synechocystis 6803 have a highly organized thylakoid membrane architecture, and it has been proposed that rearrangements in the membrane system and the spacing between membrane layers play a role in the cellular regulation of photosynthesis (10). Ideally, investigation of membrane architecture in cyanobacteria requires real-time in situ studies that can follow the time course of reorganization in response to external stimuli such as light or other environmental conditions.…”

“…Studies using light microscopy approaches such as fluorescence recovery after photobleaching and hyperspectral confocal fluorescence microscopy have led to intriguing findings regarding the mobility of photosynthetic components and the organization of photosynthetic pigments within cyanobacterial cells (9,10). SANS can also be used to characterize periodically organized biological membrane systems on smaller length scales by observing diffraction from regularly spaced structural features separated by ϳ1-200 nm (11).…”

Diblock Copolymer Micelles and Supported Films with Noncovalently Incorporated Chromophores: A Modular Platform for Efficient Energy Transfer

“…Photosynthetic membranes are highly responsive, often changing the composition and organization of LH pigment-protein components in response external stimuli, such as light intensity [38][39][40] , oxygen tension, 40 and genetic mutation. 41,42 Mimicking and expanding such responsive behavior is also highly desirable in generation of artificial light-harvesting systems.…”