title: Photosynthetic pigment localization and thylakoid membrane morphology are altered in Synechocystis 6803 phycobilisome mutants.
abstract: 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 chlorophyll-containing 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.
authors.affiliation: Department of Bioenergy and Defense Technologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA.
authors.forename: Aaron M
authors.initials: AM
authors.lastname: Collins
authors.forename: Michelle
authors.initials: M
authors.lastname: Liberton
authors.forename: Howland D T
authors.initials: HD
authors.lastname: Jones
authors.forename: Omar F
authors.initials: OF
authors.lastname: Garcia
authors.forename: Himadri B
authors.initials: HB
authors.lastname: Pakrasi
authors.forename: Jerilyn A
authors.initials: JA
authors.lastname: Timlin
mesh_terms: D015999:Multivariate Analysis; D009154:Mutation / Q000235:genetics*; D010650:Phenylalanine Ammonia-Lyase / Q000378:metabolism; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D045524:Phycobilisomes / Q000378:metabolism*; D010798:Phycocyanin / Q000378:metabolism; D013057:Spectrum Analysis; D046939:Synechocystis / Q000201:enzymology / Q000378:metabolism* / Q000648:ultrastructure*; D020524:Thylakoids / Q000648:ultrastructure*
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D045524:Phycobilisomes; C002497:allophycocyanin; D010798:Phycocyanin; D010650:Phenylalanine Ammonia-Lyase