Abstract
In photosynthetic organisms like cyanobacteria and plants, the main engines of oxygenic photosynthesis are the pigment-protein complexes photosystem I (PSI) and photosystem II (PSII) located in the thylakoid membrane. In the cyanobacterium Synechocystis sp. PCC 6803, the slr1796 gene encodes a single cysteine thioredoxin-like protein, orthologs of which are found in multiple cyanobacterial strains as well as chloroplasts of higher plants. Targeted inactivation of slr1796 in Synechocystis 6803 resulted in compromised photoautotrophic growth. The mutant displayed decreased chlorophyll a content. These changes correlated with a decrease in the PSI titer of the mutant cells, whereas the PSII content was unaffected. In the mutant, the transcript levels of genes for PSI structural and accessory proteins remained unaffected, whereas the levels of PSI structural proteins were severely diminished, indicating that Slr1796 acts at a posttranscriptional level. Biochemical analysis indicated that Slr1796 is an integral thylakoid membrane protein. We conclude that Slr1796 is a novel regulatory factor that modulates PSI titer.
Highlights
In photosynthetic organisms, a number of proteins are present that are involved in redox activity [1, 2]
Disruption of slr1796 Leads to Higher photosystem II (PSII)/photosystem I (PSI) Ratio—The protein Slr1796 was identified as a thylakoid membrane component by proteomics studies [29, 30], but the role of the protein remained unknown
Similar orthologs are predicted to be present in land plants such as Arabidopsis thaliana
Summary
In photosynthetic organisms like cyanobacteria and plants, the main engines of oxygenic photosynthesis are the pigmentprotein complexes photosystem I (PSI) and photosystem II (PSII) located in the thylakoid membrane. Oxygenic photosynthesis is performed by pigment-protein complexes located in the thylakoid membranes of cyanobacteria and chloroplasts. Redox regulation by Trx fold proteins via their oxidation, reduction, or disulfide exchange activities, depending on their redox environments, is an extensive system in almost all life forms, including oxygenic photosynthetic organisms. A few accessory protein factors involved in the stable accumulation of PSI have been identified in cyanobacteria, higher plants, and algae [6, 7]. PCC 6803 (hereafter Synechocystis 6803) lacking the protein Slr1796, which contains a Trx fold with only a single cysteine, and demonstrate that this strain has a higher PSII/PSI ratio. Our data suggest that Slr1796 is a novel thylakoid protein required for the normal accumulation of PSI
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