Abstract
Photosystem II (PSII) is a large membrane protein complex that uses light energy to convert water to molecular oxygen. This enzyme undergoes an intricate assembly process to ensure accurate and efficient positioning of its many components. It has been proposed that the Psb27 protein, a lumenal extrinsic subunit, serves as a PSII assembly factor. Using a psb27 genetic deletion strain (Deltapsb27) of the cyanobacterium Synechocystis sp. PCC 6803, we have defined the role of the Psb27 protein in PSII biogenesis. While the Psb27 protein was not essential for photosynthetic activity, various PSII assembly assays revealed that the Deltapsb27 mutant was defective in integration of the Mn(4)Ca(1)Cl(x) cluster, the catalytic core of the oxygen-evolving machinery within the PSII complex. The other lumenal extrinsic proteins (PsbO, PsbU, PsbV, and PsbQ) are key components of the fully assembled PSII complex and are important for the water oxidation reaction, but we propose that the Psb27 protein has a distinct function separate from these subunits. We show that the Psb27 protein facilitates Mn(4)Ca(1)Cl(x) cluster assembly in PSII at least in part by preventing the premature association of the other extrinsic proteins. Thus, we propose an exchange of lumenal subunits and cofactors during PSII assembly, in that the Psb27 protein is replaced by the other extrinsic proteins upon assembly of the Mn(4)Ca(1)Cl(x) cluster. Furthermore, we show that the Psb27 protein provides a selective advantage for cyanobacterial cells under conditions such as nutrient deprivation where Mn(4)Ca(1)Cl(x) cluster assembly efficiency is critical for survival.
Highlights
JOURNAL OF BIOLOGICAL CHEMISTRY protein subunits and numerous cofactors
Analysis of an Arabidopsis mutant line lacking the Psb27 protein indicated that this protein plays a role in efficient reassembly of Photosystem II (PSII) after lightinduced damage [24]
It has been previously hypothesized that the Psb27 protein functions in the PSII assembly pathway, but the precise step was unknown
Summary
Culture Conditions—Wild type (WT), ⌬psb27, ⌬psbO, and ⌬psbO, ⌬psb strains of Synechocystis 6803 were grown in BG11 medium [25] at 30 °C under 30 mol photons/m21⁄7s. ⌬psb27, ⌬psbO, and ⌬psbO,⌬psb cells were grown in BG11 medium supplemented with 10 g/ml chloramphenicol, 10 g/ml spectinomycin, and 10 g/ml chloramphenicol plus 10 g/ml spectinomycin, respectively. Cells were diluted to 5 g of Chl/ml in HNMC buffer and incubated under photoactivation light at either 13, 67, or 129 mol photons/m21⁄7s white light. After incubation at the indicated light intensity, oxygen evolution was measured at 8250 mol photons/m21⁄7s in the presence of the electron acceptors 0.5 mM 2,6-dichloro-p-benzoquinone and 1 mM potassium ferricyanide on a Clark-type electrode. For flash-induced photoactivation assays, HA-treated cells were deposited on a bare platinum electrode (Artisan Scientific), covered with an agarose disc of HNMC buffer, dark-incubated for 2 min, and subjected to a series of pre-flashes (0 –2000) at 0.3 s and a train of 20 measuring flashes at 0.3 s. PCR analysis of the psb locus was performed with the segregation analysis primers Psb27E and Psb27F (Table 1) to determine the relative amounts of WT and ⌬psb cells in the mixed cultures. The total DNA isolated from each subculture of the FEBRUARY 15, 2008 VOLUME 283 NUMBER 7
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
