Further characterization of the psbH locus of Synechocystis sp. PCC 6803: Inactivation of psbH impairs QA to QB electron transport in photosystem 2

Abstract

The psbH gene encodes a small protein which copurifies with photosystem 2. In the cyanobacterium Synechocystis sp. PCC 6803, psbH is located upstream of the cytochrome b6-f complex genes petC and petA. In striking contrast, in the genomes of plant chloroplasts, psbH is cotranscribed with petB and petD, encoding the other two major subunits of the cytochrome b6-f complex. We report that in Synechocystis sp. PCC 6803 monocistronic psbH and dicistronic petCA transcripts are probably initiated separately, each from DNA regions bearing some similarity to Escherichia coli sigma 70 promoters. Synechocystis sp. PCC 6803 psbH null mutants were generated by cartridge mutagenesis. Studies using a rapid screening procedure involving in situ complementation showed that the PsbH protein is not absolutely required for the assembly of a functionally active photosystem 2 complex since psbH insertion and deletion strains were able to grow photoautotrophically. The rate of photoautotrophic growth was, however, slower than the wild type, and studies of oxygen evolution, chlorophyll fluorescence, and thermoluminescence indicated that this reduction in growth rate is probably due mainly to an impairment in electron flow from QA to QB. We conclude, therefore, that the PsbH protein is not an absolute requirement for photosystem 2 activity but that it functions to optimize electron flow between the two secondary plastoquinone acceptors by interacting with the QB site on the D1 protein.

PsbN Is Required for Assembly of the Photosystem II Reaction Center in Nicotiana tabacum

“…Our finding that transplastomic ΔpsbN mutants are able to survive on soil only at very low light intensities is in contrast with corresponding ΔpsbN mutants in Synechocystis sp PCC 6803, which show no obvious additional effects of deleting psbN alongside psbH, and indicates that PsbN is not essential for PSII biogenesis and function in this organism (Mayers et al, 1993). However, due to the conserved C terminus, it is likely that the function of PsbN is also conserved, although the phenotype of knockout lines might differ substantially even in closely related species.…”

Metatranscriptomics of N2-fixing cyanobacteria in the Amazon River plume

“…In addition, psbH and psbK were each detected in the Trichodesmium oceanic transcriptome. In the diazotrophic cyanobacterium Synechocystis, neither psbH nor psbK were essential to photoautotrophic growth, but the loss of either resulted in reduced growth rates (Ikeuchi et al, 1991;Mayes et al, 1993). The PS-II transcript differences may reflect the morphological difference between Richelia and Trichodesmium, or indicate the Hemiaulus symbiont has reduced growth rates, as seen with heterocyst-forming cyanobacteria in other associations (Peters and Meeks, 1989;Adams et al, 2006).…”

Determination of Photosystem II Subunits by Matrix-assisted Laser Desorption/Ionization Mass Spectrometry

“…This small subunit of the PSII core was originally detected as a 9-kDa phosphoprotein in pea chloroplasts (37). The physiological roles proposed for it include regulation of photoinhibition (38), control of the electron transfer from Qa to Qb (10), and stabilization of the PSII dimer in higher plants (39,40). Interestingly, the psbh gene product from cyanobacteria is truncated at the N terminus site and lacks the phosphorylation site (19).…”

“…Preparation of Thylakoid Membranes and PSII from PSI-less Synechocystis-Thylakoids were prepared using a modified version of the procedure of Mayes et al (10). Briefly, cells were cultured as previously described (11), harvested by centrifugation (2000 ϫ g for 5 min at 4°C), and resuspended in a breaking buffer containing the following: 20 mM Mes, 5 mM MgCl 2 , 5 mM CaCl 2 , 1 mM benzamidine, 1 mM aminocaproic acid, and 25% glycerol (pH 6.35 with NaOH).…”