Abstract
Cyanobacterium Synechocystis PCC 6803 represents a favored model organism for photosynthetic studies. Its easy transformability allowed construction of a vast number of Synechocystis mutants including many photosynthetically incompetent ones. However, it became clear that there is already a spectrum of Synechocystis “wild-type” substrains with apparently different phenotypes. Here, we analyzed organization of photosynthetic membrane complexes in a standard motile Pasteur collection strain termed PCC and two non-motile glucose-tolerant substrains (named here GT-P and GT-W) previously used as genetic backgrounds for construction of many photosynthetic site directed mutants. Although, both the GT-P and GT-W strains were derived from the same strain constructed and described by Williams in 1988, only GT-P was similar in pigmentation and in the compositions of Photosystem II (PSII) and Photosystem I (PSI) complexes to PCC. In contrast, GT-W contained much more carotenoids but significantly less chlorophyll (Chl), which was reflected by lower level of dimeric PSII and especially trimeric PSI. We found that GT-W was deficient in Chl biosynthesis and contained unusually high level of unassembled D1-D2 reaction center, CP47 and especially CP43. Another specific feature of GT-W was a several fold increase in the level of the Ycf39-Hlip complex previously postulated to participate in the recycling of Chl molecules. Genome re-sequencing revealed that the phenotype of GT-W is related to the tandem duplication of a large region of the chromosome that contains 100 genes including ones encoding D1, Psb28, and other PSII-related proteins as well as Mg-protoporphyrin methylester cyclase (Cycl). Interestingly, the duplication was completely eliminated after keeping GT-W cells on agar plates under photoautotrophic conditions for several months. The GT-W strain without a duplication showed no obvious defects in PSII assembly and resembled the GT-P substrain. Although, we do not exactly know how the duplication affected the GT-W phenotype, we hypothesize that changed stoichiometry of protein components of PSII and Chl biosynthetic machinery encoded by the duplicated region impaired proper assembly and functioning of these multi-subunit complexes. The study also emphasizes the crucial importance of a proper control strain for evaluating Synechocystis mutants.
Highlights
Cyanobacteria represent excellent model organisms for photosynthesis research as they perform oxygenic photosynthesis similar to that in algae and plants while having much simpler cellular organization
The GT-W strain contained much more carotenoids and less Chl than both previously mentioned strains. When both strains were grown in the presence of 5 mM glucose, in both strains the amount of carotenoids decreased but the level of Chl decreased only in GT-P while in GT-W it increased (Figure S1)
During our previous studies dealing with the biogenesis of photosynthetic membrane complexes in Synechocystis we characterized dozens of mutants constructed in different background strains originating from various laboratories
Summary
Cyanobacteria represent excellent model organisms for photosynthesis research as they perform oxygenic photosynthesis similar to that in algae and plants while having much simpler cellular organization. Unlike the other photosynthetic bacteria they possess both types of reaction center and oxidize water to molecular oxygen by Photosystem II (PSII) and reduce NADP by Photosystem I (PSI) For both pigment-protein complexes the high resolution crystal structures are available, mostly thanks to thermophilic cyanobacteria with very stable complexes ideal for crystallographic analysis (Jordan et al, 2001; Ferreira et al, 2004; Umena et al, 2011). The glucose tolerant strain originating from the strain used in the Arizona lab of Wim Vermaas showed aberrant assembly of PSII, much lower level of PSI related to the decreased synthesis of Chl and high content of carotenoids Sequencing of the latter strain showed that its genome contains a tandem duplication of a large chromosomal region, most probably responsible for the observed phenotype
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