Identification of a Novel Vinyl Reductase Gene Essential for the Biosynthesis of Monovinyl Chlorophyll in Synechocystis sp. PCC6803

Abstract

The vast majority of oxygenic photosynthetic organisms use monovinyl chlorophyll for their photosynthetic reactions. For the biosynthesis of this type of chlorophyll, the reduction of the 8-vinyl group that is located on the B-ring of the macrocycle is essential. Previously, we identified the gene encoding 8-vinyl reductase responsible for this reaction in higher plants and termed it DVR. Among the sequenced genomes of cyanobacteria, only several Synechococcus species contain DVR homologues. Therefore, it has been hypothesized that many other cyanobacteria producing monovinyl chlorophyll should contain a vinyl reductase that is unrelated to the higher plant DVR. To identify the cyanobacterial gene that is responsible for monovinyl chlorophyll synthesis, we developed a bioinformatics tool, correlation coefficient calculation tool, which calculates the correlation coefficient between the distributions of a certain phenotype and genes among a group of organisms. The program indicated that the distribution of a gene encoding a putative dehydrogenase protein is best correlated with the distribution of the DVR-less cyanobacteria. We subsequently knocked out the corresponding gene (Slr1923) in Synechocystis sp. PCC6803 and characterized the mutant. The knock-out mutant lost its ability to synthesize monovinyl chlorophyll and accumulated 3,8-divinyl chlorophyll instead. We concluded that Slr1923 encodes the vinyl reductase or a subunit essential for monovinyl chlorophyll synthesis. The function and evolution of 8-vinyl reductase genes are discussed.Chlorophyll is an essential molecule that is utilized in photosynthetic reactions (1). Various chlorophyll species exist among photosynthetic organisms. Anoxygenic photosynthetic bacteria contain bacteriochlorophyll a, b, c, d, e, and g (2), and oxygenic photosynthetic organisms such as cyanobacteria, algae, and land plants produce chlorophyll a, b, c, d and 3,8-divinyl chlorophyll a 3 and b ( Fig. 1) (3, 4). Among them, the biosynthesis of chlorophyll a, b, c, d and all bacteriochlorophyll species requires the reduction of the 8-vinyl group (2). All of these chlorophyll molecules have a common backbone structure. Modification of the side chains on the common backbone structure gives rise to the diversity of chlorophylls (5, 6). Among the variety of chlorophyll species, bacteriochlorophyll a and monovinyl chlorophyll a (Fig. 1) are most commonly used for the photochemistry in photosynthetic organisms. The exceptions to this are a group of marine cyanobacteria, Prochlorococcus species (7), and a cyanobacterial symbiont, Acaryochloris marina (8), which use 3,8-divinyl chlorophyll a (Fig. 1) and chlorophyll d for their photochemistry, respectively. It is not clear why the majority of photosynthetic organisms prefer monovinyl chlorophylls instead of divinyl chlorophyll species. Akimoto et al. (9) recently suggested that the replacement of monovinyl chlorophylls by 3,8-divinyl chlorophylls in the dvr mutant of Arabidopsis thaliana significantly changed the antenna system and t...

slr1923 of Synechocystis sp. PCC6803 Is Essential for Conversion of 3,8-Divinyl(proto)chlorophyll(ide) to 3-Monovinyl(proto)chlorophyll(ide)

“…It was found that the absorption peak of the Soret band shifted to a longer wavelength by about 10 nm. The absorption spectrum of the same mutant reported by Ito et al (2008) showed a shift by 6 nm in the Soret band. Judged from the absorption spectrum, the peak of the mutant seems to be affected by the higher amount of carotenoids.…”

“…Judged from the absorption spectrum, the peak of the mutant seems to be affected by the higher amount of carotenoids. This may account for the difference of peak positions between our data and those of Ito et al (2008). Figure 3B shows changes of absorption spectra of the mutant cells grown under the high-light condition (300 mE m 22 s 21 ).…”

“…2B), indicating a high sensitivity to strong illumination. High sensitivity to strong illumination is reported in an Arabidopsis (Arabidopsis thaliana) mutant (Nagata et al, 2005;Nakanishi et al, 2005) and also in a Synechocystis 6803 mutant (Ito et al, 2008). Although the SD is large, slr1923 M showed a tendency to grow slower than the wild type under low CO 2 (0.03%).…”

“…1C, arrow), further analysis is necessary to determine whether the three open reading frames form operons or not. Ito et al (2008) addressed this problem by inactivating slr1924 and slr1925. They found no difference in phenotypes in the two mutants.…”

“…Recently, Ito et al (2008) considered slr1923 as a candidate of cyanobacterial DVR from bioinformatics analyses. They also knocked out the gene and characterized the mutant.…”