Involvement of sulfoquinovosyl diacylglycerol in DNA synthesis in Synechocystis sp. PCC 6803

Abstract

Background: Sulfoquinovosyl diacylglycerol (SQDG) is present in the membranes of cyanobacteria and their postulated progeny, plastids, in plants. A cyanobacterium, Synechocystis sp. PCC 6803, requires SQDG for growth: its mutant (SD1) with the sqdB gene for SQDG synthesis disrupted can grow with external supplementation of SQDG. However, upon removal of SQDG from the medium, its growth is retarded, with a decrease in the cellular content of SQDG throughout cell division, and finally ceases. Concomitantly with the decrease in SQDG, the maximal activity of photosynthesis at high-light intensity is repressed by 40%.

Findings: We investigated effects of SQDG-defect on physiological aspects in Synechocystis with the use of SD1. SD1 cells defective in SQDG exhibited normal photosynthesis at low-light intensity as on culturing. Meanwhile, SD1 cells defective in SQDG were impaired in light-activated heterotrophic growth as well as in photoautotrophic growth. Flow cytometric analysis of the photoautotrophically growing cells gave similar cell size histograms for the wild type and SD1 supplemented with SQDG. However, the profile of SD1 defective in SQDG changed such that large part of the cell population was increased in size. Of particular interest was the microscopic observation that the mitotic index, i.e., population of dumbbell-like cells with a septum, increased from 14 to 29% in the SD1 culture without SQDG. Flow cytometric analysis also showed that the enlarged cells of SD1 defective in SQDG contained high levels of Chl, however, the DNA content was low.

Conclusions: Our experiments strongly support the idea that photosynthesis is not the limiting factor for the growth of SD1 defective in SQDG, and that SQDG is responsible for some physiologically fundamental process common to both photoautotrophic and light-activated heterotrophic growth. Our findings suggest that the SQDG-defect allows construction of the photosynthetic machinery at an elevated level for an increase in cell mass, but represses DNA synthesis. SQDG may be essential for normal replication of chromosomal DNA for completion of the cell cycle.

Figure 1

Effect of the SQDG-defect on the light-intensity dependency of photosynthesis or LAHG in SD1. (A) SD1 cells precultured with supplementation of SQDG were transferred to SQDG-replete or -deprived conditions for 3 days. Then, photosynthetic activity was measured with illumination with different light intensities. The values are averages ± SD of three independent measurements. Closed circles, SD1-SQDG; closed triangles, SD1 + SQDG. (B) Wild-type and SD1 cells were precultured under photoautotrophic conditions with supplementation of SQDG. Wild type cells were further grown under the same conditions, while SD1 cells were grown under photoautotrophic conditions with or without SQDG-supplementation. Open circles, wild type; closed circles, SD1-SQDG; closed triangles, SD1 + SQDG. (C) Wild-type and SD1 cells were precultured under LAHG conditions with supplementation of SQDG. Wild type cells were further grown under the same conditions, while SD1 cells were grown under LAHG conditions with or without SQDG-supplementation. Open circles, wild type; closed circles, SD1-SQDG; closed triangles, SD1 + SQDG.

Figure 2

Effect of the SQDG-defect on the cell morphology of Synechocystis or Synechococcus. Wild-type cells were normally cultured whereas SD1 cells precultured with supplementation of SQDG were transferred to SQDG-replete or -deprived conditions for 3 days. (A) Wild type cells of Synechocystis. (B) SD1 cells replete of SQDG. (C) SD1 cells defective in SQDG. (D) Wild type cells of Synechococcus. (E) SDL1 cells. Scale bars indicate 10 μm. Insets show two-fold magnified images. Shown are the results representative of three independent experiments. (F) Distribution of cell lengths in the wild type Synechococcus or SDL1.

Figure 3

Effect of the SQDG-defect on the cell size or Chl content. Wild-type cells were normally cultured whereas SD1 cells precultured with supplementation of SQDG were transferred to SQDG-replete or -deprived conditions for 3 days. In individual cells of the wild type or SD1, forward light scattering or Chl fluorescence was measured with a flow cytometer, as described under Materials and Methods. Shown are the results representative of three independent experiments. (A) Wild-type cells. (B) SD1 cells replete of SQDG. (C) SD1 cells defective in SQDG.

Figure 4

Effect of the SQDG-defect on the cytogram of Chl fluorescence vs. cell size or DNA content vs. cell size. Wild-type cells were normally cultured whereas SD1 cells precultured with supplementation of SQDG were transferred to SQDG-replete or -deprived conditions for 3 days. In individual cells of the wild type or SD1, Chl fluorescence vs. forward light scattering (A-C), or DNA content vs. forward light scattering (D-F) was measured with a flow cytometer, as described in Materials and methods. The results were visualized in 2D color density plot such that blue-to-red color changes in the plot indicate dot-density changes from low to high. Shown are the results representative of three independent experiments. (A), (D): Wild-type cells. (B), (E): SD1 cells replete of SQDG. (C), (F): SD1 cells defective in SQDG.