Abstract
The isocyclic ring (E-ring) is a common structural feature of chlorophylls. The E-ring is formed by two structurally unrelated Mg-protoporphyrin IX monomethylester (MPE) cyclase systems, oxygen-dependent (AcsF), and oxygen-independent (BchE) systems, which involve incorporation of an oxygen atom from molecular oxygen and water into the C-13(1) position of MPE, respectively. Which system operates in cyanobacteria that can thrive in a variety of anaerobic environments remains an open question. The cyanobacterium Synechocystis sp. PCC 6803 has two acsF-like genes, sll1214 (chlA(I)) and sll1874 (chlA(II)), and three bchE-like genes, slr0905, sll1242, and slr0309. Five mutants lacking one of these genes were isolated. The DeltachlA(I) mutant failed to grow under aerobic conditions with anomalous accumulation of a pigment with fluorescence emission peak at 595 nm, which was identified 3,8-divinyl MPE by high-performance liquid chromatography-mass spectrometry analysis. The growth defect of DeltachlA(I) was restored by the cultivation under oxygen-limited (micro-oxic) conditions. MPE accumulation was also detected in DeltachlA(II) grown under microoxic conditions, but not in any of the bchE mutants. The phenotype was consistent with the expression pattern of two chlA genes: chlA(II) was induced under micro-oxic conditions in contrast to the constitutive expression of chlA(I). These findings suggested that ChlA(I) is the sole MPE cyclase system under aerobic conditions and that the induced ChlA(II) operates together with ChlA(I) under micro-oxic conditions. In addition, the accumulation of 3,8-divinyl MPE in the DeltachlA mutants suggested that the reduction of 8-vinyl group occurs after the formation of E-ring in Synechocystis sp. PCC 6803.
Highlights
Cyanobacteria are prokaryotes performing oxygenic photosynthesis similar to plants
Ancient cyanobacteria are thought to have started to carry out oxygenic photosynthesis about 2.7 giga years ago [15, 16, 17], and later one lineage of cyanobacteria to have become endosymbionts of protoeukaryotic cells leading to chloroplasts of plants [18]
Chlorophyll E-ring Formation in Cyanobacteria are usually associated with aerobic environments, many strains thrive in environments with a variety of oxygen levels including anaerobic environments such as microbial mats, lake sediments, and soil [19, 20]
Summary
Chlorophyll; MPE, magnesium protoporpyrin IX monomethylester; Pchlide, protochlorophyllide. Chlorophyll E-ring Formation in Cyanobacteria are usually associated with aerobic environments, many strains thrive in environments with a variety of oxygen levels including anaerobic environments such as microbial mats, lake sediments, and soil [19, 20]. Most cyanobacteria face a diurnal light and dark cycle, and the oxygen level in natural environments undergoes dynamic changes from anaerobic to aerobic throughout the day. Little information is available regarding which E-ring formation system operates and how it is regulated under oxygen-fluctuated environments in cyanobacteria. We report the identification of two acsF-like genes, chlAI and chlAII, in the cyanobacterium Synechocystis sp. We found that ChlAI is the sole E-ring formation system under aerobic conditions and that the second isoform ChlAII operates together with ChlAI under oxygen-limited (micro-oxic) conditions in Synechocystis sp.
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