¹⁵N-labeling to determine chlorophyll synthesis and degradation in Synechocystis sp. PCC 6803 strains lacking one or both photosystems

Rates of chlorophyll synthesis and degradation were analyzed in Synechocystis sp. PCC 6803 wild type and mutants lacking one or both photosystems by labeling cells with (¹⁵NH₄) ₂SO₄ and Na¹⁵NO₃. Pigments extracted from cells were separated by HPLC and incorporation of the ¹⁵N label into porphyrins was subsequently examined by MALDI-TOF mass spectrometry. The life time (τ) of chlorophyll in wild-type Synechocystis grown at a light intensity of 100 μmol photons m^-2 s^-1 was determined to be about 300 h, much longer than the cell doubling time of about 14 h. Slow chlorophyll degradation (τ ∼200-400 h) was also observed in Photosystem I-less and in Photosystem II-less Synechocystis mutants, whereas in a mutant lacking both Photosystem I and Photosystem II chlorophyll degradation was accelerated 4-5 fold (τ ∼50 h). Chlorophyllide and pheophorbide were identified as intermediates of chlorophyll degradation in the Photosystem I-less/Photosystem II-less mutant. In comparison with the wild type, the chlorophyll synthesis rate was five-fold slower in the Photosystem I-less strain and about eight-fold slower in the strain lacking both photosystems, resulting in different chlorophyll levels in the various mutants. The results presented in this paper demonstrate the presence of a regulation that adjusts the rate of chlorophyll synthesis according to the needs of chlorophyll-binding polypeptides associated with the photosystems.