A Novel Oxygen-induced Greening Process in a Cyanobacterial Mutant Lacking the Transcriptional Activator ChlR Involved in Low-oxygen Adaptation of Tetrapyrrole Biosynthesis

Background: ChlR activates the transcription of genes encoding low-oxygen-type enzymes in response to hypoxia in cyanobacteria. Results: The chlR-lacking mutant showed a novel oxygen-induced greening process upon exposure to air. Conclusion: The contents of photosystems were correlated well with the chlorophyll contents in the greening process. Significance: Oxygen-induced greening provides a promising alternative system to investigate the biogenesis of photosystems. ChlR activates the transcription of the chlA(II)-ho2-hemN operon in response to low-oxygen conditions in the cyanobacterium Synechocystis sp. PCC 6803. Three genes in the operon encode low-oxygen-type enzymes to bypass three oxygen-dependent reactions in tetrapyrrole biosynthesis. A chlR-lacking mutant, chlR, shows poor photoautotrophic growth due to low chlorophyll (Chl) content under low-oxygen conditions, which is caused by no induction of the operon. Here, we characterized the processes of etiolation of chlR cells in low-oxygen conditions and the subsequent regreening of the etiolated cells upon exposure to oxygen, by HPLC, Western blotting, and low-temperature fluorescence spectra. The Chl content of the etiolated chlR cells incubated under low-oxygen conditions for 7 days was only 10% of that of the wild-type with accumulation of almost all intermediates of the magnesium branch of Chl biosynthesis. Both photosystem I (PSI) and photosystem II (PSII) were significantly decreased, accompanied by a preferential decrease of antenna Chl in PSI. Upon exposure to oxygen, the etiolated chlR cells resumed to produce Chl after a short lag (approximate to 2 h), and the level at 72 h was 80% of that of the wild-type. During this novel oxygen-induced greening process, the PSI and PSII contents were largely increased in parallel with the increase in Chl contents. After 72 h, the PSI content reached approximate to 50% of the wild-type level in contrast to the full recovery of PSII. chlR provides a promising alternative system to investigate the biogenesis of PSI and PSII.