Inorganic Carbon Limitation and Light Control the Expression of Transcripts Related to the CO2-Concentrating Mechanism in the Cyanobacterium Synechocystis sp. Strain PCC6803

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Abstract
The cyanobacterium Synechocystis sp. strain PCC6803 possesses three modes of inorganic carbon (Ci) uptake that are inducible under Ci stress and that dramatically enhance the efficiency of the CO2-concentrating mechanism (CCM). The effects of Ci limitation on the mRNA transcript abundance of these inducible uptake systems and on the physiological expression of the CCM were investigated in detail in this cyanobacterium. Transcript abundance was assessed with semiquantitative and real-time reverse transcriptase-polymerase chain reaction techniques. Cells aerated with CO2-free air for 30 min in the light, but not in the dark, depleted the total [Ci] to near zero levels. Under these conditions, the full physiological expression of the CCM was apparent within 2 h. Transcripts for the three inducible Ci uptake systems,ndhF3, sbtA, and cmpA, showed near-maximal abundance at 15 min under Ci limitation. The transcriptional regulators, cmpR andndhR, were more moderately expressed, whereas therbcLXS and ccmK-N operons andndhF4/ndhD4/chpX and ccaAgenes were insensitive to the low-Ci treatment. The combined requirement of low Ci and light for the expression of several CCM-related transcripts was examined using real-time reverse transcriptase-polymerase chain reaction. CmpA,ndhF3, and sbtA were strongly expressed in the light, but not in the dark, under low-Ci conditions. We could find no evidence for induction of these or other CCM-related genes by a high-light treatment under high-CO2 conditions. This provided evidence that high-light stress alone could not trigger the expression of CCM-related transcripts in Synechocystissp. PCC6803. Potential signals triggering induction of the high-affinity state of the CCM are discussed.

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