Inorganic Carbon Limitation Induces Transcripts Encoding Components of the CO₂-Concentrating Mechanism in Synechococcus sp. PCC7942 through a Redox-Independent Pathway

The cyanobacterial CO₂-concentrating mechanism (CCM) allows photosynthesis to proceed in CO₂-limited aquatic environments, and its activity is modulated in response to inorganic carbon (Ci) availability. Real-time reverse transcriptase-PCR analysis was used to examine the transcriptional regulation of more than 30 CCM-related genes in Synechococcus sp. strain PCC7942 with an emphasis on genes encoding high-affinity Ci transporters and carboxysome-associated proteins. This approach was also used to test hypotheses about sensing of Ci limitation in cyanobacteria. The transcriptional response of Synechococcus sp. to severe Ci limitation occurs rapidly, being maximal within 30 to 60 min, and three distinct temporal responses were detected: (a) a rapid, transient induction for genes encoding carboxysome-associated proteins (ccmKLMNO, rbcLS, and icfA) and the transcriptional regulator, cmpR; (b) a slow sustained induction of psbAII; and (c) a rapid sustained induction of genes encoding the inducible Ci transporters cmpABCD, sbtA, and ndhF3-D3-chpY. The Ci-responsive transcripts investigated had half-lives of 15 min or less and were equally stable at high and low Ci. Through the use of a range of physiological conditions (light and Ci levels) and inhibitors such as 3-(3,4-dichlorophenyl)-1,1dimethylurea, glycolaldehyde, dithiothreitol, and ethoxyzolamide, we found that no strict correlation exists between expression of genes known to be induced under redox stress, such as psbAII, and the expression of the Ci-responsive CCM genes. We argue that redox stress, such as that which occurs under high-light stress, is unlikely to be a primary signal for sensing of Ci limitation in cyanobacteria. We discuss the data in relation to current theories of CO₂ sensing in cyanobacteria.