High light enhances the expression of low-CO₂-inducible transcripts involved in the CO₂-concentrating mechanism in Synechocystis sp. PCC6803

The relative abundances of key gene transcripts encoding proteins involved in inducible inorganic carbon (C_i) transport in the cyanobacterium Synechocystis PCC6803 were quantified by real-time reverse-transcriptase- polymerase chain reaction under conditions of varying C_i and light levels. Significant increases in CO₂-concentrating mechanism (CCM)-related transcript abundance were observed only in cells aerated with CO₂-free air for 30 min in the light, but not in the dark, relative to illuminated cells grown at high CO₂ levels (high-C_i cells). Cells were incubated under precisely defined conditions in a cuvette attached to a mass spectrometer to allow for the measurement of photosynthetic gas exchange rates, [C_i] and [O₂], in combination with quantitative analysis of transcripts. Under conditions of increasing irradiance and low [C_i], or under conditions of decreasing [C_i] at constant irradiance, the abundances of cmpA (encoding part of the BCT1 HCO ₃^- transporter), ndhF3 (encoding subunit of high-affinity CO₂ uptake system) and sbtA (encoding Na⁺-dependent HCO₃^- transporter) transcripts tended to increase, relative to illuminated cells grown at high-CO₂. The cmpA transcript appeared to be less responsive to decreasing [C_i] than either ndhF3 or sbtA. The induction of cmpA and ndhF3 transcripts was completely inhibited by 10 μM 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) compared to untreated control cells. This inhibitor had no effect on sbtA expression. In the presence of 2,6-dichlorobenzoquinone (DCBQ), the expression of the cmpA transcript tracked the apparent rate of O₂ evolution from photosystem II closely as the irradiance was increased, reaching maximum levels of expression approximately 16-fold higher than control cells. Under the same conditions, the ndhF3 transcript increased by two- to three-fold, whereas the sbtA transcript did not respond to this treatment The regulation of CCM induction in this strain is discussed in relation to current hypotheses on the sensing of C_i limitation.