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

Abstract

The relative abundances of key gene transcripts encoding proteins involved in inducible inorganic carbon (Ci) transport in the cyanobacterium Synechocystis PCC6803 were quantified by real-time reverse-transcriptase- polymerase chain reaction under conditions of varying Ci 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-Ci 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, [Ci] and [O2], in combination with quantitative analysis of transcripts. Under conditions of increasing irradiance and low [Ci], or under conditions of decreasing [Ci] 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 [Ci] 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 Ci limitation.