Photoreversibility of the effect of red and green light pulses on the accumulation in darkness of mRNAs coding for phycocyanin and phycoerythrin in Fremyella diplosiphon

DNA fragments encoding a red light-inducible phycocyanin gene and a green light-inducible phycoerythrin gene have been used to investigate the effect of red and green pulses on the accumulation of phycocyanin and phycoerythrin mRNA in subsequent darkness. A red pulse promotes phycocyanin and suppresses phycoerythrin mRNA accumulation while a green pulse has an opposite effect on both transcript levels. The effect of a saturating light pulse is canceled by a subsequently given pulse of the other light quality. For a given mRNA, the positive and negative effects require the same fluence for saturation, whereas to saturate the phycoerythrin mRNA response requires at least twice as much light as to saturate the phycocyanin mRNA response. Calculations of the apparent extinction coefficients for the pigments mediating the light-regulated mRNA increase and decrease are of the order of 2 × 104 for phycocyanin mRNA and less than 104 for phycoerythrin mRNA. The data are consistent with the hypothesis that the light-induced increase and decrease of a particular phycobiliprotein mRNA is controlled by a single red/green photoreversible photosystem, but that phycoerythrin and phycocyanin mRNA levels are either controlled by two distinct photoreversible systems or that marked differences occur in the chain of events leading from photoperception to gene activation. These system(s) differ from most phytochrome systems in several ways: First, they remain fully on or off depending upon the light quality of the terminal irradiation. Second, they can be completely reversed by light of the appropriate wavelength after several hours of darkness without diminution of the effectiveness of the reversing light pulse. These two features argue against the existence of dark reversion or dark destruction of the biologically active moiety. Third, signal transduction is rapid--measurable mRNA changes occur even during a 10 minute irradiation.