Electron transport controls transcription of the thioredoxin gene (trxA) in the cyanobacterium Synechocystis sp PCC 6803

The trxA gene encoding one of the different thioredoxins of the facultative heterotrophic cyanobacterium Synechocystis sp. PCC 6803 is transcribed as a single mRNA of 450 nucleotides. Transcript accumulation is similar in all standard growth conditions but strongly decreases after transferring cell cultures from light to darkness. In steady-state conditions, trxA transcrip tion is reduced at high (150-500 mu E m(-2) s(-1)) compared with moderate ( 10-50 mu E m(-2) s(-1)) light intensities. The stability of the trxA transc ript was similar at different light intensities, and also in darkness. Phot osynthetic electron transport inhibitors, as well as glucose starvation in a mutant strain lacking photosystem II, promote a strong decline in the lev el of trxA transcript. Primer extension analysis suggests that trxA is tran scribed from two proximal promoters containing a -10 TATA box similar to th e Escherichia coli consensus promoters. Unlike the trxA mRNA, the amount of thioredoxin protein was not reduced in the dark, neither at high light int ensities, indicating that thioredoxin protein is very stable. Our results i ndicate that the thioredoxin encoded by the trxA gene is likely to be prima rily regulated at the transcriptional level, rather than at the protein lev el, by the electron transport generated photosynthetically or from glucose metabolism.