Expression and regulation of the crucial plant‐like ferredoxin of cyanobacteria

Summary The Synechocystis fedI gene ( petF , ssl0020) was found to be strongly expressed under the negative control of H 2 O 2 or heavy metals, and the positive control of light fluence (regulation dependent on active photosynthesis) or carbon availability [under the control of NdhR, the regulator of the ndh3 operon encoding N A D (P) H dehydrogenase subunits]. The b asic and constitutive p romoter (BP) of fedI extending from −62 to +25 (relative to the transcription start point) is weakly active, presumably because it harbours a long (30 bp) spacer between the two crucial motifs: the −10 box (5′‐TAgtAT‐3′, −13 to −8) and the ‘−35’ box (5′‐TTGctA‐3′, −49 to −44). BP strength is strongly enhanced by the two upstream regions, −113 to −82 and −151 to −114, mediating the 30‐fold constitutive stimulation and the fourfold light activation respectively. Three well‐conserved transcriptional elements were characterized for the first time, namely the −19 box (5′‐TTTT‐3′) that is essential to transcription, and the two twice repeated elements that are both critical to light induction: the TTGyCA‐3′ box (−35 to −30, and −125 to −120) and the 5′‐ATTTyA‐3′ box (−55 to −50, and −134 to −129). That two of these light induction motifs (5′‐TTGtCA‐3′, −35 to −30; 5′‐ATTTcA‐3′, −55 to −50) occur in the constitutive BP promoter indicate that in the fedI gene light activation and transcription per se are closely interacting. Interestingly, the fedI gene from marine strains was found to lack the three transcriptional elements presently described, as well as the 5′‐AGGA‐3′ Shine–Dalgarno sequence, which are all conserved among the fedI from non‐marine strains.