Iron-dependent protection of theSynechococcus ferredoxin I transcript against nucleolytic degradation requirescis-regulatory sequences in the 5′ part of the messenger RNA

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Abstract
We have previously reported that the ferredoxin I gene fromSynechococcus sp. PCC 7942 is regulated by iron at the level of differential mRNA stability. To identify iron-responsive elements in theSynechococcus ferredoxin transcript, we have tested chimaeric constructs containing translational fusions between theSynechococcus and theAnabaena sp. PCC 7937 ferredoxin genes for iron-dependent expression in transgenicSynechococcus strains. This strategy was based on the observation that the level of theAnabaena ferredoxin mRNA did not increase upon iron addition inSynechococcus. Our results show that the presence of the first 207 nucleotides of theSynechococcus ferredoxin transcript is sufficient to confer iron responsiveness to the chimaeric transcripts. This iron responsiveness was accomplished by an increased stability of the chimaeric transcript in the presence of iron, as was found for the intactSynechococcus ferredoxin gene. Addition of the translation inhibitor chloramphenicol to the cultures led to a rapid stabilization, in low- and high-iron conditions, of the wild-typeSynechococcus ferredoxin transcript as well as all chimaeric ferredoxin transcripts tested. These results suggest the existence of a constitutively expressed nuclease capable of degrading the ferredoxin transcripts. They further support the suggestion that the first 207 nucleotides of theSynechococcus transcript contain a specific sequence that is recognized by an iron-responsive factor and that this interaction leads to protection against degradation.