IRON-DEPENDENT PROTECTION OF THE SYNECHOCOCCUS FERREDOXIN-I TRANSCRIPT AGAINST NUCLEOLYTIC DEGRADATION REQUIRES CIS-REGULATORY SEQUENCES INTHE 5' PART OF THE MESSENGER-RNA

We have previously reported that the ferredoxin I gene from Synechococ cus sp. PCC 7942 is regulated by iron at the level of differential mRN A stability. To identify iron-responsive elements in the Synechococcus ferredoxin transcript, we have tested chimaeric constructs containing translational fusions between the Synechococcus and the Anabaena sp. PCC 7937 ferredoxin genes for iron-dependent expression in transgenic Synechococcus strains. This strategy was based on the observation that the level of the Anabaena ferredoxin MRNA did not increase upon iron addition in Synechococcus. Our results show that the presence of the f irst 207 nucleotides of the Synechococcus ferredoxin transcript is suf ficient to confer iron responsiveness to the chimaeric transcripts. Th is iron responsiveness was accomplished by an increased stability of t he chimaeric transcript in the presence of iron, as was found for the intact Synechococcus ferredoxin gene. Addition of the translation inhi bitor chloramphenicol to the cultures led to a rapid stabilization, in low- and high-iron conditions, of the wild-type Synechococcus ferredo xin transcript as well as all chimaeric ferredoxin transcripts tested. These results suggest the existence of a constitutively expressed nuc lease capable of degrading the ferredoxin transcripts. They further su pport the suggestion that the first 207 nucleotides of the Synechococc us transcript contain a specific sequence that is recognized by an iro n-responsive factor and that this interaction leads to protection agai nst degradation.