IdiA (iron-deficiency-induced protein A) is a protein expressed at highly elevated levels in sp. strains PCC 6301 and PCC 7942 under Fe-or Mn-limiting growth conditions. Besides being similar to two bacterial Fe-binding proteins, SfuA and FbpA, IdiA shows similarity to two ORFs ( and ) of sp. PCC 6803. Northern blot analysis detected one transcript of about 1300 nt in RNA extracted from sp. PCC 6301 and PCC 7942 grown under Fe deficiency. The intensity of this transcript was considerably reduced in Fe-sufficient culture. It could be further shown that the regulation of IdiA expression is at the transcriptional level and that transcription and translation of IdiA are closely linked. Primer extension analysis indicated a single transcriptional start site 193 nt upstream of the first presumed translational start codon. Moreover, molecular characterization of the entire 5·8 kb chromosomal III DNA fragment carrying the gene from sp. PCC 6301 led to the identification of six long ORFs in addition to The two genes adjacent to and located 2018 nt downstream of were insertionally inactivated in sp. PCC 7942 and the corresponding mutants were partially characterized. These experiments provide evidence that the gene products of located immediately downstream of and of are involved in the activation of IdiA expression, since the absence of each of these two gene products prevents the greatly elevated expression of IdiA under nutrient deficiency.


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