In the cyanobacteria Synechococcus PCC 6301 and PCC 7942 a protein with an apparent molecular mass of about 34 kDa (called IdiA for iron-deficiency-induced protein A) accumulates under iron and managanese limitation. IdiA from Synechococcus PCC 6301 was partially sequenced, showing that the N-terminal amino acid is an alanine. Moreover, the gene encoding this protein in Synechococcus PCC 6301 has been identified and completely sequenced. The idiA gene codes for a protein starting with valine and consisting of 330 amino acid residues. Thus, IdiA is apparently synthesized as a precursor protein of 36.17 kDa and cleaved to its mature form of 35.01 kDa between two alanine residues at positions 9 and 10. IdiA is a highly basic protein having an isoelectric point of 10.55 (mature protein). Comparison of the amino acid sequence of IdiA with protein sequences in the database revealed that IdiA has similarities to two basic bacterial iron-binding proteins, SfuA from Serratia marcescens and Fbp from Neisseria gonorrhoeae. Insertional inactivation of the idiA gene in Synechococcus PCC 7942 resulted in a mutant which was unable to grow under iron-or manganese-limiting conditions. Manganese limitation of the mutant strain led to a drastic reduction of photosystem II activity (O2 evolution) within less than 48 h, while wild-type cells required a prolonged cultivation in Mn-deficient medium before an effect on photosystem II was observed. Thus, IdiA is a protein involved in the process of providing photosystem II with manganese.