IMMUNOCYTOCHEMICAL LOCALIZATION OF IDIA, A PROTEIN EXPRESSED UNDER IRON OR MANGANESE LIMITATION IN THE MESOPHILIC CYANOBACTERIUM SYNECHOCOCCUS PCC-6301 AND THE THERMOPHILIC CYANOBACTERIUM SYNECHOCOCCUS-ELONGATUS

Iron-deficiency-induced protein A (IdiA) with a calculated molecular m ass of 35 kDa has previously been shown to be essential under manganes e- and iron-limiting conditions in the cyanobacteria Synechococcus PCC 6301 and PCC 7942. Studies of mutants indicated that in the absence o f IdiA mainly photosystem II becomes damaged, suggesting that the majo r function of IdiA is in Mn and not Fe metabolism (Michel et al. 1996, Microbiology 142: 2635-2645). To further elucidate the function of Id iA, the immunocytochemical localization of IdiA in the cell was examin ed. These investigations provided evidence that under mild Fe deficien cy IdiA is intracellularly localized and is mainly associated with the thylakoid membrane in Synechococcus PCC 6301. The protein became dist ributed throughout the cell under severe Fe limitation when substantia l morphological changes had already occurred. For additional verificat ion of a preferential thylakoid membrane association of IdiA, these in vestigations were extended to the thermophilic Synechococcus elongatus . In this cyanobacterium Mn deficiency could be obtained more rapidly than in the mesophilic Synechococcus PCC 6301 and PCC 7942, and the th ylakoid membrane structure proved to be more stable under limiting gro wth conditions. The immunocytochemical investigations with this cyanob acterium clearly supported a thylakoid membrane association of IdiA. I n addition, evidence was obtained for a localization of IdiA on the cy toplasmic side of the thylakoid membrane. All available data support a function of IdiA as an Mn-binding protein that facilitates transport of Mn via the thylakoid membrane into the lumen to provide photosystem II with Mn. A possible explanation for the observation that IdiA a as not only expressed under Mn deficiency but also under Fe deficiency i s given in the discussion.