Reflections on the Function of IsiA, a Cyanobacterial Stress-inducible, Chl-binding Protein

The isiA gene encodes a protein that is similar to the Photosystem II chlorophyll-binding protein CP43, but lacks the entire large lumenal loop of over 100 amino acids. What is the function of this IsiA protein? Research on IsiA has traveled a long and interesting path since it was first discovered by its large accumulation during growth under iron-limited conditions. What appeared to be a simple on-off switch for isiA based on iron concentration has developed into a much richer and more intriguing set of possibilities that involve its expression and function. We provide an overview of isiA transcriptional regulation by many environmental factors and its proposed functions. We also describe the response to oxidative stress by cells that lack the IsiA protein. It is now clear that isiA expression can be de-repressed in the presence of normal iron levels and that the regulatory mechanisms can be linked to the inter-relationship between iron homeostasis and oxidative stress. The de facto transcriptional control of isiA expression has expanded to include regulation at both the transcriptional and post-transcriptional levels.