Adaptation of cyanobacterial photosynthesis to metal constraints

Metals are essential elements for life, but they are particularly necessary for photosynthetic organisms. Here, we focus on how cyanobacterial photosynthesis responds to metal constrains, with especial emphasis on copper and iron. In particular, the photosynthetic machinery requires large amounts of iron, either as free ions or in heme and iron-sulfur cofactors. In response to the limitations and bioavailability of iron, photosynthetic organisms have developed alternatives to replace iron-containing proteins. Thus, in the photosynthetic chain of most cyanobacteria, flavodoxin (a flavoprotein) replaces ferredoxin (an iron-sulfur protein) on the donor side of photosystem I under iron-limiting conditions. In addition, when copper is available, plastocyanin (a copper protein) substitutes the heme-protein cytochrome c6 in the lumen of the thylakoid as the electron carrier from cytochrome b6f to photosystem I, and in connecting the photosynthetic and respiratory chains. In each of these pairs, both proteins have developed equivalent functional areas, resulting in a functional equivalence. However, the plastocyanin/cytochrome c6 couple exhibits intriguing parallel differences in different cyanobacteria. The ferredoxin/flavodoxin and plastocyanin/cytochrome c6 expression is precisely regulated as a function of variations in iron or copper concentrations. Recently, the system regulating the copper-dependent cytochrome c6/plastocyanin switch has been described as composed by a transcriptional factor and a protease, the latter regulating the levels of the expression factor in response to the presence of copper.