Immunocytochemical localization of the stress-induced DpsA protein in the cyanobacterium Synechococcus sp. strain PCC 7942.

Proteins of the Dps family are divergent ferritins that have been shown to bind DNA with high affinity during periods of nutrient and oxidative stress. Such binding protects the chromosome from peroxide attack. Surprisingly, we show by immunocytochemistry that the cyanobacterial Dps homolog, DpsA, localizes preferentially to the thylakoid membrane in Synechococcus sp. strain PCC7942. We propose that two DpsA pools are functioning in this species--an insoluble fraction bound to the chromosome, and a soluble fraction acting as a ferritin involved metal homeostasis of the photosynthetic apparatus. This model is presented in light of recent work on the E. coli Dps protein showing that DNA binding is regulated by the metal-binding capacity of the Dps complex (Frenkiel-Krispin et al. 2001). Additionally, the pattern of DpsA localization in cells as they progress through the growth curve suggests that the DpsA complex may be involved in metal ion transport across the cell envelope.