A novel role for a TonB-family protein and photoregulation of iron acclimation in Fremyella diplosiphon.

Photosynthetic organisms display adaptations to changes in light and nutrient availability. Iron, which is required for the function of photosynthetic photosystems and other important biochemical processes, is an essential mineral that consequently impacts not only overall photosynthetic efficiency, but also the physiology of organisms in general. Our recent study represents the first functional characterization of a cyanobacterial TonB protein. TonB proteins classically are membrane proteins that support the transport of iron and vitamin B12 into cells. TonB proteins thus generally serve a critical role in organismal iron acclimation. We recently identified FdTonB, a TonB-family protein, in the filamentous freshwater cyanobacterium Fremyella diplosiphon. FdTonB contains conserved TonB residues and domains, as well as novel protein domains. Our recent study, however, supports a novel function for this protein in the photoregulation of morphology, rather than iron acclimation, in F. diplosiphon. Our detailed investigations into the responses of SF33 wild-type and ΔtonB mutant strains did not support a role for FdTonB in organismal responses to iron limitation. However, close examination of our recent results did highlight a novel interaction between light and iron acclimation in F. diplosiphon.