Coordinated transporter activity shapes high-affinity iron acquisition in cyanobacteria

Abstract

Iron bioavailability limits biological activity in many aquatic and terrestrial environments. Broad scale genomic meta-analyses indicated that within a single organism, multiple iron transporters may contribute to iron acquisition. Here, we present a functional characterization of a cyanobacterial iron transport pathway that utilizes concerted transporter activities. Cyanobacteria are significant contributors to global primary productivity with high iron demands. Certain cyanobacterial species employ a siderophore-mediated uptake strategy; however, many strains possess neither siderophore biosynthesis nor siderophore transport genes. The unicellular, planktonic, freshwater cyanobacterium, Synechocystis sp. PCC 6803, employs an alternative to siderophore-based uptake-reduction of Fe(III) species before transport through the plasma membrane. In this study, we combine short-term radioactive iron uptake and reduction assays with a range of disruption mutants to generate a working model for iron reduction and uptake in Synechocystis sp. PCC 6803. We found that the Fe(II) transporter, FeoB, is the major iron transporter in this organism. In addition, we uncovered a link between a respiratory terminal oxidase (Alternate Respiratory Terminal Oxidase) and iron reduction -suggesting a coupling between these two electron transfer reactions. Furthermore, quantitative RNA transcript analysis identified a function for subunits of the Fe(III) transporter, FutABC, in modulating reductive iron uptake. Collectively, our results provide a molecular basis for a tightly coordinated, high-affinity iron transport system.

New insights into iron acquisition by cyanobacteria: an essential role for ExbB-ExbD complex in inorganic iron uptake

“…Fe 0 may be able to enter the cells by passive transport via OM porins at a very low rate, but the majority would be absorbed by energy-coupled active transport. As it crosses the OM, Fe 0 maybe reduced in the periplasmic space before transport across the cyanobacterial plasma membrane (Kranzler et al, 2011(Kranzler et al, , 2014. The findings in the present study provide new insights into the iron acquisition pathway of photosynthetic cyanobacteria in aqueous environments.…”

“…As many cyanobacterial species other than Synechocystis 6803 do not excrete siderophores, the importance of siderophore-mediated iron uptake in cyanobacteria has been questioned. Hopkinson and Morel (2009) and Kranzler et al (2011Kranzler et al ( , 2014 suggested that cyanobacteria probably use uptake pathways involving a reduction step, rather than direct internalization of siderophores and other iron chelates. We hypothesize that organic chelators such as siderophores may provide iron pools, which maintain iron solubility and recruit iron in dilute water environments, while cyanobacteria prefer free iron and efficiently sequester it with high rate via TonB-ExbB-ExbD-dependent active transport system (see a hypothesized model shown in Figure 5).…”

“…Outer membrane (OM) transport of siderophore-Fe is also under control with the TonB-ExbB-ExbDdependent transport system despite its low rate. Once Fe(III) crosses the OM, it is probably bound to periplasmic protein FutA2 and then reduced to Fe(II) by a reductive iron uptake pathway (Kranzler et al, 2014). Immediately, Fe(II) could be transported into CM by CMlocated FeoB transporter.…”

“…Hopkinson and Morel (2009) suggested that the role of siderophores in marine environments is probably overestimated but could reach no definitive conclusion, because the iron acquisition mechanisms of cyanobacteria are poorly understood. Kranzler et al (2011Kranzler et al ( , 2014 proposed an alternative reduction-based uptake strategy by which cyanobacteria can sequester iron from multiple complexes in dilute aquatic environments.…”

Long-Term Acclimation to Iron Limitation Reveals New Insights in Metabolism Regulation of Synechococcus sp. PCC7002

“…For instance, they undergo major architectural changes in photosynthetic membranes involving the over-expression of photosynthetic pigments (Ryan-Keogh et al, 2012), the use Fe-economic pathways for ATP synthesis (Bailey et al, 2008), and the reduction of their Fe requirements (Behrenfeld and Milligan, 2013). In addition, some cyanobacteria have also developed specific high-affinity Fe uptake strategies (Morrissey and Bowler, 2012;Kranzler et al, 2014) such as production of siderophores that enhance Fe bioavailability in their immediate environment (Sunda, 2012). Thus, these complex biological interactions with Fe chemistry challenge the quantification of Fe bioavailability in marine environments.…”