A novel function of type IV pili in iron uptake in Synechococcus sp. PCC 7002

Iron is abundant in the Earth’s crust, but in marine environments, iron is often the limiting micronutrient for cyanobacterial growth. Marine cyanobacteria such as Synechococcus sp. PCC 7002 (hereafter Synechococcus) require iron for their photosynthetic machinery, especially Photosystem I, and have therefore developed efficient iron acquisition strategies. Recent research on type IV pili suggests that these so-called “nanowires” can increase metal bioavailability by facilitating reduction of extracellular electron acceptors such as iron oxides (1,2). These pili consist of tightly stacked pilA1 proteins and seem to have conductive properties (1). We deleted the gene encoding pilA1 in Synechococcus and studied the growth of the deletion strain on different iron minerals with varying bioavailability. The ΔpilA1 strain shows reduced growth compared to wildtype Synechococcus when grown on iron oxides with low bioavailability. We analyzed pili expression with SEM, TEM and mass spectrometry. Our results indicate a clear effect of pilA1 deletion on the expression of alternative pilins. Finally, we assessed the state of the photosynthetic complexes by spectroscopic methods. Spectral analysis of the ΔpilA1 strain indicate changes of the photosynthetic complexes consistent with iron stress. Our results indicate a novel function of type IV pili in iron acquisition in marine cyanobacteria. Heterologous pilA expression and deletion of genes involved in other parts of the external electron transport chain will further elucidate how intracellular electron transport is coupled to extracellular reduction. (1) Gorby YA, Yanina S, McLean JS, et al (2006) Electrically conductive bacterial nanowires produced by Shewanella oneidensis strain MR-1 and other microorganisms. Proc Natl Acad Sci U S A 103:11358–63. doi: 10.1073/pnas.0604517103 (2) Lamb JJ, Hill RE, Eaton-Rye JJ, Hohmann-Marriott MF (2014) Functional role of PilA in iron acquisition in the cyanobacterium Synechocystis sp. PCC 6803. PLoS One 9:e105761. doi: 10.1371/journal.pone.0105761