Iron acquisition across outer membrane: The role of major outer membrane protein Slr1908 under iron supplemented conditions inSynechocystis6803

Abstract

The Ton-B dependent outer membrane (OM) transporters are responsible for active iron (Fe) import in Synechocystis sp. strain PCC 6803 (S. 6803 or WT) under Fe depletion. However, the mechanism of Fe acquisition under Fe supplemented conditions remains uncharacterised. In the present study, functional role of OMP Slr1908 in S. 6803 was addressed by insertional mutagenesis. The Δslr1908 cells exhibited slower growth in the first week in comparison to the WT and displayed an absorption and 77K fluorescence spectrum typical of Fe deficiency. Indeed, the mutant had ∼ 80% less Fe as confirmed by atomic absorption spectroscopy and 55Fe-radiotracer uptake. The iron deficiency was paralleled with low Mn content. The mutant had low SOD content as well as activity, less cytochromes, less chlorophyll content, less Fv/ Fm, lower ETRII and high oxidative stress in comparison to the WT at the end of first week. Interestingly, the mutant showed transcriptional upregulation of iron stress induced protein isiA and isiB signifying intracellular Fe deficiency. Upregulation of OMP Slr0042 was also observed at RNA and protein level. The results indicate that Slr1908 is a major Fe uptake OMP in S. 6803 the deletion of which leads to initial slow growth that gets partially offset by induction of other Fe importing OMPs.