Elucidation of the coping strategy in an OMP homozygous knockout mutant of Synechocystis 6803 defective in iron uptake.

The TonB-dependent outer membrane (OM) transporters import iron (Fe) under Fe-deficient growth conditions in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 (S. 6803 or WT). The present study characterises the mechanisms employed by a homozygous Δslr1908 (Slr1908, an OM protein) mutant of S. 6803 to adapt to intracellular iron deficiency imposed due to lack of Slr1908 when grown under Fe sufficient conditions for short (1 week) and long term (3 weeks). Although the homozygous Δslr1908 cells showed symptoms of stress such as slow growth, high ROS and altered ultrastructure, they were transcriptionally as well as translationally modulating their physiology to abrogate the Fe limitation. The mutant displayed only 20% of the Fe content in comparison to the WT after 1 week of growth, which increased to 50% by the end of the third week. The increase in intracellular Fe is ascribed to higher transcripts as well as corresponding protein of slr0042 which is the alternate OM route for Fe uptake. The mutant was also allocating its available Fe pools judiciously among Fe requiring proteins such as catalase, superoxide dismutase and cytochromes. The results elucidate the mechanism employed to tide over the Fe deficiency by Δslr1908 mutant and will be important in explaining the fine-tuning of Fe homeostasis in this model cyanobacterium.