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C. MrgA—Oxidative Stress and Fe Storage

An interesting current project concerns the MrgA protein.  We first identified this protein in our oxidative stress experiment and found this to be part of the PerR regulon.  We are currently studying this protein from two directions.  First, in a DmrgA mutant, among the gene changes that occurred in the mutant relative to the wild-type, there were two major gene clusters that appear to be involved with the biosynthesis of exopolysaccharides (EPS) and other proteins destined for the cell wall.  Thus, we began studying the relationship between oxidative stress damage and the EPS.  In order to do so, we have needed to develop some new techniques, including the study of various stains via fluorescence microscopy.  These experiments are being done in connection with Dr. Jamie Foster, a former postdoc in the lab.  Our working hypothesis is that DmrgA cells are less sensitive to peroxide killing as the culture approaches stationary phase than while growing exponentially due to the large increase in EPS that occurs during growth.  To test this hypothesis, we have knocked out one of the gene clusters involved with extracellular EPS production and are using that in our studies.  Our preliminary data supports our working hypothesis.

We have provided the DmrgA strain, as well as the above mutant strains, to Dr. Nir Keren of the Hebrew University in Israel.  He had been studying Fe storage in the bacterioferritin proteins in Synechocystis.  However, the deletion of both Bfr genes indicated that other proteins were obviously involved and we decided to investigate the importance of MrgA as an Fe-storage protein.  Work done in both laboratories indicates that DmrgA cultures grow less well in a Fe-deplete medium, and experiments indicated that there is an impeded utilization of the intracellular iron.  Our current results suggest that MrgA plays an important role in the transport of intracellular Fe from storage in the bacterioferritins, to biosynthesis of metal cofactors during cell growth.

At the current time we have two hypotheses for the overall role of MrgA in Synechocystis.  First, it appears to be a critical Fe storage/transfer protein that likely interacts with the bacterioferritins in order to make Fe bioavailable to the cell.  In addition, the lack of MrgA in the cell appears to once again affect the external surface of the cell.  More exopolysaccharide is produced and excreted and, with time, this helps cells become less susceptible to peroxide stress.