The REPI region is part of an iron transport system-replication unit that we found to be conserved in bacterial plasmids carrying the aerobactin iron transport genes. The REPI replication region endows the cell with incompatibility properties corresponding to the IncFI group and must have been instrumental in the preservation and epidemiological spread of the aerobactin system. This replicon is frequently found in bacterial plasmids, however little is known on the mechanism controlling its replication and maintenance, as well as on the mechanisms that result in the expression of the IncFI phenotype. We successfully analyzed and characterized the DNA sequences and product(s) involved in the control of replication of REPI-containing plasmids; purified and characterized the trans- acting protein RepI, essential for REPI replication, and dissected the REPI incompatibility determinants. Furthermore, we have recently found that the iron concentration of the cell and expression of the fur (ferric uptake regulation) gene are part of a novel regulatory circuit that acts in the control of copy number of REPI. We are now performing structure-function studies on the RepI protein, and analyzing the mechanism of regulation of the copy number REPI replicons by Fur and the iron status of the cell. %%% This system stands out as a model to study the regulation of initiation of replication and its relationship to environmental factors.
