The calcium dependence region of the 47 megadalton plasmid of Yersinia pestis is required for growth within macrophages.

Yersinia pestis cells bearing the 6,47 and 61 megadalton (Mdal) plasmids have been shown previously to be more resistant to novobiocin and to take up less gentian violet than cells lacking the 47 Mdal plasmid. Mutants with lesions in the calcium dependence region of the 47 Mdal plasmid, which are known to have resistance to high levels of novobiocin, were found to have rates of gentian violet and deoxycholate sensitivities identical to cells with intact plasmids. Cells bearing the 6, 47 and 61 Mdal plasmids, but not cells without the 47 Mdal plasmid, were known to grow within peritoneal macrophages. Cells with mutations in the calcium dependence region of the 47 Mdal plasmid had greatly reduced virulence in iron compromised mice and were unable to proliferate in the peritoneal macrophages of these mice. Cells lacking both the 6 and 61 Mdal plasmids, selected in the presence of gentian violet, were found to have reduced gentian violet uptake but unexpectedly had increased sensitivity to novobiocin and deoxycholate. These cells had an LD50 in iron compromised mice similar to that of cells with all three plasmids and proliferated in the peritoneal macrophages of iron compromised mice.