YopJ is an effector protein with acetyltransferase activity that is secreted into host cells. Compared with CO92, the
Lys→Glu substitution in YopJ of the Justinian (and Black Death) strains increases the antihost activity of this effector. Additionally, using our new pan-array we were able to identify a roughly 15 kb genomic island in the Justinian strains and also in the Black Death strain that we previously sequenced (appendix 1 p 16). This genomic island, referred to as DFR 4
(different region 4), was lost in some Y pestis strains, such as CO92. DFR 4 contains several genes with potential roles in virulence, including ccm2A, which encodes a multidrug ABC transporter ATPase.