Increased plasmid copy number is essential for Yersinia T3SS function and virulence

Plasmid copy number promotes plague The virulence of the plague bacterium, Yersinia pestis , is encoded on a plasmid. Normally, the bacterium only tolerates one copy of the plasmid, which provides the molecular machinery for injecting toxins into host cells. Wang et al. have found that Y. pestis can only be pathogenic if plasmid numbers are boosted and express enough protein components to assemble a functional virulence apparatus. Problematically, the synthetic activity of the plasmid has such a high metabolic cost that it can impede growth. Thus, the bacterium only allows the plasmid to replicate when it senses a potential host cell. Science , this issue p. 492