A parasitic plasmid poaches a centromeric protein from its host, as shown by Hajra et al. on page 779. The protein helps the plasmid segregate during mitosis.
The plasmid is a benign yeast parasite, called the 2-μm plasmid, that replicates and segregates precisely to maximize its numbers while restricting itself to nontoxic levels. The plasmid's segregation locus, STB, contains chromatin-remodeling proteins, suggesting that STB's architecture is different from that of the rest of the plasmid—much as the centromere is unique within a host chromosome.
The new results reveal how the plasmid makes the STB locus distinctive: it marks it with the same histone H3 variant, Cse4p, that its host uses to tag centromeres. Cse4p is a rapidly degraded protein but is somehow protected from the proteasome at the STB locus, as it is at yeast centromeres. Plasmid mutants that do not bind...
