Yersinia pestis, the flea-borne agent of bubonic plague, is associated with two plasmids, the roles of which were not known. In 26 April Science, B. Joseph Hinnebusch and colleagues from National Institute of Allergy and Infectious Diseases, show that a plasmid-encoded phospholipase D is required for survival of Y. pestis in the midgut of its principal vector, the rat flea Xenopsylla cheopis (Science 2002, 296:733-735).

Hinnebusch et al. found that the activity of intracellular phospholipase D (previously characterized as Yersinia murine toxin, or ymt), seems to protect Y. pestis from a cytotoxic digestion product of blood plasma in the flea gut. By enabling colonization of the flea midgut, acquisition of this phospholipase D may have precipitated the transition of Y. pestis to obligate arthropod-borne transmission.

"Introduction of this single gene [ymt] into Y. pestis was likely a crucial step in the recent evolutionary process that led, uniquely...

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