Osmoregulated periplasmic glucans (OPGs) are oligosaccharides found in the periplasm of many Gram-negative bacteria. Glucose is the sole constitutive sugar and this backbone may be substituted by various kinds of molecules depending on the species. In E. coli, OPG are substituted by phosphoglycerol and phosphoethanolamine derived from membrane phospholipids and by succinyl residues. In this study, we describe the isolation of the opgE gene encoding the phosphoethanolamine transferase by a screen previously used for the isolation of the opgB gene encoding the phosphoglycerol transferase. Both genes show structural and functional similarities without sequence similarity.

fThe opgGH operon encodes glucosyltransferases that synthesize osmoregulated periplasmic glucans (OPGs) from UDP-glucose, using acyl carrier protein (ACP) as a cofactor. OPGs are required for motility, biofilm formation, and virulence in various bacteria. OpgH also sequesters FtsZ in order to regulate cell size according to nutrient availability. Yersinia pestis (the agent of fleaborne plague) lost the opgGH operon during its emergence from the enteropathogen Yersinia pseudotuberculosis. When expressed in OPG-negative strains of Escherichia coli and Dickeya dadantii, opgGH from Y. pseudotuberculosis restored OPGs synthesis, motility, and virulence. However, Y. pseudotuberculosis did not produce OPGs (i) under various growth conditions or (ii) when overexpressing its opgGH operon, its galUF operon (governing UDP-glucose), or the opgGH operon or Acp from E. coli. A ⌬opgGH Y. pseudotuberculosis strain showed normal motility, biofilm formation, resistance to polymyxin and macrophages, and virulence but was smaller. Consistently, Y. pestis was smaller than Y. pseudotuberculosis when cultured at >37°C, except when the plague bacillus expressed opgGH. Y. pestis expressing opgGH grew normally in serum and within macrophages and was fully virulent in mice, suggesting that small cell size was not advantageous in the mammalian host. Lastly, Y. pestis expressing opgGH was able to infect Xenopsylla cheopis fleas normally. Our results suggest an evolutionary scenario whereby an ancestral Yersinia strain lost a factor required for OPG biosynthesis but kept opgGH (to regulate cell size). The opgGH operon was presumably then lost because OpgH-dependent cell size control became unnecessary. Yersinia pestis is the bacterium that causes plague, a fatal disease that cycles between mammalian and flea hosts (1). After Y. pestis is taken up into a flea's gut during a blood meal, the bacterium forms a biofilm that ultimately obstructs the digestive tract. The "blocked" (and thus starving) flea will bite a new host many times in an effort to feed. During these unproductive attempts to feed, some bacteria are dislodged from the biofilm and regurgitated into the dermal biting site (2-4). Although it is not clear whether these Y. pestis organisms are truly phagocytized, it generally assumed that they replicate initially within phagocytes and produce antiphagocytic factors; this leads to extracellular replication throughout the draining lymph node and ultimately in the blood and other deep tissues (5).Y. pestis emerged from Yersinia pseudotuberculosis, a widely spread environmental bacterium that causes a mild bowel disease in humans following ingestion of contaminated foods (6). During its emergence, Y. pestis accreted a small amount of genetic material via horizontal transfer but also lost a large number of functional genes (7). Early investigations showed that the emergence of plague can be explained (at least in part) by the acquisition of genetic material (8-11). Hence, a stepwise scenario in which sequential gene losses led to a flea-b...