One of the most virulent and feared bacterial pathogens is Yersinia pestis, the aetiologic agent of bubonic plague. Characterization of the O‐antigen gene clusters of 21 serotypes of Yersinia pseudotuberculosis and the cryptic O‐antigen gene cluster of Y. pestis showed that the plague bacillus is most closely related to and has evolved from Y. pseudotuberculosis serotype O:1b. The nucleotide sequences of both gene clusters (about 20.5 kb each) were determined and compared to identify the differences that caused the silencing of the Y. pestis gene cluster. At the nucleotide sequence level, the loci were 98.9% identical and, of the 17 biosynthetic genes identified from the O:1b gene cluster, five were inactivated in the Y. pestis cluster, four by insertions or deletions of one nucleotide and one by a deletion of 62 nucleotides. Apparently, the expression of the O‐antigen is not beneficial for the virulence or to the lifestyle of Y. pestis and, therefore, as one step in the evolution of Y. pestis, the O‐antigen gene cluster was inactivated.

Yersinia enterocolitica O:9 strain Ruokola/71-c-FR1-37-R possesses mainly rough-type lipopolysaccaride (LPS) and smaller amounts of S-form LPS. Structural analysis of the former is reported here. After deacylation of the LPS, the phosphorylated carbohydrate backbone of the inner core-lipid A region could be isolated by using highperformance anion-exchange chromatography. Its structure was determined by means of compositional and methylation analyses and 1 H-, 13 C-, and 31 P-NMR spectroscopy as:and Kdo is 3-deoxy-d-manno-oct-2-ulopyranosonic acid. All hexoses are pyranoses.

L-Glycero-D-manno-heptopyranose is a characteristic compound of many lipopolysaccharide (LPS) core structures of Gram-negative bacteria. In Escherichia coli two heptosyltransferases, namely WaaC and WaaF, are known to transfer L-glycero-D-manno-heptopyranose to Re-LPS and Rd 2 -LPS, respectively. It had been proposed that both reactions involve ADPL-glycero-D-manno-heptose as a sugar donor; however, the structure of this nucleotide sugar had never been completely elucidated. In the present study, ADPL-glycero-D-manno-heptose was isolated from a heptosyltransferasedeficient E. coli mutant, and its structure was determined by nuclear magnetic resonance spectroscopy and matrix-assisted laser-desorption/ionization time-of-flight mass spectrometry as ADPL-glycero-β-D-manno-heptopyranose. This compound represented the sole constituent of the bacterial extract that was accepted as a sugar donor by heptosyltransferases I and II in vitro.