We identify a function-controlling O antigen chain length for a plasmid-borne gene, cldpHS-2, harboured by Flexneri strains of Escherichia coli known to cause reactive arthritis. The predicted amino acid sequence of the gene product is very similar to those of other cld genes and that of fepE, thought to be part of the enterobactin iron uptake system of E. coli. The predicted proteins are compared with rfb-associated chain length determinants as a family of related genes.

-B, and -C; IVA and -B; and VA and -B), is due to differences in sugar composition and arrangement of the remaining 0 unit (35).In both S. enterica and Y pseudotuberculosis, DDH are formed by the same biosynthetic-reaction sequences. Four DDH, abequose, tyvelose, paratose, and ascarylose, are synthesized by a common pathway proceeding from CDP-D-glucose ( Fig. 1) (28); the fifth, colitose, is derived from GDP-D-mannose presumably via a similar reaction sequence (11). All enzymes involved in DDH biosynthesis in S. enterica are encoded within the rib gene cluster, which is * Corresponding author. responsible for the formation of the O-specific subunit of the LPS (26).The rfb gene clusters of several DDH-containing S. enterica strains have been cloned and sequenced (7,8,17,24,45,50), and most of the DDH-related genes have been identified. In all cases investigated, the arrangement of the DDH pathway genes and their relative positions within the rib region are conserved: immediately downstream of the rhamnose pathway genes, a block of four highly conserved genes is found, comprising rfbF and rfbG as well as two open reading frames (ORFs), orf7.6 and orflO.4, which are thought to correspond to the postulated DDH pathway genes rflI and OJbH (8). Abequose-forming strains of serogroups B and C2 (serovars typhimurium and muenchen, respectively) were shown to possess another gene, rfbJ, coding for abequose synthase, which is located adjacent to this highly conserved block; although secondary structure predictions indicated very similar proteins, DNA and amino acid sequences of the two rfbl genes had only low levels of similarity (only 36% identity at the amino acid level [8]). In strains forming tyvelose and paratose (serovars typhi and paratyphi of serogroups D and A, respectively), the rfbJ gene was replaced by a paratose synthase gene, rUbS, and a tyvelose epimerase gene, rfbE (46). The tyvelose epimerase gene in the paratose-producing group A strain, however, was found to be inactive because of a single point mutation.While rfbS still showed a low degree of similarity to rfib at the DNA sequence level, no counterpart to rfbE was found in the rfb region of the abequose-producing strains of S. enterica. Sequence analysis revealed unusually low G+C contents for all rib regions investigated, suggesting a relatively recent transfer of the gene cluster to S. enterica from a nonenterobacterial donor with a low G+C content (8,17,22,48).We have previously reported the cloning of the Y pseudotuberculosis serogroup IIA rib region (19). Hybridization studies of this abequose-producing strain (M85) had shown that at least some of the M85 rib genes are related to DDH pathway genes (rfbF and rCbG) of S. enterica LT2. In this study, we present the sequence and detailed analysis of the DDH pathway gene region of this Y pseudotuberculosis strain. Most of its DDH pathway genes were clearly related

An unstable determinant encoding resistance to tetracycline and minocycline has been cloned and characterized. It could be demonstrated that this determinant shares extensive homology with the otrA gene identified in Streptomyces rimosus. Tetracycline-sensitive variants of Streptomyces lividans derive mostly by deletion of this resistance determinant and neighbouring DNA regions.

The rfb region of Yersinia pseudotuberculosis serogroup IIA has been cloned and expression of 0 antigen in Escherichia coli K12 was demonstrated. Transposon mutagenesis analysis confined the DNA region required for 0 antigen expression to a 19.3 kb fragment, and the 0 antigen expressed was visualized by SDS-PAGE and silver staining. Southern hybridization analysis demonstrated significant levels of similarity between the Yersiniu rcfb region and the 3,6-dideoxyhexose pathway genes rfbF and rfbG, previously isolated from Salmonella entericu LT2, but no similarity to the abequose synthase gene rfb J of the same strain or the paratose synthase gene rfbS isolated from S. enterica Ty2. The evolutionary relationship between the abequose biosynthetic genes of the two species of Salmonella and Yersinia is discussed.