Molecular analysis of the rfb gene cluster of Salmonella serovar muenchen (strain M67): the genetic basis of the polymorphism between groups C2 and B

Abstract

The rfb (O antigen) gene cluster of group C2 Salmonella differs from that of group B in a central region of 12.4 kb: we report the sequencing of this region of strain M67 (group C2) and a subsequent comparison with the central region of strain LT2 (group B). We find a block of seven open reading frames unique to group C2 which encode the O antigen polymerase (rfc) and the transferases responsible for assembly of the group C2 O antigen. The remaining rfb genes are common to strains M67 and LT2, but rfbJ (CDP-abequose synthase) and rfbM and rfbK (GDP-mannose synthesis), which are immediately adjacent to the central region, are highly divergent. All these genes have a low G+C content and appear to have been recent additions to Salmonella enterica. We discuss the evolutionary significance of the arrangement and divergence of the genes in the polymorphism of the rfb cluster.

Molecular analysis of the 3,6-dideoxyhexose pathway genes of Yersinia pseudotuberculosis serogroup IIA

“…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).…”

“…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).…”

“…2E and F). The presence of orf7.6 and orflO.4 exclusively in DDH-forming strains (8,17,22,24,48) strongly suggests their involvement in DDH formation, and they are, therefore, assumed to represent the so far unidentified genes rfbI (coding for CDP-6-deoxy-A3'4-glucoseen reductase) and rfbH (coding for CDP-4-keto-6-deoxy-D-glucose-3-dehydrase).…”

“…Seven ORFs were found within this region; they were provisionally named orfO. 8 the Y pseudotuberculosis M85 DDH pathway genes" below); it was, therefore, renamed orf6.3 and, as it is probably not functional, is not included in the following general discussion of ORFs. All ORFs found were transcribed in the same direction, from orfO.8 towards orf7.4.…”

“…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).…”