Lateral transfer of rfb genes: a mobilizable ColE1-type plasmid carries the rfbO:54 (O:54 antigen biosynthesis) gene cluster from Salmonella enterica serovar Borreze

Abstract

Plasmid pWQ799 is a 6.9-kb plasmid isolated from Salmonella enterica serovar Borreze. Our previous studies have shown that the plasmid contains a functional biosynthetic gene cluster for the expression of the O:54 lipopolysaccharide O-antigen of this serovar. The minimal replicon functions of pWQ799 have been defined, and a comparison with nucleotide and protein databases revealed this replicon to be virtually identical to ColE1. This is the first report of involvement of ColE1-related plasmids in O-antigen expression. The replicon of pWQ799 is predicted to encode two RNA molecules, typical of other ColE1-type plasmids. RNAII, the putative replication primer from pWQ799, shares regions of homology with RNAII from ColE1. RNAI is an antisense regulator of DNA replication in ColE1-related plasmids. The coding region for RNAI from pWQ799 shares no homology with any other known RNAI sequence but is predicted to adopt a secondary structure characteristic of RNAI molecules. pWQ799 may therefore represent a new incompatibility group within this family. pWQ799 also possesses cer, rom, and mob determinants, and these differ minimally from those of ColE1. The plasmid is mobilizable in the presence of either the broad-host-range helper plasmid pRK2013 or the IncI 1 plasmid R64drd86. Mobilization and transfer of pWQ799 to other organisms provides the first defined mechanism for lateral transfer of O-antigen biosynthesis genes in S. enterica and explains both the distribution of related plasmids and coexpression of the O:54 factor with other O-factors in different Salmonella serovars. The base composition of the pWQ799 replicon sequences gives an average percent G؉C value typical of Salmonella spp. In contrast, the percent G؉C value is dramatically lower within rfb O:54 , consistent with the possibility that the cluster was acquired from an organism with a much lower G؉C composition.

Lipopolysaccharide Endotoxins

“…The plasmid-encoded O:54 antigen of S. enterica serovar Borreze is currently the only known example of a synthase-dependent O polysaccharide (265,332,333). Synthesis of the poly-N-acetylmannosamine homopolymer that comprises the O:54 antigen is initiated by WecA (265) (Figure 15).…”

A Novel Pathway for O-Polysaccharide Biosynthesis in Salmonella enterica Serovar Borreze

“…Gels were silver-stained using the method of Tsai and Frasch (25). O:54 -Previous analysis of the nucleotide sequence and genetic organization of pWQ799 indicated that approximately half of the 6915-base pair plasmid (nucleotides 72-3384) is involved in plasmid replication and mobilization (4). These regions are related to ColE1 and possess an average G ϩ C content of 50 -53% (Fig.…”

“…1. The construction of plasmids pWQ800 and pWQ802 has been previously reported (3,4). pWQ819 contains a 2.3-kb EcoRIKpnI fragment from pWQ799 cloned in pTrc99A (Pharmacia Biotech Inc.).…”

“…We have studied the 6.9-kb plasmid pWQ799 from serovar Borreze to determine the role of these plasmids in expression of the O:54 antigen (3). pWQ799 is a novel ColE1-related plasmid carrying the entire rfb O:54 biosynthetic cluster (4). This is the only reported example of a ColE1-related plasmid carrying genes for the synthesis of cell-surface antigens.…”

The Sweet Tooth of Bacteria: Common Themes in Bacterial Glycoconjugates

“…enterica serovar Borreze O:54 synthesizes its O antigen by using a synthase-dependent pathway (42,178,179) (Fig. 6C).…”