Molecular analysis of capsular polysaccharide biosynthesis in Escherichia coli K5

• Main Author: Smith, Annabel N.
• Published: University of Leicester 1995
• Subjects: 570
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.737555

Escherichia coli can produce over 70 structurally distinct acidic capsular polysaccharides (K antigens) and these have been divided into two groups. The genetic determinants required for the expression of group II K antigens map at kps and have previously been shown to have a conserved genetic organization consisting of three functional regions. The nucleotide sequence of region 3 of the K5 kps locus was determined and this revealed the presence of two genes, kpsM and kpsT. KpsM and KpsT have characteristics in common with members of the ABC family of membrane transporters and since region 3 had previously been implicated in polymer translocation across the inner membrane, it is proposed that KpsM and KpsT constitute an inner-membrane capsular polysaccharide export complex. Proteins homologous to KpsM and KpsT are encoded by the capsule loci of Haemophilus Influenzae and Neisseria meningitidis, possibly suggesting that capsule production in these unrelated species has a common evolutionary origin. Region 2 of the kps locus has previously been shown to comprise a cassette of polymer synthesis genes. Determination of the nucleotide sequence of region 2 of the K5 kps locus revealed the presence of at least four genes, kfiA, kfiB, kfiC and kfiD. The KfiC protein may act as a glycosyltransferase and KfiD may be a UDP-glucose dehydrogenase. The functions of KfiA and KfiB are unknown. It was demonstrated that region 2 could direct the synthesis and polymerization of the K5 polymer in the absence of region 1 and region 3 encoded proteins. Region 2 has an atypically low G+C content and may have been acquired from another microorganism, possibly through recombination events occurring in the conserved flanking genes, kpsS and kpsT. These genes have a variable 3' end, which may have arisen as a result of in-frame fusions with incoming region 2 elements.