RELATIONSHIPS BETWEEN RFB GENE CLUSTERS REQUIRED FOR BIOSYNTHESIS OF IDENTICAL D-GALACTOSE-CONTAINING O ANTIGENS IN KLEBSIELLA-PNEUMONIAE SEROTYPE-O1 AND SERRATIA-MARCESCENS SEROTYPE-O-16

The lipopolysaccharide O antigens of Klebsiella pneumoniae serotype O1 and Serratia marcescens serotype O16 both contain a repeating unit disaccharide of [-->3)-beta-D-Galf-(1-->3)-alpha-D-Galp-(1-->]; the resulting polymer is known as D-galactan I. In K. pneumoniae serotype O1, the genes responsible for the synthesis of D-galactan I are found in the rfb gene cluster (rfb(KpO1)). We report here the cloning and analysis of the rfb cluster from S. marcescens serotype O16 (rfb(SmO16)). This is the first rfb gene cluster examined for the genus Serratia. Synthesis of D-galactan I is an rfe-dependent process for both K. pneumoniae serotype O1 and S. marcescens serotype O16. Hybridization experiments with probes derived from each of the six rfb(KpO1) genes indicate that the cloned rfb(SmO16) cluster contains homologous genes arranged in the same order. However, the degree of homology at the nucleotide sequence level was sufficiently low that hybridization was detected only under low-stringency conditions. rfbAB(SmO16) genes were subcloned and shown to encode an ABC-2 (ATP-binding cassette) transporter which is functionally identical to the one encoded by the corresponding rfb genes from K. pneumoniae serotype O1. The amino acid sequences of the predicted RfbA and RfbB homologs showed identities of 75.7% (87.9% total similarity) and 78.0% (86.5% total similarity), respectively, The last gene of the rfb(KpO1) cluster, rfbF(KpO1), encodes a bifunctional galactosyltransferase which initiates the formation of D-galactan I. RfbF(KpO1) and RfbF(SmO16) are 57.6% identical (with 71.1% total similarity), and both show similarity with RfpB, the galactosyltransferase involved in the synthesis of Shigella dysenteriae type I O-polysaccharide, The G+C contents of the rfbAB genes from each organism are quite similar, and values are lower than those typical for the species. However, the G+C content of rfbF(SmO16) (47.6%) was much higher than that of rfbF(KpO1) (37.3%), despite the fact that the average for each species (52 to 60%) falls within the same range.