Szabo M, Bronner D, Whitfield C
Canadian Bacterial Diseases Network, Department of Microbiology, University of Guelph, Ontario, Canada.
J Bacteriol. 1995 Mar;177(6):1544-53. doi: 10.1128/jb.177.6.1544-1553.1995.
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 (rfbKpO1). We report here the cloning and analysis of the rfb cluster from S. marcescens serotype O16 (rfbSmO16). 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 rfbKpO1 genes indicate that the cloned rfbSmO16 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. rfbABSmO16 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 rfbKpO1 cluster, rfbFKpO1, encodes a bifunctional galactosyltransferase which initiates the formation of D-galactan I. RfbFKpO1 and RfbFSmO16 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 rfbFSmO16 (47.6%) was much higher than that of rfbFKpO1 (37.3%), despite the fact that the average for each species (52 to 60%) falls within the same range.
肺炎克雷伯菌O1血清型和粘质沙雷氏菌O16血清型的脂多糖O抗原均含有一个重复单元二糖[→3)-β-D-半乳糖-(1→3)-α-D-半乳糖-(1→]；由此产生的聚合物被称为D-半乳聚糖I。在肺炎克雷伯菌O1血清型中，负责合成D-半乳聚糖I的基因位于rfb基因簇（rfbKpO1）中。我们在此报告了来自粘质沙雷氏菌O16血清型（rfbSmO16）的rfb基因簇的克隆和分析。这是首次对沙雷氏菌属进行检测的rfb基因簇。对于肺炎克雷伯菌O1血清型和粘质沙雷氏菌O16血清型，D-半乳聚糖I的合成都是一个依赖rfe的过程。用源自六个rfbKpO1基因中的每一个的探针进行杂交实验表明，克隆的rfbSmO16基因簇包含按相同顺序排列的同源基因。然而，核苷酸序列水平的同源程度足够低，以至于仅在低严谨条件下才能检测到杂交。rfbABSmO16基因被亚克隆，并显示编码一种ABC-2（ATP结合盒）转运蛋白，其功能与肺炎克雷伯菌O1血清型相应rfb基因编码的转运蛋白相同。预测的RfbA和RfbB同源物的氨基酸序列分别显示出75.7%（总相似性为87.9%）和78.0%（总相似性为86.5%）的同一性。rfbKpO1基因簇的最后一个基因rfbFKpO1编码一种双功能半乳糖基转移酶，它启动D-半乳聚糖I的形成。RfbFKpO1和RfbFSmO16的同一性为57.6%（总相似性为71.1%），并且两者都与参与痢疾志贺氏菌I型O-多糖合成的半乳糖基转移酶RfpB显示出相似性。每种生物体的rfbAB基因的G+C含量相当相似，并且其值低于该物种的典型值。然而，rfbFSmO16的G+C含量（47.6%）远高于rfbFKpO1的G+C含量（37.3%），尽管每个物种的平均值（52%至60%）落在相同范围内。
