Stevenson G, Andrianopoulos K, Hobbs M, Reeves P R
Department of Microbiology, University of Sydney, New South Wales, Australia.
J Bacteriol. 1996 Aug;178(16):4885-93. doi: 10.1128/jb.178.16.4885-4893.1996.
Colanic acid (CA) is an extracellular polysaccharide produced by most Escherichia coli strains as well as by other species of the family Enterobacteriaceae. We have determined the sequence of a 23-kb segment of the E. coli K-12 chromosome which includes the cluster of genes necessary for production of CA. The CA cluster comprises 19 genes. Two other sequenced genes (orf1.3 and galF), which are situated between the CA cluster and the O-antigen cluster, were shown to be unnecessary for CA production. The CA cluster includes genes for synthesis of GDP-L-fucose, one of the precursors of CA, and the gene for one of the enzymes in this pathway (GDP-D-mannose 4,6-dehydratase) was identified by biochemical assay. Six of the inferred proteins show sequence similarity to glycosyl transferases, and two others have sequence similarity to acetyl transferases. Another gene (wzx) is predicted to encode a protein with multiple transmembrane segments and may function in export of the CA repeat unit from the cytoplasm into the periplasm in a process analogous to O-unit export. The first three genes of the cluster are predicted to encode an outer membrane lipoprotein, a phosphatase, and an inner membrane protein with an ATP-binding domain. Since homologs of these genes are found in other extracellular polysaccharide gene clusters, they may have a common function, such as export of polysaccharide from the cell.
柯氏酸（CA）是大多数大肠杆菌菌株以及肠杆菌科其他物种产生的一种细胞外多糖。我们已经确定了大肠杆菌K-12染色体上一个23kb片段的序列，该片段包含产生CA所需的基因簇。CA基因簇由19个基因组成。位于CA基因簇和O抗原基因簇之间的另外两个已测序基因（orf1.3和galF）被证明对CA的产生是不必要的。CA基因簇包括CA前体之一GDP-L-岩藻糖合成的基因，并且通过生化分析鉴定了该途径中一种酶（GDP-D-甘露糖4,6-脱水酶）的基因。六个推断的蛋白质与糖基转移酶具有序列相似性，另外两个与乙酰转移酶具有序列相似性。另一个基因（wzx）预计编码一种具有多个跨膜区段的蛋白质，并且可能在类似于O单元输出的过程中参与将CA重复单元从细胞质输出到周质中。该基因簇的前三个基因预计编码一种外膜脂蛋白、一种磷酸酶和一种具有ATP结合结构域的内膜蛋白.由于这些基因的同源物存在于其他细胞外多糖基因簇中，它们可能具有共同功能，例如从细胞中输出多糖。
