Henderson I R, Meehan M, Owen P
Department of Microbiology, Moyne Institute of Preventive Medicine, Trinity College Dublin, Ireland.
Adv Exp Med Biol. 1997;412:349-55. doi: 10.1007/978-1-4899-1828-4_56.
Antigen 43 (Ag43) is a prominent hetero-oligomeric protein complex in the outer membrane of Escherichia coli. It is composed of two subunits. alpha 43 (M(r) 60, 000) and beta 43 (M(r) 53, 000) in 1:1 stoichiometry. alpha 43 is surface expressed, extends beyond the O-side chains of smooth lipopolysaccharide and is bound to the cell surface through an interaction with beta 43, itself an integral outer membrane protein, alpha 43 shows limited sequence homology with some enterobacterial adhesins. Expression of Ag43 is subject to reversible phase variation, the rates of variation from the Ag43+ve to Ag43-ve states in liquid minimal medium being approximately 2.2 x 10(-3), the corresponding rates from Ag43-ve to Ag43+ve states being approximately 1 x 10(-3). Phase switching of genes encoding Ag43 are transcriptionally regulated by DNA methylation (deoxyadenosine methylase [dam] mutants being "locked OFF") and by OxyR (oxyR mutants being "locked ON"). It is proposed that OxyR acts as a repressor of Ag43 transcription by binding to unmethylated GATC sites in the regulatory region of the gene. Sequencing and mapping has identified Ag43 as the likely product of the metastable flu gene first described in 1980 by Diderichsen and responsible for colony form variation in E. coli.
抗原43（Ag43）是大肠杆菌外膜中一种显著的异源寡聚蛋白复合物。它由两个亚基组成，α43（分子量60,000）和β43（分子量53,000），化学计量比为1:1。α43在表面表达，延伸超出光滑脂多糖的O侧链，并通过与β43（本身是一种外膜整合蛋白）相互作用而与细胞表面结合。α43与一些肠杆菌粘附素显示出有限的序列同源性。Ag43的表达受到可逆的相变调控，在液体基本培养基中从Ag43阳性状态转变为Ag43阴性状态的速率约为2.2×10⁻³，从Ag43阴性状态转变为Ag43阳性状态的相应速率约为1×10⁻³。编码Ag43的基因的相变由DNA甲基化（脱氧腺苷甲基化酶[dam]突变体“锁定关闭”）和OxyR（oxyR突变体“锁定开启”）进行转录调控。有人提出，OxyR通过与基因调控区域中未甲基化的GATC位点结合，作为Ag43转录的阻遏物。测序和定位已确定Ag43可能是1980年由迪德里克森首次描述的不稳定流感基因的产物，并与大肠杆菌的菌落形态变异有关。
