Yamamoto T, Tamura T, Yokota T
J Biol Chem. 1984 Apr 25;259(8):5037-44.
Heat-labile enterotoxin of Escherichia coli pathogenic for humans (LTh) or for piglets (LTp) and Vibrio cholerae enterotoxin (CT) are structurally and functionally similar toxins. We have determined the complete nucleotide sequence of the toxA gene which encodes the subunit A of LTh (LTh A). The deduced amino acid sequence consists of 258 residues including a signal peptide of 18 residues. According to the previously completed LTh B sequence (103 residues), the predicted holotoxin (1A5B) of LTh comprises 755 residues and has Mr = 87,866. With respect to LTh A and LTh B, secondary structures, local hydrophilicity, and sites for antigenic determinants were predicted. Both codon usage and G + C content of the toxA gene and the LTh B gene (toxB) were markedly different from those observed with several E. coli chromosomal genes. Its relatively low G + C content was rather close to that of the V. cholerae chromosome. Although the toxA gene shares a common ancestor with the LTp A gene (eltA), the two genes are apparently distinguishable from each other in their sequences. Like LTh B reported previously, the predicted sequence of the catalytic fragment LTh A1 also showed more homology to that of CT A1 than did that of LTp A1. In contrast, unique sequences were found in LTh A2.
对人类致病的大肠杆菌热不稳定肠毒素（LTh）或对仔猪致病的大肠杆菌热不稳定肠毒素（LTp）以及霍乱弧菌肠毒素（CT）是结构和功能相似的毒素。我们已经确定了编码LTh亚基A（LTh A）的toxA基因的完整核苷酸序列。推导的氨基酸序列由258个残基组成，包括一个18个残基的信号肽。根据先前完成的LTh B序列（103个残基），预测的LTh全毒素（1A5B）包含755个残基，分子量为87,866。针对LTh A和LTh B，预测了二级结构、局部亲水性和抗原决定簇位点。toxA基因和LTh B基因（toxB）的密码子使用情况和G + C含量与几个大肠杆菌染色体基因明显不同。其相对较低的G + C含量与霍乱弧菌染色体的G + C含量相当接近。尽管toxA基因与LTp A基因（eltA）有共同的祖先，但这两个基因在序列上显然彼此有区别。与先前报道的LTh B一样，催化片段LTh A1的预测序列与CT A1的序列比与LTp A1的序列显示出更多的同源性。相反，在LTh A2中发现了独特的序列。
