Cholera toxin is composed of two subunits (Cholera toxin, CT), namely A subunit (CTA) and B subunit (CTB). The A subunit is the active part of the cholera toxin, and the main function of the B subunit is to allow the A subunit to enter the cell by binding to the monosialoganglioside GM1 on the mammalian intestinal epithelial cells. Cholera toxin is encoded by the cholera operon (ctx), in the 5 to 3 direction, followed by the recognition sequence of the counteracting factor ToxR, the promoter, the A gene (ctxA), the B gene (ctxB), and the transcription termination independent of the r factor. series. It is now widely accepted that there is only one promoter in the operon, and ctxA and ctxB are located in the same transcription unit. In Vibrio cholerae, the expression level of ctxB gene is 5 times that of ctxA gene, and when the cloned ctx operon is expressed in E. coli, the expression level of B gene is more than 6 times that of A gene.
However, Cao Cheng et al. found that there is a promoter-active sequence in the Xbal~Clal restriction fragment upstream of the cholera toxin B subunit gene (ie, inside the cholera toxin A subunit structural gene). The cholera toxin B sub-gene can be highly expressed under the control of this promoter, and the presence of this promoter may be due to the fact that the expression yield of the CTB subunit in Vibrio cholerae and E. coli is 6-fold and 7-fold, respectively, of the CTA subunit.
The cholera toxin B subunit () gene (CtxB) has a sequence length of about 380 bp and encodes a total of 124 amino acids, of which the first 21 amino acids are signal peptide sequences. The 124 amino acid sequence encoded by the CtxB gene was subjected to computer software secondary structure prediction analysis. The first 17 amino acids were significantly hydrophobic, forming an a-helix and a b-fold. Gly and Thr at position 21 are more hydrophilic and are exposed on the surface of the molecule, which is beneficial to the cleavage of signal peptidase.
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