[The upstream sequence of cholera toxin B subunit gene: effect on CTB expression].

In this work, we have studied the effect of cholera toxin A structure gene on the expression of the distal ctxB gene by the methods of deletion and frame-shift mutation. The results showed that: The expression level of Plasmid pUC19CTB, which was constructed by cloning the XbaI-EcoRI restriction fragment into pUC19 and ctxA gene was out-frame with lacZ' gene, is about 30 micrograms/ml; If a frame shift mutation was introduced at XbaI site of pUC19CTB so that the cholera toxin A gene was inframe with lacZ' and could be translated, the expression level of ctxB was decreased to 12 micrograms/ml; When A further deletion from XbaI to ClaI of cholera toxin A gene (about 550bp) was made and ctxA was outframe with LacZ', ctxB expression was decreased two fold compared to pUC19CTB; If the ctxA was inframe with LacZ' so ctxA gene could be translated, the expression level of CTB is much lower than the plasmid outframe with lacZ'. These observations could not be explained by the current knowledge about genetical regulation of cholera toxin operon. The promoter we found located in the cholera toxin A subunit gene, which is responsible for the expression of cholera toxin B subunit, may answer the question why the 550bp non-coding sequence could enhance the expression of cholera toxin B subunit.