Transcriptional control plays an important role for the production of heat-labile enterotoxin in enterotoxigenic Escherichia coli of human origin.

The production of heat-labile enterotoxin (LT) in 76 strains of human enterotoxigenic Escherichia coli (ETEC) varied by a factor of 100. Three ETEC strains that differ in the levels of LT production were chosen for the cloning of LT genes (toxAB) into plasmid pBR322, and the gene structure and expression were compared in E. coli HB101. The recombinant of the low LT-producing strain produced LT at the same level as that of the moderate LT-producing strain, but that of the high-level producer continued to produce at a level 14-21 times higher than the others. The restriction maps of the coding regions of the cloned LT genes (toxAB) were identical, but the flanking regions were dissimilar. The content of LT mRNA per cell, examined by Northern blot analysis, was higher in the high producer than the others by 6 times. The promoter strengths of the recombinants were all alike. LT mRNA of the high producer was more stable than that of the moderate one by 1.3 times, but the difference was not large enough to explain the difference of the content of LT mRNA. It was shown that LT production can be controlled at a transcriptional step, and DNA structure of the flanking regions may be involved in the control of the LT gene expression.