SUPPRESSION OF TRANSCRIPTION POLARITY IN THE ESCHERICHIA-COLI HEMOLYSIN OPERON BY A SHORT UPSTREAM ELEMENT SHARED BY POLYSACCHARIDE AND DNATRANSFER DETERMINANTS

Expression of the Escherichia coil hlyCABD operon encoding synthesis, maturation and export of haemolysin toxin was strongly dependent upon a 35 bp DNA sequence, spanning the element GGCGGTAG, located 2 kbp ups tream. When the hly operon was placed under the control of the inducib le tao promoter, expression remained dependent upon this element, when transcribed in its native orientation 3' of the promoter. The increas e in ptac-directed transcription was strongest for the distal, export genes of the hly operon, and was particularly striking when ptac and t he element were placed far upstream. The element did not influence tra nscript stability, and we suggest that it is a key component of a nove l regulatory mechanism may suppresses transcription polarity within op erons, The mechanism that be of widespread importance in bacterial gen e expression because the 8 bp element is present in many Gram-negative species as an upstream component of operons encoding the production o f toxins and the surface assembly of polysaccharides and components re quired for the conjugal transfer of DNA. We name it the ops element fo r operon polarity suppressor.