ESCHERICHIA-COLI HHA MUTANTS, DNA SUPERCOILING AND EXPRESSION OF THE HEMOLYSIN GENES FROM THE RECOMBINANT PLASMID PANN202-312

The hha gene of Escherichia coli was identified as modulating the expr ession of the haemolysin (hly) genes encoded by the recombinant plasmi d pANN202-312. hha mutants harbouring plasmid pANN202-312 showed incre ased haemolysin production. The product of the hha gene, the Hha prote in, shows strong homology to the YmoA protein of Yersinia enterocoliti ca, which plays a role in the thermoregulation of various Y. enterocol itica virulence genes. We show in this study that the Hha protein modu lates the expression of haemolysin at the transcriptional level in cel ls harbouring plasmid pANN202-312. In addition, hha mutants show alter ations in the level of plasmid DNA supercoiling. This suggests that th e hha mutation increases haemolysin expression through changes in the DNA topology. This hypothesis is supported by our finding that gyr mut ations, inhibitors of DNA gyrase such as novobiocin, or growth in cond itions reported to reduce levels of negative supercoiling, such as low osmolarity medium, increase haemolysin production.