The Hha protein from Escherichia coli is highly similar (82%) to the YmoA protein from Yersinia enterocolitica. Both are members of a new class of proteins that modulates gene expression, probably by influencing DNA topology. In this paper, complementation of the hha mutation in E. coli by the ymoA gene from Y. enterocolitica has been studied. We show that the ymoA gene complements one of the phenotypic properties of hha mutants (high level of haemolysin production when they carry the recombinant plasmid pANN202-312) when cloned in a medium-copy-number plasmid but not when carried in a low-copy-number plasmid. Western blot analysis of the expression of YmoA in E. coli rules out inefficient expression of the protein. Surprisingly, the hha gene itself fails to complement the hha mutation when cloned in a medium-copy-number vector and causes genetic rearrangements of the E. coli chromosome as a consequence of insertion sequences mobilization.