addition to hlyCABD, also contains the 5' upstream hlyR region. Therefore the hha mutation appears to compensate for the absence of hlyR. An additional characteristic of the hha mutation is its pleiotropic effect, increasing the expression of other heterologous proteins cloned in multicopy plasmids (Blanco et al., 1991, FEMS Microbiol Lett 81: 221-226). In a recent paper we reported the cloning of the hha gene and the identification of the Hha protein, a 8.6kDa protein, which appears to modulate haemolysin transcription: hha mutants showed increased levels of the hlyCA transcript (Nieto efa/., 1991, Mo/M;cro£>/o/5:1285-1293). In parallel, Cornelis and co-workers reported the identification and characterization of the ymoA gene of Yersinia enterocolitica (Cornelis ef al., 1991, Mol Microbiol 5: 1023-1034). A comparison of the amino acid sequences of the Hha and YmoA proteins showed 82% identity (Fig. 1). The YmoA protein of Y. enterocolitica regulates the expression of different Y. enterocolitica virulence genes in response to temperature. The ymoA mutants exhibit a phenotype similar to that of hns mutants (Higgins et al., 1990, Mol Microbiol 4: 2007-2012) and it has been suggested that YmoA is a histone-like protein (Cornelis et a/., 1991, ibid.). Both hha and ymoA mutants share some properties, such as pleiotropy (Nieto etal., 1987, ibid.; Cornelis etal., 1991, ibid.), alterations in the topology of the DNA (M. Carmona et al., unpublished; Cornelis et al., 1991; ibid.) and increase of the expression of either the pANN202-312 encoded hly genes or the yop genes at high osmolarity (Carmona etal., unpublished results; Cornelis etal., 1991, ibid). We suggest that both the Hha protein and the YmoA protein are a new class of proteins which modulate bacterial gene expression influencing DNA topology in response to environmental stimuli.