TRANSCRIPTION OF THE SALMONELLA-TYPHIMURIUM SPV VIRULENCE LOCUS IS REGULATED NEGATIVELY BY THE NUCLEOID-ASSOCIATED PROTEIN H-NS

The possibility that the pleiotropic transcriptional regulator H-NS mi ght play a role in regulating expression of the spv virulence locus of Salmonella typhimurium was investigated. A transposon insertion mutat ion in hns, the gene encoding H-NS, resulted in enhanced transcription of the spvR regulatory gene and the spvB structural gene in stationar y phase cultures. Enhanced transcription was not detected prior to sta tionary phase, indicating that H-NS makes a negative contribution that is growth phase-specific to the control of spv transcription. When H- NS was over-expressed from a multicopy plasmid, the normal stationary phase induction of spv transcription seen in wild-type cells was aboli shed, spv transcription was also found to be modulated by growth mediu m osmolarity, a feature common to many H-NS-regulated genes. In additi on, transcription of the spv genes was reduced in mutants with abnorma l levels of DNA supercoiling.