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 might play a role in regulating expression of the spv virulence locus of Salmonella typhimurium was investigated. A transposon insertion mutation in hns, the gene encoding H-NS, resulted in enhanced transcription of the spvR regulatory gene and the spvB structural gene in stationary phase cultures. Enhanced transcription was not detected prior to stationary 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 abolished. spv transcription was also found to be modulated by growth medium osmolarity, a feature common to many H-NS-regulated genes. In addition, transcription of the spv genes was reduced in mutants with abnormal levels of DNA supercoiling.