Coupling of Escherichia coli hns mRNA levels to DNA synthesis by autoregulation: implications for growth phase control.

The H-NS protein of enteric bacteria is one of the major proteins of the bacterial nucleoid and seems to play an important role in nucleoid structure. Transcription of the hns gene encoding the H-NS protein appears to be negatively regulated by H-NS itself both in vitro and in vivo. We have examined the role of this mode of regulation in wild-type cells in vivo. We find that hns transcription is down-regulated when DNA synthesis is blocked in growing cells, in a manner that is dependent upon continuing H-NS protein synthesis. These data suggest that hns autoregulation serves to match de novo H-NS synthesis to the demands of DNA synthesis and may maintain a relatively constant H-NS:DNA ratio. It has previously been suggested that hns transcription is activated as cells enter stationary phase, which would require a complete relaxation of autoregulatory control given that DNA synthesis decreases at this time. However, we show here that levels of hns mRNA in fact decline at the onset of stationary phase in a manner fully consistent with the autoregulation model. We also fail to detect any significant accumulation of the H-NS protein in stationary phase.