COUPLING OF ESCHERICHIA-COLI HNS MESSENGER-RNA 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 t he bacterial nucleoid and seems to play an important role in nucleoid structure. Transcription of the hns gene encoding the H-NS protein app ears to be negatively regulated by H-NS itself both in vitro and in vi vo. 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 D NA synthesis is blocked in growing cells, in a manner that is dependen t upon continuing H-NS protein synthesis. These data suggest that hns autoregulation selves to match de novo H-NS synthesis to the demands o f 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 relaxa tion of autoregulatory control given that DNA synthesis decreases at t his time. However, we show here that levels of hns mRNA in fact declin e at the onset of stationary phase in a manner fully consistent with t he autoregulation model. We also fail to detect any significant accumu lation of the H-NS protein in stationary phase.