H-NS OVER-EXPRESSION INDUCES AN ARTIFICIAL STATIONARY-PHASE BY SILENCING GLOBAL TRANSCRIPTION

Bacteria organize their chromosomes in a complex interwound supercoile d structure called the nucleoid through the action of topoisomerases a nd a set of small (10-20 kDa) proteins. The two most abundant nucleoid -associated proteins are HU and H-NS. H-NS increases in abundance duri ng stationary phase. Over-expression of HU is well tolerated and compa tible with transcription and cell growth. Increasing the concentration of H-NS leads to a rapid silencing of global transcription and produc es a growth-arrested state reminiscent of stationary phase. H-NS over- expression also induces a substantial loss of supercoiling in plasmid DNA during the time that transcription is arrested. Comparing the effe cts of over-expression of these two proteins gives some insight into t he differential roles of these proteins in the activity of the chromos ome. These observations are interpreted in a model of nucleoid organiz ation.