H-NS - A MODULATOR OF ENVIRONMENTALLY REGULATED GENE-EXPRESSION

H-NS is a small chromatin-associated protein found in enterobacteria. H-NS has affinity for all types of nucleic acids but binds preferentia lly to intrinsically curved DNA. The major role of H-NS is to modulate the expression of a large number of genes, mostly by negatively affec ting transcription. Many of the H-NS-modulated genes are regulated by environmental signals, and expression of most of these genes is positi vely regulated by specific transcription factors. Therefore one of the purposes of H-NS could be to repress expression of some genes under c onditions characteristic of a non-intestinal environment, but allow ex pression of specific genes in response to certain stimuli in the intes tinal environment. The hns gene is autoregulated. In vivo the H-NS to DNA ratio is fairly constant except during cold shock, when it increas es three-to fourfold. In this review we propose that only the preferen tial binding to intrinsically curved DNA plays a role under normal gro wth conditions, and we discuss the different mechanisms by which H-NS might affect gene expression and how H-NS Could be involved in the res ponse to different stress situations. Finally, we summarize the evolut ionary and functional relationship between H-NS and the homologous Stp A.