OSMOTIC REGULATION OF GENE-EXPRESSION

Cells react to increased osmolality with numerous changes in gene expr ession, The specific genes affected differ between species, but the kn own osmoprotective effects of the gene products are remarkably similar , particularly with regard to cellular accumulation of compatible orga nic osmolytes, Here we concentrate on the molecular basis for osmotic regulation of gene expression, emphasizing certain genes expressed in bacteria, yeast, and the mammalian renal medulla because their express ion is best understood, Thus, we emphasize I) bacterial and yeast two- component histidine kinase systems, each consisting of a membrane osmo lality sensor and a separate cytoplasmic response regulator that, when phosphorylated, alters transcription, 2) volume regulatory increases in cellular K+ salts that can prompt increased gene transcription in b acteria through direct effects on DNA and that in mammalian renal cell s increase transcription, seemingly via trans-activating proteins, 3) a yeast kinase cascade that transmits an osmotic signal to the gene re gulating the level of glycerol, and 4) in mammalian cells, several hom ologous cascades that are activated by hypertonicity, but whose osmore gulatory targets are not yet known.