The nucleoid-associated protein StpA binds curved DNA, has a greater DNA-binding affinity than H-NS and is present in significant levels in hns mutants

The StpA protein is closely related to H-NS, the well-characterised global regulator of gene expression which is a major component of eubacterial chro matin. Despite sharing a Very high degree of sequence identify and having b iochemical properties in common with H-NS, the physiological function of St pA remains unknown. We show that StpA exhibits similar DNA-binding activiti es to H-NS. Although both display a strong preference for binding to curved DNA, StpA binds DNA with a four-fold higher affinity than H-NS, with K(d)s of 0.7 muM and 2.8 muM, respectively. It has previously been reported that expression of stpA is derepressed in an hns mutant. We have quantified the amount of StpA protein produced under this condition and find it to be onl y one-tenth the level of H-NS protein in wild-type cells. Our findings expl ain why the presence of StpA does not compensate for the lack of H-NS in an hns mutant, and why the characteristic pleiotropic hits mutant phenotype i s observed. (C) 2001 Societe francaise de biochimie et biologie moleculaire / Editions scientifiques et medicales Elsevier SAS.