H-NS Is a Repressor of the Proteus mirabilis Urease Transcriptional Activator Gene ureR

Abstract
Expression of Proteus mirabilis urease is governed by UreR, an AraC-like positive transcriptional activator. A poly(A) tract nucleotide sequence, consisting of A 6 TA 2 CA 2 TGGTA 5 GA 6 TGA 5 , is located 16 bp upstream of the ς 70 -like ureR promoter P2. Since poly(A) tracts of DNA serve as binding sites for the gene repressor histone-like nucleoid structuring protein (H-NS), we measured β-galactosidase activity of wild-type Escherichia coli MC4100 (H-NS + ) and its isogenic derivative ATM121 ( hns ::Tn 10 ) (H-NS ) harboring a ureR-lacZ operon fusion plasmid (pLC9801). β-Galactosidase activity in the H-NS host strain was constitutive and sevenfold greater ( P < 0.0001) than that in the H-NS + host. A recombinant plasmid containing cloned P. mirabilis hns was able to complement and restore repression of the ureR promoter in the H-NS host when provided in trans. Deletion of the poly(A) tract nucleotide sequence from pLC9801 resulted in an increase in β-galactosidase activity in the H-NS + host to nearly the same levels as that observed for wild-type pLC9801 harbored by the H-NS host. Urease activity in strains harboring the recombinant plasmid pMID1010 (encoding the entire urease gene cluster of P. mirabilis ) was equivalent in both the H-NS background and the H-NS + background in the presence of urea but was eightfold greater ( P = 0.0001) in the H-NS background in the absence of urea. We conclude that H-NS represses ureR expression in the absence of urea induction.