Mutations in a Gene Encoding a New Hsp7O Suppress Rapid DNA Inversion and bgl Activation, but Not proU Derepression, in hns-1 Mutant Escherichia coli

Mutations in hns, the gene encoding the nucleoid-associated protein H-NS, affect both the expression of many specific unlinked genes and the inversion rate of the DNA segment containing the piL4 promoter in Escherichia col. A second-site mutation, termed hscAl, compensated for the effect of an hns-1 mutant allele on the piL4 promoter inversion rate and on activation of the bgl operon. The proU operon, induced in an hns-1 background, remained derepressed in an hns-1 hscAl strain and was induced at an intermediate level in an hns hscAl strain. An insertion mutant allele, hscA2-cat, conferred the same partial hns-l compensatory phenotype as the hscAl allele. The hscA gene encoded a 66-kDa protein product that is a member of the Hsp7O protein class. The gene encoding this product is part of a bicistronic operon that is preceded by a possible r32 promoter and also encodes a 21-kDa protein with significant homology to the DnaJ protein family. The mutation defining the hscAl allele resulted in a phenylalanine substituting a conserved serine residue located in the ATP-binding region of other Hsp7O proteins. H-NS (H1) is an approximately 16-kDa neutral protein that is a major nucleoid component in Escherichia coli (10, 35). Homodimers of this protein bind tightly to double-stranded DNA, conferring increased thermal stability to the double