THE DNA-BINDING DOMAIN OF THE HEXAMERIC ARGININE REPRESSOR

The arginine repressor of Escherichia coli is a classical feedback reg ulator, signalling the availability of L-arginine inside the cell. it differs from most other bacterial repressors in functioning as a hexam er, but structural details have been lacking and its shares no clear s equence homologies with other transcriptional regulators. Analysis of the amino acid residue sequence and proteolytic cleavage pattern of th e repressor was used to identify a region predicted to house the DNA-b inding function. When this protein fragment is overexpressed from a cl one of the corresponding gene fragment, it represses ornithine transca rbamylase levels in vivo, and binds to the operator DNB in vitro, both in an arginine-independent manner. Sedimentation equilibrium and gel filtration indicate that the purified protein fragment is a monomer in solution. The results thus define the domain organization of the repr essor at low resolution, suggesting that the N and C-terminal portions of the polypeptide chain are separated by a structural and functional border that decouples hexamerization and arginine binding from DNA bi nding. (C) 1995 Academic Press Limited