NahR

Overview

The nahR gene originated from the 83 kb naphthalene degradation plasmid NAH7 of Pseudomonas putida encodes a 34 kDa protein which binds to nah and sal promoters to activate transcription of the degradation genes in response to the inducer salicylate. This plasmid encodes enzymes for the metabolism of naphthalene or salicylate as the sole carbon source (Fig. 1a) ^[1]. The 14 genes encoding the enzymes for this metabolism are organized in two operons: nah (nahA-F), encoding six enzymes required for metabolism for naphthalene to salicylate and pyruvate, and sal (nahG-M), encoding eight enzymes which metabolize salicylate to intermediates of TCA cycle (Fig. 1b) ^[2].















Previous work have confirmed that the cloned nah, sal, and nahR genes can be expressed and normally regulated in heterologous host Escherichia coli resembling the situation in Pseudomonas putida [3]. NahR is a member of LysR-type transcriptional factors, which have a conserved N-terminal segment that contains the helix-turn-helix DNA-binding motif. It is sigma70 dependent and functions via contacting the α-unit of RNAP [4]. Mutagenesis experiments also largely facilitated the localization of functional domains in the NahR protein [5,6]. N terminal portion (residues 23-45) accounts for binding DNA. Interestingly, the discovery of C terminal (residues 239-291) mutants unable to bind DNA suggested that the DNA binding requires multimerization through a different protein domain [6]. Gel filtration analysis done by Mark A. Schell showed that the active NahR transcription factor may be a tetramer [5]. Additionally, mutations among residues 140-200 and 207-266 largely affected specificity of inducers, indicating that those residues might serve as a ligand-binding crevice (Fig. 2) [6].