Evidence that the transcription activator encoded by the Pseudomonas putida nahR gene is evolutionarily related to the transcription activators encoded by the Rhizobium nodD genes.

The nahR gene of the 83-kilobase naphthalene degradation plasmid NAH7 of Pseudomonas putida encodes a 34-kilodalton polypeptide which binds to the nah and sal promoters to activate transcription of the degradation genes in response to the inducer salicylate. The DNA sequence of the nahR gene was determined, and a derived amino acid sequence of the NahR protein was obtained. A computer search for homologous proteins showed that within the first 124 amino-terminal residues, NahR has approximately 35% identity with the transcriptional activator proteins encoded by the nodD genes of Rhizobium species. Allowing for ultraconservative amino acid substitutions, greater than 47% overall similarity was found between NahR and NodD, while 32% similarity was found between NahR and another transcription activator, LysR of Escherichia coli. The region of greatest similarity among all three proteins contained a probable helix-turn-helix DNA-binding motif as suggested by homology with the proposed consensus sequence for Cro-like DNA-binding domains. The high level of amino acid identity between NahR and NodD, in conjunction with the observations that nahR and nodD are 45% homologous in DNA sequence, are divergently transcribed from homologous promoters near the structural genes they control, and have similar DNA-binding sites, strongly suggests that these two genes evolved from a common ancestor.