Nucleic Acids Research, 1994, Vol. 22, No. 10 1821-1829
© 1994
MOLECULAR BIOLOGY |
The tryptophan repressor sequence is highly conserved among the Enterobacteriaceae
California Institute of Biological Research 11099 N. Torrey Pines Road, La Jolla, CA 92037 1Department of Microbiology and Immunology, Oregon Health Sciences University Portland, OR 97201 2Biology Department, Brookhaven National Laboratory Upton, NY 11973, USA
*To whom correspondence should be addressed
Received February 11, 1994. Revised March 28, 1994. Accepted March 28, 1994.
Tryptophan biosynthesis in Escherichia coli is regulated by the product of the trpR gene, the tryptophan (Trp) repressor. Trp aporepressor binds the corepressor, L-tryptophan, to form a holorepressor complex, which binds trp operator DNA tightly, and inhibits transcription of the tryptophan biosynthetic operon. The conservation of trp operator sequences among enteric Gram-negative bacteria suggests that trpR genes from other bacterial species can be cloned by complementation in E.coli. To clone trpR homologues, a deletion of the E.coli trpR gene, 
trpR504, was made on a plasmid by site-directed mutagenesis, then crossed onto the E.coli genome. Plasmid clones of the trpR genes of Enterobacter aerogenes and Enterobacter cloacae were isolated by complementation of the trpR504 allele, scored as the ability to repress ß-galactosidase synthesis from a prophage-borne trpE–lacZ gene fusion. The predicted amino acid sequences of four enteric TrpR proteins show differences, clustered on the backside of the folded repressor, opposite the DNA-binding helix-turn-helix substructures. These differences are predicted to have little effect on the interactions of the aporepressor with tryptophan, holorepressor with operator DNA, or tandemly bound holorepressor dimers with one another. Although there is some variation observed at the dimer interface, interactions predicted to stabilize the interface are conserved. The phylogenetic relationships revealed by the TrpR amino acid sequence alignment agree with the results of others.
+Present address: Department of Microbiology, University of Washington, Seattle, WA 98195, USA