THE TRYPTOPHAN REPRESSOR SEQUENCE IS HIGHLY CONSERVED AMONG THE ENTEROBACTERIACEAE

Tryptophan biosynthesis in Escherichia coli is regulated by the produc t of the trpR gene, the tryptophan (Trp) repressor. Trp aporepressor b inds the corepressor, L-tryptophan, to form a holorepressor complex, w hich binds trp operator DNA tightly, and inhibits transcription of the tryptophan biosynthetic operon. The conservation of trp operator sequ ences among enteric Gram-negative bacteria suggests that trpR genes fr om other bacterial species can be cloned by complementation in E. coli . To clone trpR homologues, a deletion of the E. coli trpR gene, Delta trpR504, was made on a plasmid by site-directed mutagenesis, then cro ssed onto the E. coli genome. Plasmid clones of the trpR genes of Ente robacter aerogenes and Enterobacter cloacae were isolated by complemen tation of the Delta trpR504 allele, scored as the ability to repress b eta-galactosidase synthesis from a prophage-borne trpE- lacZ gene fusi on. The predicted amino acid sequences of four enteric TrpR proteins s how differences, clustered on the backside of the folded repressor, op posite the DNA-binding helix-turn-helix substructures. These differenc es are predicted to have little effect on the interactions of the apor epressor with tryptophan, holorepressor with operator DNA, or tandemly bound holorepressor dimers with one another. Although there is some v ariation observed at the dimer interface, interactions predicted to st abilize the interface are conserved. The phylogenetic relationships re vealed by the TrpR amino acid sequence alignment agree with the result s of others.