Sequence of a class E tetracycline resistance gene from Escherichia coli and comparison of related tetracycline efflux proteins.

We determined the nucleotide sequence of the class E tetA gene on plasmid pSL1456 from Escherichia coli SLH1456A. The deduced amino acid sequence of the class E TetA protein shows 50 to 56% identity with the sequences of five related TetA proteins (classes A through D and G). Hydrophobicity profiles identify 12 putative transmembrane segments with similar boundaries in all six TetA sequences. The N-terminal alpha domain of the six sequences is more highly conserved than the C-terminal beta domain; the central hydrophilic loop connecting the alpha and beta domains is the least conserved region. Amino acid residues that have been shown to be important for class B (Tn10) TetA function are conserved in all six TetA sequences. Unlike the class B tetA gene, the class D and E tetA genes do not exhibit a negative gene dosage effect when present on multicopy plasmids derived from pACYC177.