Characterization of the tet(M) determinant of Tn916: evidence for regulation by transcription attenuation.

The nucleotide sequence of the tetracycline resistance determinant tet(M), located on conjugative transposon Tn916 of Enterococcus faecalis, was determined and found to encode a 72,486-dalton protein exhibiting a high degree of homology with other tet(M) determinants. A short open reading frame corresponding to a 28-amino-acid peptide and containing a number of inverted repeat sequences was noted immediately upstream of tet(M), suggesting that regulation might occur by a mechanism involving transcriptional attenuation. Transcription analyses found this to indeed be the case, showing that the expression of tet(M) resulted from an extension of a small transcript representing the upstream leader region into the resistance determinant. Exposure of cells to tetracycline resulted in a significant increase in the amount of tet(M) transcription; this increase could be explained on the basis of increased transcriptional read-through from the upstream transcript. A model suggesting how transcriptional attenuation might operate in this system is presented.