Tandem termination sites in the tryptophan operon of Escherichia coli

In vivo, transcription of tryptophan (trp) operon mRNA appears to terminate at a site (trp t) 36 nucleotides after the last structural gene, and efficient function at this site requires the protein factor rho. However, distal nucleotide sequences also seem to play a role in modulating termination at trp t. We report here our in vitro studies of DNA fragments carrying portions of the trp termination region. Transcription of these DNA fragments in a purified system demonstrates that RNA polymerase actually recognizes two different termination sites. Termination at the previously characterized site, trp t, is only 25% efficient, and it is unaffected by the presence of rho factor in vitro. However, addition of rho to the transcription reaction mixture reveals that termination also occurs within a region that we have designated trp t', located about 250 bases past trp t. These two sites behave independently in vitro, whether in the tandem configuration or cloned separately, and their structural features and functional characteristics are quite different. This contrasts with the observation that termination of transcription at the end of the trp operon in vivo appears to require a rho-mediated interaction between trp t and trp t'. The possible involvement of other factors and the significance of multiple termination sites is discussed.