leu operon of Salmonella typhimurium is controlled by an attenuation mechanism.

The nucleotide sequence of the control region of the leu operon of Salmonella typhimurium was determined. A prominent feature of this region is a signal for termination of transcription. In vitro, transcription does terminate at this site, yielding a leader RNA of about 160 nucleotides as a major product. This leader RNA is potentially translatable into a peptide containing 28 amino acids, 4 of which are adjacent leucine residues. Several regions of base complementarity exist within the leader, positioned such that pairing of one region precludes pairing of another. The position of the four leucine codons relative to two regions of base complementarity suggest a model for the regulation of the leu operon similar to that proposed by Yanofsky and coworkers for the trp operon. In addition, a third region of base complementarity was identified which, when incorporated into the model, explains why premature termination is the usual outcome when transcription is initiated in vitro by purified RNA polymerase.