Regulation of the threonine operon: tandem threonine and isoleucine codons in the control region and translational control of transcription termination.

The DNA sequence of 178 base pairs preceding the first structural gene of the threonine operon of Escherichia coli has been determined. A region of perfect 2-fold rotational symmetry, involving 28 base pairs, precedes the first structural gene. The structural similarity of this sequence to known RNA polymerase termination sites suggests that this region is the termination site of the threonine operon leader RNA. Moreover a mutation (thr 79-20), which confers a depressed, constitutive phenotype, was sequenced and found to be a G.C insertion in the putative terminator. A potential coding region for a 21-amino acid leader peptide ends approximately 18 base pairs before the terminator. This peptide contains eight threonine and four isoleucine codons. Eleven of these codons are in tandem. A model for threonine operon regulation, involving alternative secondary RNA structures and translation of leader RNA, is discussed.