Translational coupling in the threonine operon of Escherichia coli K-12

In an attempt to express the two distal genes of the Escherichia coli threonine operon, the majority of the first gene in the operon, thrA, was removed and a series of transcriptional fusions were constructed placing the thrB and thrC genes downstream of either the trp or hybrid tac promoter. Analysis of the proteins produced by cells containing these fusions revealed that although the distal gene, thrC, was efficiently expressed, the proximal gene, thrB, was not expressed at a detectable level. A translational fusion was constructed which fused the cat gene in phase to the last 800 base pairs of thrA followed by thrB and thrC. Cells containing this fusion produced high levels of both the thrB and thrC gene products, showing that translation of thrB requires translation through thrA; thus, thrA and thrB are translationally coupled. In addition, it was found that a sequence between 220 and 57 base pairs before the start of thrB was necessary to allow translational coupling to occur.