Threonine Operon (Molecular Biology)

The four carbon atoms of aspartic acid in Escherichia coli are the precursors of the four carbon atoms of threonine. In addition, with pyruvic acid they contribute to the biosynthesis of lysine and diaminopimelate and, with the b-carbon of serine, to the biosynthesis of methionine (1). The reactions leading from aspartate to threonine are as follows:

tmp10D-26_thumb[1]

Reaction 1 is catalyzed by the enzyme aspartate kinase I, encoded by the gene thrA; reaction 2 by aspartate semialdehyde dehydrogenase (asd); reaction 3 by homoserine dehydrogenase, encoded by thrA; reaction 4 by homoserine kinase ( thrB); and reaction 5 by threonine synthase (thrC).

Note that both reactions 1 and 3 are catalyzed by a single bifunctional protein, aspartate kinaseI-homoserine dehydrogenase! encoded by thrA. Another bifunctional enzyme, aspartate kinase II-homoserine dehydrogenase II, is encoded by metL, whereas aspartate kinase III is encoded by lys C. The three aspartate kinases catalyze the same reaction and differ in the mode of regulating their activity and their synthesis (1).


The genes thrA, thrB, thrC are organized in this order in an operon, localized at min 0 on the E. coli chromosome (2, 3). The full-length messenger RNA is 4860 nucleotides long (4, 6). The coding sequences of thrA and thrB are separated by only one nucleotide, whereas those of thrB and thrC are contiguous (5). Therefore the Shine-Dalgarno sequence of thrB is within the coding sequence of thrA, and that of thrC is within the coding sequence of thrB.

A strong promoter is located 190 bp upstream of the translation start of thrA. In addition to the transcripts initiated at this promoter, an internal promoter at the 3′ end of thrA allows the transcription of thrB, although with much lower efficiency (7). Translational coupling between thrA and thrB has been demonstrated (8). A detailed genetic map of thrA shows that the gene is indeed composed of two segments corresponding, respectively, to aspartate kinase I and homoserine dehydrogenase I (9).

Expression of the threonine operon depends on the intracellular concentrations of both threonine and isoleucine (10) by a mechanism called multivalent repression (11). In diploids, some derepressed mutations were cis-acting, while other mutations acted in trans. The trans-acting mutations affected either the threonyl-tRNA synthetase (12, 13) or the isoleucyl-t-RNA synthetase (14) (see Aminoacyl tRNA Synthetases). The cis-acting mutations were first thought to be operator mutants (15, 16) but were localized in a region of the gene that had all the characteristics of an attenuation mechanism: (1) the mRNA leader sequence shows the possibility of several mutually exclusive secondary structures; (2) there is a r-independent signal for transcription termination; (3) finally, the leader peptide sequence contains numerous threonine and isoleucine codons: Thr-Thr-Ile-Thr-Thr-Thr-Ile-Thr-Ile-Thr-Thr-Thr (17). Further study of derepressed mutants supports the attenuation mechanism (18). In particular, a mutant carrying a deletion of the leader sequence is derepressed (19).

The organization of the three genes is different in other species, such as Bacillus subtilis (20), Corynebacterium glutamicum (21), and Pseudomonas aeruginosa (22). In some of these organisms, there is a single aspartate kinase that is not covalently linked with homoserine dehydrogenase in a multifunctional protein (23).

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