The three-dimensional crystal structure of met repressor, in the presence or absence of bound corepressor (S-adenosylmethionine), shows a dimer of intertwined monomers, which do not have the helix-turn-helix motif characteristic of other bacterial repressor and activator structures. We propose that the interaction of met repressor with DNA occurs through either a pair of symmetry-related alpha-helices or a pair of beta-strands, and suggest a model for binding of several dimers to met operator regions.

The metC gene of Escherichia coli K-12 was cloned and the nucleotide sequence of the metC gene and its flanking regions was determined. The translation initiation codon was identified by sequencing the NH2-terminal part of 13-cystathionase, the MetC gene product. The meIC gene (1185 nucleotides) encodes a protein having 395 amino acid residues.The 5' noncoding region was found to contain a "Met box" homologous to sequences suggestive of operator structures upstream from other methionine genes that are controlled by the product of the pleiotropic regulatory metJ gene. The deduced amino acid sequence of (3-cystathionase showed extensive homology with that of the MetB protein (cystathionine y-synthase) that catalyzes the preceding step in methionine biosynthesis. The homology strongly suggests that the structural genes for the MetB and MetC proteins evolved from a common ancestral gene.

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We present biochemical and genetic data to support the hypothesis that the Escherichia coli met repressor, MetJ, binds to synthetic and natural operator sequences in tandem arrays such that repression depends not only on the affinity of the DNA-protein interaction, but also on protein-protein contacts along the tandem array. This represents a novel form of regulatory switch. Furthermore, there seems to be homology between the organization of the met and trp operators.

The thrA gene of Escherichia coli codes for a single polypeptide chain having two enzymatic activities required for the biosynthesis of threonine, aspartokinase I and homoserine dehydrogenase I. This gene was cloned in a bacterial plasmid and its complete nucleotide sequence was established. It contains 2460 base pairs that encode for a polypeptide chain of 820 amino acids. The previously determined partial amino acid sequence of this protein is in good agreement with that predicted from the nucleotide sequence. The gene contains an internal sequence that resembles the structure of bacterial ribosome-binding sites, with an AUG preceded by four triplets, each of which can be converted to a nonsense coon by a single mutation. This suggests that the single polypeptide chain was formed by the fusion of two genes and that initiation of translation may occur inside the gene to give a protein fragment having only the homoserine dehydrogenase activity.The thrA gene is the first structural gene of the threonine operon of Escherichia coil K-12 (1, 2). It is composed of two parts, thrAl and thrA2 and codes for a bifunctional enzyme, aspartokinase I-homoserine dehydrbgenase I (EC 2.7.2.4 and EC 1.1.1.3). The native enzyme (3) is a tetramer with each chain carrying, on discrete domains, the aspartokinase I and homoserine dehydrogenase I activities, which are regulated allosterically by L-threonine. Limited proteolysis of the native enzyme leads to a homodimeric fragment having the same COOHterminal sequence as the native enzyme having only the dehydrogenase activity and no longer inhibited by threonine (3). On the other hand, a polypeptide chain synthesized by an ochre mutant that has the same NH2 terminus as the native enzyme assembles as a tetramer having only the aspartokinase activity, still regulated by threonine (3). The determination of the primary structure of aspartokinase I homoserine-dehydrogenase I seemed warranted for a number of reasons. Sequence information was important to understand enzyme structure-function relationships and to elucidate the allosteric properties of the enzyme. It should permit the study of possible evolutionary relationships between the different proteins coded by the threonine operon and the homology with the isofunctional enzymes in E. coli, aspartokinase II-homoserine dehydrogenase II, coded by metL, and aspartokinase III coded by lysC.