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The deoxyribonucleotide sequence of pyrB, the cistron encoding the catalytic subunit of aspartate transcarbamoylase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2), has been determined. The pyrB gene encodes a polypeptide of 311 amino acid residues initiated by an NH2-terminal methionine that is not present in the catalytically active polypeptide. The DNA sequence analysis revealed the presence of an eightamino-acid sequence beginning at Met-219 that was not detected in previous analyses of amino acid sequence. This octapeptide sequence provides an additional component of the disordered loop in the equatorial domain of the catalytic polypeptide. It had been found previously that the catalytic polypeptide is expressed from a bicistronic operon that also produces the regulatory polypeptide encoded by pyrI. A single transcriptional control region precedes the structural gene of the catalytic polypeptide and a simple 15-base-pair region separates its COOH terminus from the structural gene of the regulatory polypeptide. The chain-terminating codon of the catalytic polypeptide may contribute to the ribosomal binding site for the regulatory polypeptide and thus assist coordinate expression of the two cistrons.The aspartate transcarbamoylase holoenzyme (ATCase; aspartate carbamoyltransferase; carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) is a dodecamer composed of two catalytic trimers (c3) and three regulatory dimers (r2) (1). This oligomer, 2(c3)'3(r2), is so arranged that each catalytic monomer is in contact with three other catalytic chains and two regulatory chains, while each regulatory monomer interfaces with one other regulatory chain and two catalytic chains (2). Perbal and Herv6 demonstrated that the two polypeptides are produced in approximately balanced biosyntheses (3), and we recently demonstrated that the two genes were closely linked and encoded on a 6.0-kilobase (kb) fragment isolated from AdargI'pyrB+ transducing phage obtained from N. Glansdorff (4). The structural gene that encodes the catalytic polypeptide was designated pyrB (5) and the structural gene encoding the regulatory polypeptide has been designated pyrl (4,6). Recently, we demonstrated that pyrBI is organized as a bicistronic operon possessing a single control region, which includes a pindependent attenuator sequence, a region of dyad symmetry overlapping transcriptional initiation, and a presumed leader polypeptide whose termination overlaps the ribosomal binding site for the translation of the catalytic polypeptide (7).The architecture of ATCase from Escherichia coli has been extensively examined because the regulatory controls that affect the catalytic sites are mediated by distinct allosteric sites located on separate regulatory polypeptides. ATCase catalyzes the first reaction unique to pyrimidine biosynthesis, the condensation of carbamoyl phosphate and aspartate to produce carbamoyl-L-aspartate and orthophosphate (8). The enzyme is subject to allosteric inhibition by CTP, one of the ulti...

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The argI gene from E. coli K12 has been sequenced. It contains an open reading frame of 1002 bases which encodes a polypeptide of 334 amino acids. Three such polypeptides are required to form the functional catalytic trimer (c3) of ornithine transcarbamoylase (OTCase-1, EC 2.1.3.3). The molecular mass of the mature trimer deduced from the amino acid sequence is 114,465 daltons. An altered form of argI was produced when a 1.6 kilobase DdeI fragment was subcloned into the HincII site of plasmid pUC8 extending the open reading frame an additional 20 nucleotides. It has been previously reported that the amino-terminal region of the respective polypeptides of argI, argF, and pyrB of E. coli possessed significant homology. In contrast, the homologous promoter/operator regions of argI and argF did not appear to share any homologies with pyrB. However, a closer scrutiny of the nucleotide sequence immediately preceding the pyrBI attenuator revealed a remarkable similarity to the argI and argF control region.

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