No abstract

Plasmids have an important role in the pathogenicity of certain bacterial species, and Escherichia coli provides the most complete example of the relationship involved. Enterotoxigenic strains of E. coli, in addition to producing heat-stable and/or heat-labile enterotoxins, may also produce a haemolysin and fimbriate cell surface antigens which facilitate the adherence of the bacterial cell to the mucosa of the small bowel. Numerous studies have shown that these properties are plasmid-mediated and that the plasmids act in concert to confer on the host bacterium the ability to produce enteric disease in man and in animals. Moreover, studies with invasive strains of E. coli have shown that the Col V plasmid, which codes for the synthesis of colicin V, significantly enhances the pathogenicity of its host bacterium. Although the relationship between Col V plasmids and virulence is unclear, reports indicate that Col V-containing strains of E. coli are better able to survive in the alimentary tract and that colicine V itself inhibits macrophage function. It is probable that bacterial virulence is a complex phenomenon involving both chromosomal and plasmid genes. We describe here a virulence plasmid which mediates tissue invasiveness in human pathogenic strains of Yersinia enterocolitica.

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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...

The nucleotide sequences of the genes encoding the enzyme aspartate transcarbamoylase (ATCase) from Pseudomonas putida have been determined. Our results confirm that the P. putida ATCase is a dodecameric protein composed of two types of polypeptide chains translated coordinately from overlapping genes. The P. putida ATCase does not possess dissociable regulatory and catalytic functions but instead apparently contains the regulatory nucleotide binding site within a unique N-terminal extension of the pyrB-encoded subunit. The first gene, pyrB, is 1,005 bp long and encodes the 334-amino-acid, 36.4-kDa catalytic subunit of the enzyme. The second gene is 1,275 bp long and encodes a 424-residue polypeptide which bears significant homology to dihydroorotase (DHOase) from other organisms. Despite the homology of the overlapping gene to known DHOases, this 44.2-kDa polypeptide is not considered to be the functional product of the pyrC gene in P. putida, as DHOase activity is distinct from the ATCase complex. Moreover, the 44.2-kDa polypeptide lacks specific histidyl residues thought to be critical for DHOase enzymatic function. The pyrC-like gene (henceforth designated pyrC) does not complement Escherichia coli pyrC auxotrophs, while the cloned pyrB gene does complement pyrB auxotrophs. The proposed function for the vestigial DHOase is to maintain ATCase activity by conserving the dodecameric assembly of the native enzyme. This unique assembly of six active pyrB polypeptides coupled with six inactive pyrC polypeptides has not been seen previously for ATCase but is reminiscent of the fused trifunctional CAD enzyme of eukaryotes.

The repressive effects of exogenous cytidine on growing cells was examined in a specially constructed strain in which the pool sizes of endogenous uridine 5'diphosphate and uridine 5'-triphosphate cannot be varied by the addition of uracil and/or uridine to the medium. Five enzymes of the pyrimidine biosynthetic pathway and one enzyme of the arginine biosynthetic pathway were assayed from cells grown under a variety of conditions. Cytidine repressed the synthesis of dihydroorotase (encoded by pyrC), dihydroorotate dehydrogenase (encoded by pyrD), and ornithine transcarbamylase (encoded by argI). Moreover, aspartate transcarbamylase (encoded bypyrB) became further derepressed upon cytidine addition, whereas no change occurred in the levels of the last two enzymes (encoded by pyrE and pyrF) of the pyrimidine pathway. Quantitative nucleotide pool determinations have provided evidence that any individual riboor deoxyribonucleoside mono-, di-, or triphosphate of cytosine or uracil is not a repressing metabolite for the pyrimidine biosynthetic enzymes. Other nucleotide derivatives or ratios must be considered.

The

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.