The aspartate transcarbamylase domain of a mammalian multifunctional protein expressed as an independent enzyme in Escherichia coli

Expression, purification, crystallization and preliminary X-ray diffraction analysis of the aspartate transcarbamoylase domain of human CAD

Aspartate transcarbamoylase (ATCase) catalyzes the synthesis of N-carbamoyl-L-aspartate from carbamoyl phosphate and aspartate in the second step of the de novo biosynthesis of pyrimidines. In prokaryotes, the first three activities of the pathway, namely carbamoyl phosphate synthetase (CPSase), ATCase and dihydroorotase (DHOase), are encoded as distinct proteins that function independently or in noncovalent association. In animals, CPSase, ATCase and DHOase are part of a 243 kDa multifunctional polypeptide named CAD. Up-regulation of CAD is essential for normal and tumour cell proliferation. Although the structures of numerous prokaryotic ATCases have been determined, there is no structural information about any eukaryotic ATCase. In fact, the only detailed structural information about CAD is that it self-assembles into hexamers and trimers through interactions of the ATCase domains. Here, the expression, purification and crystallization of the ATCase domain of human CAD is reported. The recombinant protein, which was expressed in bacteria and purified with good yield, formed homotrimers in solution. Crystallization experiments both in the absence and in the presence of the inhibitor PALA yielded small crystals that diffracted X-rays to 2.1 Å resolution using synchrotron radiation. The crystals appeared to belong to the hexagonal space group P6(3)22, and Matthews coefficient calculation indicated the presence of one ATCase subunit per asymmetric unit, with a solvent content of 48%. However, analysis of the intensity statistics suggests a special case of the P21 lattice with pseudo-symmetry and possibly twinning.

Diversity of aspartate carbamoyltransferase genes of Trypanosoma cruzi

The pyrimidine-biosynthetic (pyr) gene cluster, a tandem array of pyr1-pyr3-pyr6/5-pyr2(ACT)-pyr4 from the 5' terminus, encodes all the six enzymes of de novo pyrimidine biosynthesis and occurs as a polycistronic transcription unit in Trypanosoma cruzi. The gene encoding aspartate carbamoyltransferase (ACT), the second enzyme of the pathway, was characterised using a laboratory-reared Tulahuen strain and Tulahuen-derived clones of T. cruzi. Three loci with different restriction maps that contain ACT1, ACT2, and ACT3 were identified. ACT1 and ACT2 are involved in the pyr gene cluster on two different chromosomal DNA molecules of 1,000 and 800 kb, respectively, whereas ACT3 is linked with pyr4 alone. There are 29 nucleotide substitutions out of 981 positions in these three ACTs, yielding 13 amino acid replacements, and a deletion of triplet nucleotides in ACT1 entails a lack of single amino acid residue. Transcription of the three ACTs takes place in the three developmental stages of the parasite, epimastigotes, trypomastigotes, and amastigotes. Pulsed field gel electrophoresis and Southern blot analyses demonstrated that the cloned T. cruzi stocks, Y-02, CAN III/1, Sylvio-X10/4, and possibly Esmeraldo/3, also possess two complete sets of the pyr gene cluster including ACT, accompanied by additional incomplete clusters. These results suggest that the marked intra-species diversity in the copy number and chromosomal localisations of ACT and other pyr genes may have resulted from partial duplications and subsequent translocations of the polycistronic pyr gene cluster.

Pseudomonas aeruginosa aspartate transcarbamoylase. Characterization of its catalytic and regulatory properties

Aspartate transcarbamoylase from Pseudomonadaceae is a class A enzyme consisting of six copies of a 36-kDa catalytic chain and six copies of a 45-kDa polypeptide of unknown function. The 45-kDa polypeptide is homologous to dihydroorotase but lacks catalytic activity. Pseudomonas aeruginosa aspartate transcarbamoylase was overexpressed in Escherichia coli. The homogeneous His-tagged protein isolated in high yield, 30 mg/liter of culture, by affinity chromatography and crystallized. Attempts to dissociate the catalytic and pseudo-dihydroorotase (pDHO) subunits or to express catalytic subunits only were unsuccessful suggesting that the pDHO subunits are required for the proper folding and assembly of the complex. As reported previously, the enzyme was inhibited by micromolar concentrations of all nucleotide triphosphates. In the absence of effectors, the aspartate saturation curves were hyperbolic but became strongly sigmoidal in the presence of low concentrations of nucleotide triphosphates. The inhibition was unusual in that only free ATP, not MgATP, inhibits the enzyme. Moreover, kinetic and binding studies with a fluorescent ATP analog suggested that ATP induces a conformational change that interferes with the binding of carbamoyl phosphate but has little effect once carbamoyl phosphate is bound. The peculiar allosteric properties suggest that the enzyme may be a potential target for novel chemotherapeutic agents designed to combat Pseudomonas infection.

Substitutions in the aspartate transcarbamoylase domain of hamster CAD disrupt oligomeric structure

Aspartate transcarbamoylase (ATCase; EC 2.1.3.2) is one of three enzymatic domains of CAD, a protein whose native structure is usually a hexamer of identical subunits. Alanine substitutions for the ATCase residues Asp-90 and Arg-269 were generated in a bicistronic vector that encodes a 6-histidine-tagged hamster CAD. Stably transfected mammalian cells expressing high levels of CAD were easily isolated and CAD purification was simplified over previous procedures. The substitutions reduce the ATCase V(max) of the altered CADs by 11-fold and 46-fold, respectively, as well as affect the enzyme's affinity for aspartate. At 25 mM Mg(2+), these substitutions cause the oligomeric CAD to dissociate into monomers. Under the same dissociating conditions, incubating the altered CAD with the ATCase substrate carbamoyl phosphate or the bisubstrate analogue N-phosphonacetyl-L-aspartate unexpectedly leads to the reformation of hexamers. Incubation with the other ATCase substrate, aspartate, has no effect. These results demonstrate that the ATCase domain is central to hexamer formation in CAD and suggest that the ATCase reaction mechanism is ordered in the same manner as the Escherichia coli ATCase. Finally, the data indicate that the binding of carbamoyl phosphate induces conformational changes that enhance the interaction of CAD subunits.

Aspartate-90 and arginine-269 of hamster aspartate transcarbamylase affect the oligomeric state of a chimaeric protein with an Escherichia coli maltose-binding domain

Residues Asp-90 and Arg-269 of Escherichia coli aspartate transcarbamylase seem to interact at the interface of adjacent catalytic subunits. Alanine substitutions at the analogous positions in the hamster aspartate transcarbamylase of a chimaeric protein carrying an E. coli maltose-binding domain lead to changes in both the kinetics of the enzyme and the quaternary structure of the protein. The Vmax for the Asp-90-->Ala and Arg-269-->Ala substitutions is decreased to 1/21 and 1/50 respectively, the [S]0.5 for aspartate is increased 540-fold and 826-fold respectively, and the [S]0.5 for carbamoyl phosphate is increased 60-fold for both. These substitutions decrease the oligomeric size of the protein. Whereas the native chimaeric protein behaves as a pentamer, the Asp-90 variant is a trimer and the Arg-269 variant is a dimer. The altered enzymes also exhibit marked decreases in thermal stability and are inactivated at much lower concentrations of urea than is the unaltered enzyme. Taken together, these results are consistent with the hypothesis that both Asp-90 and Arg-269 have a role in the enzymic function and structural integrity of hamster aspartate transcarbamylase.

Ribosome-mediated translational pause and protein domain organization

Because regions on the messenger ribonucleic acid differ in the rate at which they are translated by the ribosome and because proteins can fold cotranslationally on the ribosome, a question arises as to whether the kinetics of translation influence the folding events in the growing nascent polypeptide chain. Translationally slow regions were identified on mRNAs for a set of 37 multidomain proteins from Escherichia coli with known three-dimensional structures. The frequencies of individual codons in mRNAs of highly expressed genes from E. coli were taken as a measure of codon translation speed. Analysis of codon usage in slow regions showed a consistency with the experimentally determined translation rates of codons; abundant codons that are translated with faster speeds compared with their synonymous codons were found to be avoided; rare codons that are translated at an unexpectedly higher rate were also found to be avoided in slow regions. The statistical significance of the occurrence of such slow regions on mRNA spans corresponding to the oligopeptide domain termini and linking regions on the encoded proteins was assessed. The amino acid type and the solvent accessibility of the residues coded by such slow regions were also examined. The results indicated that protein domain boundaries that mark higher-order structural organization are largely coded by translationally slow regions on the RNA and are composed of such amino acids that are stickier to the ribosome channel through which the synthesized polypeptide chain emerges into the cytoplasm. The translationally slow nucleotide regions on mRNA possess the potential to form hairpin secondary structures and such structures could further slow the movement of ribosome. The results point to an intriguing correlation between protein synthesis machinery and in vivo protein folding. Examination of available mutagenic data indicated that the effects of some of the reported mutations were consistent with our hypothesis.

The molecular biology of multidomain proteins. Selected examples

The aim of this review is to give an overview of the contribution molecular biology can make to an understanding of the functions and interactions within multidomain proteins. The contemporary advantages ascribed to multidomain proteins include (a) the potential for metabolite channelling and the protection of unstable intermediates; (b) the potential for interactions between domains catalysing sequential steps in a metabolic pathway, thereby giving the potential for allosteric interactions; and (c) the facility to produce enzymic activities in a fixed stoichiometric ratio. The alleged advantages in (a) and (b) however apply equally well to multi-enzyme complexes; therefore, specific examples of these phenomena are examined in multidomain proteins to determine whether the proposed advantages are apparent. Some transcription-regulating proteins active in the control of metabolic pathways are composed of multiple domains and their control is exerted and modulated at the molecular level by protein-DNA, protein-protein and protein-metabolite interactions. These complex recognition events place strong constraints upon the proteins involved, requiring the recognition of and interaction with different classes of cellular metabolites and macromolecules. Specific examples of transcription-regulating proteins are examined to probe how their multidomain nature facilitates a general solution to the problem of multiple recognition events. A general unifying theme that emerges from these case studies is that a basic unitary design of modules provided by enzymes is exploited to produce multidomain proteins by a complex series of gene duplication and fusion events. Successful modules provided by enzymes are co-opted to new function by selection apparently acting upon duplicated copies of the genes encoding the enzymes. In multidomain transcription-regulating proteins, former enzyme modules can be recruited as molecular sensors that facilitate presumed allosteric interactions necessary for the molecular control of transcription.

UMP synthase activity expressed in deficient hamster cells by separate transferase and decarboxylase proteins or by linker-deleted bifunctional protein

Segments of the human UMP synthase cDNA coding for the orotate phosphoribosyl transferase (OPRT) and orotidylate decarboxylase (ODC) domains of the bifunctional protein UMP synthase were produced by polymerase chain reaction techniques and cloned into a eukaryotic expression vector. The separate OPRT and ODC vectors, along with a selectable marker, were cotransfected into UMP synthase-deficient hamster cells (Urd-C) that require exogeneous uridine for growth. Transfected Urd-C cells surviving selection in media without added uridine were isolated and designated transferase decarboxylase Urd-C (TDU). All of the selected colonies contained DNA corresponding to the OPRT and ODC expression vectors. Two cell lines (TDU3 and TDU5) integrated many more copies of the OPRT and ODC vectors into their genomes compared to the other TDU lines. A 28.6-kDa ODC protein band and a 24.4-kDa OPRT band were detected on western blots with UMP synthase-specific polyclonal antiserum. The OPRT activity of the TDU lines was up to 8.7 times the OPRT activity of control CHL cells, and the ODC activity was up to 12.5 times control levels. Both OPRT and ODC activities in the monofunctional proteins were less heat stable than in the bifunctional UMP synthase protein. The monofunctional OPRT protein was less stable than the ODC protein at 45 degrees C. Growth of transfected cells in 6-azauridine resulted in striking increases in activity and temperature stability for the monofunctional ODC protein. A UMP synthase bifunctional protein was constructed with a deletion of the suspected linker region joining the two catalytic domains. The linker-deleted UMP synthase showed no significant change in either OPRT or ODC activity or temperature stability. The increased stability of the bifunctional protein may be a factor in its evolutionary selection in mammalian cells.

Proteolytic cleavage of the multienzyme polypeptide CAD to release the mammalian aspartate transcarbamoylase. Biochemical comparison with the homologous Escherichia coli catalytic subunit

We have demonstrated biochemically that the conformation of the proteolytic fragment (mammalian aspartate transcarbamoylase) from the C-terminus of the 240-kDa multienzyme polypeptide carrying the activities carbamoyl phosphate synthetase II, aspartate transcarbamoylase and dihydroorotase (CAD) is similar to that of the catalytic subunits from Escherichia coli aspartate transcarbamoylase. We have measured the extent of unfolding of the mammalian aspartate transcarbamoylase in guanidinium chloride solutions, and have also demonstrated that the protein cross-reacts with antibodies raised against the E. coli enzyme. CAD is digested by low concentrations of trypsin in the presence of 0.2 mM UTP to release an active aspartate transcarbamoylase domain and a 195-kDa 'nicked CAD' molecule containing active carbamoyl phosphate synthetase. These two products are easily separated by ion-exchange chromatography. Similar proteolytic cleavage and trimming by elastase releases a family of aspartate transcarbamoylase fragments. Direct N-terminal sequencing of the aspartate transcarbamoylase fragments confirms predictions of the most accessible residues in the region linking the aspartate transcarbamoylase and dihydroorotase domains. Only the largest of the four fragments generated by elastase retains phosphorylation site 2. When this largest fragment is phosphorylated, the family of aspartate transcarbamoylase fragments is eluted together from ion-exchange columns in a different fraction from the completely unphosphorylated preparation, demonstrating the affinity of the domains for each other.

The multifunctional folic acid synthesis fas gene of Pneumocystis carinii encodes dihydroneopterin aldolase, hydroxymethyldihydropterin pyrophosphokinase and dihydropteroate synthase

The nucleotide sequence of a folic acid synthesis (fas) gene from Pneumocystis carinii contains an open reading frame (ORF) that predicts a protein of 740 amino acids with an M(r) of 83,979. A recombinant baculovirus was constructed which directed expression of the predicted Fas740 polypeptide in cultured Spodoptera frugiperda (SF9) insect cells. The overexpressed 'full-length' protein migrated anomalously in sodium dodecyl sulfate/polyacrylamide gels, with an apparent molecular mass of 71.5 kDa. An abundant 69-kDa species was also recognized by polyclonal sera specific for the Fas protein in immunoblotting analyses. Dihydroneopterin aldolase, dihydropterin pyrophosphokinase and dihydropteroate synthase activities were readily detected in SF9 extracts in which the 71.5/69-kDa immunoreactive species were overproduced, demonstrating that three enzyme functions involved in catalysing three sequential steps of the folate biosynthetic pathway are encoded by a single gene in P. carinii. Importantly, the polyclonal sera recognize a single 69-kDa species in P. carinii extracts suggesting that the three activities are indeed properties of a single polypeptide, although the nature of the suggested post-translational modification is unknown. Location of the individual enzyme domains with the Fas polypeptide based upon amino acid sequence similarity to their bacterial counterparts is discussed. Furthermore, expression of various truncated fas gene constructs demonstrates that the complete fas ORF, including the N-terminus of the predicted polypeptide (FasA domain) whose enzyme function is unknown, must be expressed for maximum dihydroneopterin aldolase (FasB domain) and dihydropteroate synthase (FasD domain) activities. This suggests interactions between the domains within the larger polypeptide to stabilize the functions of these two enzymes. The FasC domain, which contains 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase activity, is able to fold and function independently of the other domains. The requirement by mammalian cells for preformed folates, and the absence of dihydroneopterin aldolase, 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase and dihydropteroate synthase from these tissues opens up the possibility of designing highly selective drugs which inhibit these unique targets.

The evolutionary history of the first three enzymes in pyrimidine biosynthesis

Some metabolic pathways are nearly ubiquitous among organisms: the genes encoding the enzymes for such pathways must therefore be ancient and essential. De novo pyrimidine biosynthesis is an example of one such metabolic pathway. In animals a single protein called CAD carries the first three steps of this pathway. The same three enzymes in prokaryotes are associated with separate proteins. The CAD gene appears to have evolved through a process of gene duplication and DNA rearrangement, leading to an in-frame gene fusion encoding a chimeric protein. A driving force for the creation of eukaryotic genes encoding multienzymatic proteins such as CAD may be the advantage of coordinate expression of enzymes catalyzing steps in a biosynthetic pathway. The analogous structure in bacteria is the operon. Differences in the translational mechanisms of eukaryotes and prokaryotes may have dictated the different strategies used by organisms to evolve coordinately regulated genes.

Expression of catalytic domains of human UMP synthase in uridine auxotrophic bacteria

Orotate phosphoribosyltransferase (OPRT) and orotidine-5'-monophosphate decarboxylase (ODC), which catalyze the last two steps in de novo UMP biosynthesis, are two distinct monofunctional proteins in bacteria and lower eukaryotes. In mammals, OPRT and ODC activities are contained in a single bifunctional protein labeled UMP synthase. The human UMP synthase cDNA was separated into the predicted OPRT and ODC domains using polymerase chain reaction techniques and the domains inserted into pUC19 expression vectors. Following transformation into OPRT- and ODC-deficient E. coli, the strains were able to grow on minimal media without uridine. The ODC-transformed bacteria expressed up to 24 times the level of activity found in a wild-type E. coli line. The OPRT-transformed E. coli contained only 4-9% of wild-type activity. Western blot analysis with antiserum to human UMP synthase demonstrates that OPRT and ODC domains are being produced in the deficient cells by the respective vectors. The level of the domain protein approximates the level of enzyme activity. The complementation of the OPRT and ODC activities in the transformed deficient E. coli strains demonstrates that human UMP synthase can be separated into active monofunctional domains that will function in the bacterial cell environment.

Complete hamster CAD protein and the carbamylphosphate synthetase domain of CAD complement mammalian cell mutants defective in de novo pyrimidine biosynthesis

The mammalian CAD gene codes for a 240-kDa multifunctional protein that catalyzes the first three steps of de novo pyrimidine biosynthesis. Previously, the longest cDNA construct available was missing approximately 500 bp of coding sequence at the 5' end, thereby lacking the sequence to encode the entire carbamylphosphate synthetase (CPSase) domain. Here, a complete CAD hamster cDNA is constructed, placed into a mammalian expression vector, and transfected into hamster cells deficient in CAD. Transfectants show coordinately restored levels of all three enzyme activities and the presence of full-length CAD protein. A derivative construct of the CAD cDNA was generated that should encode only the CPSase domain. When transfected into mammalian cells, a protein was synthesized that had significant CPSase activity both in vivo and in vitro. The two constructs generated in this study will facilitate the study of CAD structure, function, and allosteric regulation.

Synthesis of the nonconserved dihydroorotase domain of the multifunctional hamster CAD protein in Escherichia coli

CAD is the multifunctional protein of higher eukaryotes which catalyzes the first three steps of pyrimidine biosynthesis. Its enzymatic activities exist as independent domains in the order: N terminus-carbamylphosphate synthetase II(CPSase)-dihydroorotase(DHOase)-aspartate transcarbamylase(ATCase)-C terminus. To functionally define the minimum hamster cDNA region required to encode an active DHOase, expression constructs were generated. Many such constructs complement Escherichia coli mutants defective not only in DHOase but also in ATCase. Constructs deleted for most of the sequence encoding the ATCase domain continue to complement E. coli mutants defective in DHOase. All of these smaller constructs also lack the region encoding CPSase. Therefore, a 'genetic cassette', containing information for neither the CPSase nor the ATCase domain, can direct the synthesis of a polypeptide with DHOase activity. Interestingly, inclusion of a portion of the DHOase-ATCase interdomain bridge appears to be required for optimum activity.

Organization and nucleotide sequence of the 3' end of the human CAD gene

Aspartate transcarbamylase (ATCase) is found as a monofunctional protein in prokaryotes and as a part of a multifunctional protein in fungi and animals. In mammals, this enzyme along with carbamyl phosphate synthetase II and dihydroorotase (DHOase) is encoded by a single gene called CAD. To determine the relationship between gene structure and the enzymatic domains of human CAD, we have isolated genomic clones of the human gene and sequenced the region corresponding to the 3' end of the gene. This includes exons encoding the end of the domain for DHOase, the complete domain for ATCase, and the bridge region connecting the two enzymatic domains. Three findings emerged. First, in comparing the human coding sequence to that obtained for other species that have a CAD gene, the length of the bridge region is conserved but its sequence is not. This is in contrast to the strong degree of positional identity observed for the segments of CAD encoding the DHOase and ATCase domains. Second, sets of exons appear to correspond to specific domains and subdomains of the encoded protein. Third, while overall there is a strong conservation of protein sequence among the ATCases of all species, reflecting conservation in catalytic function, two particular regions of the enzyme are more highly conserved among species where ATCase is a domain of a multifunctional protein as opposed to species where it is a monofunctional protein. Such findings may indicate regions of the ATCase domain that provide important structural contacts or functional channels when part of a multifunctional protein.

A microtiter plate assay for aspartate transcarbamylase

A discontinuous, colorimetric method for the assay of aspartate transcarbamylase has been adapted for use with 96-well microtiter plates. The method is based on that of L.M. Prescott and M.E. Jones (1969 Anal. Biochem. 32, 408-419) for the detection of ureido compounds, using monoxime and antipyrine. The enzymatic reaction is carried out in a volume of 150 microliters and is stopped by the addition of 100 microliters of a color mix. After development, the absorbance at 460 nm is directly proportional to the quantity of N-carbamyl-L-aspartate up to at least 0.125 mumol and to the quantity of Escherichia coli aspartate transcarbamylase up to about 7 ng. Kinetic parameters obtained from saturation curves for L-aspartate in 50 mM Tris-acetate, pH 8.0, are indistinguishable from those previously obtained: Vmax = 26,225 mumol h-1 mg-1; S0.5 = 14.7 mmol liter-1; hill constant = 2.5.

Organization of the yeast URA2 gene: identification of a defective dihydroorotase-like domain in the multifunctional carbamoylphosphate synthetase-aspartate transcarbamylase complex

The 6636 bp of the yeast URA2 gene encoding the carbamoylphosphate synthetase-aspartate transcarbamylase complex have been sequenced. The protein is organized into four regions, three of which are functional domains as indicated previously by genetic analysis. The fourth domain corresponds to a defective dihydroorotase called DHOase-like. The URA2 gene complex with the same organization as the equivalent genes in higher eukaryotes suggests an evolution from a common ancestral gene.

Mammalian aspartate transcarbamylase (ATCase): sequence of the ATCase domain and interdomain linker in the CAD multifunctional polypeptide and properties of the isolated domain

Mammalian aspartate transcarbamylase (ATCase; carbamoyl-phosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) is part of a 240-kDa multifunctional polypeptide called CAD, which also has carbamoyl-phosphate synthetase and dihydroorotase activities. We have sequenced selected restriction fragments of a Syrian hamster CAD cDNA that are clearly homologous to three prokaryotic ATCases. These studies, combined with previous sequence data, showed that the ATCase domain of CAD is encoded by 924 base pairs and has a mass of 34,323 Da and a pI of 9.8. While the bacterial pyrimidine biosynthetic enzymes are separate proteins, in mammals the ATCase domain is fused to the carboxyl end of the CAD chimera via a 133-amino acid (14-kDa) linker with an unusual amino acid composition, a pI of 10.2, and pronounced hydrophilic character. The fully active domain isolated from proteolytic digests was characterized by partial amino acid sequencing and amino acid analysis. Trypsin cleavage produced the ATCase domain with a 20-residue amino-terminal extension. Hydrodynamic studies showed that the isolated domain is a 110-kDa trimer with a Stokes radius of 41 A. The mammalian ATCase domain and the prokaryotic enzymes have virtually identical active-site residues and are likely to have the same tertiary fold.

Molecular evolution of enzyme structure: construction of a hybrid hamster/Escherichia coli aspartate transcarbamoylase

Aspartate transcarbamoylase (ATCase, EC 2.1.3.2) is the first unique enzyme common to de novo pyrimidine biosynthesis and is involved in a variety of structural patterns in different organisms. In Escherichia coli, ATCase is a functionally independent, oligomeric enzyme; in hamster, it is part of a trifunctional protein complex, designated CAD, that includes the preceding and subsequent enzymes of the biosynthetic pathway (carbamoyl phosphate synthetase and dihydroorotase). The complete complementary DNA (cDNA) nucleotide sequence of the ATCase-encoding portion of the hamster CAD gene is reported here. A comparison of the deduced amino acid sequences of the hamster and E. coli catalytic peptides revealed an overall 44% amino acid similarity, substantial conservation of predicted secondary structure, and complete conservation of all the amino acids implicated in the active site of the E. coli enzyme. These observations led to the construction of a functional hybrid ATCase formed by intragenic fusion based on the known tertiary structure of the bacterial enzyme. In this fusion, the amino terminal half (the "polar domain") of the fusion protein was provided by a hamster ATCase cDNA subclone, and the carboxyl terminal portion (the "equatorial domain") was derived from a cloned pyrBI operon of E. coli K-12. The recombinant plasmid bearing the hybrid ATCase was shown to satisfy growth requirements of transformed E. coli pyrB- cells. The functionality of this E. coli-hamster hybrid enzyme confirms conservation of essential structure-function relationships between evolutionarily distant and structurally divergent ATCases.