Structure and properties of the putrescine carbamoyltransferase of Streptococcus faecalis

Polyamine and Ethanolamine Metabolism in Bacteria as an Important Component of Nitrogen Assimilation for Survival and Pathogenicity

Nitrogen is an essential element required for bacterial growth. It serves as a building block for the biosynthesis of macromolecules and provides precursors for secondary metabolites. Bacteria have developed the ability to use various nitrogen sources and possess two enzyme systems for nitrogen assimilation involving glutamine synthetase/glutamate synthase and glutamate dehydrogenase. Microorganisms living in habitats with changeable availability of nutrients have developed strategies to survive under nitrogen limitation. One adaptation is the ability to acquire nitrogen from alternative sources including the polyamines putrescine, cadaverine, spermidine and spermine, as well as the monoamine ethanolamine. Bacterial polyamine and monoamine metabolism is not only important under low nitrogen availability, but it is also required to survive under high concentrations of these compounds. Such conditions can occur in diverse habitats such as soil, plant tissues and human cells. Strategies of pathogenic and non-pathogenic bacteria to survive in the presence of poly- and monoamines offer the possibility to combat pathogens by using their capability to metabolize polyamines as an antibiotic drug target. This work aims to summarize the knowledge on poly- and monoamine metabolism in bacteria and its role in nitrogen metabolism.

Sources and Fates of Carbamyl Phosphate: A Labile Energy-Rich Molecule with Multiple Facets

Carbamyl phosphate (CP) is well-known as an essential intermediate of pyrimidine and arginine/urea biosynthesis. Chemically, CP can be easily synthesized from dihydrogen phosphate and cyanate. Enzymatically, CP can be synthesized using three different classes of enzymes: (1) ATP-grasp fold protein based carbamyl phosphate synthetase (CPS); (2) Amino-acid kinase fold carbamate kinase (CK)-like CPS (anabolic CK or aCK); and (3) Catabolic transcarbamylase. The first class of CPS can be further divided into three different types of CPS as CPS I, CPS II, and CPS III depending on the usage of ammonium or glutamine as its nitrogen source, and whether N-acetyl-glutamate is its essential co-factor. CP can donate its carbamyl group to the amino nitrogen of many important molecules including the most well-known ornithine and aspartate in the arginine/urea and pyrimidine biosynthetic pathways. CP can also donate its carbamyl group to the hydroxyl oxygen of a variety of molecules, particularly in many antibiotic biosynthetic pathways. Transfer of the carbamyl group to the nitrogen group is catalyzed by the anabolic transcarbamylase using a direct attack mechanism, while transfer of the carbamyl group to the oxygen group is catalyzed by a different class of enzymes, CmcH/NodU CTase, using a different mechanism involving a three-step reaction, decomposition of CP to carbamate and phosphate, transfer of the carbamyl group from carbamate to ATP to form carbamyladenylate and pyrophosphate, and transfer of the carbamyl group from carbamyladenylate to the oxygen group of the substrate. CP is also involved in transferring its phosphate group to ADP to generate ATP in the fermentation of many microorganisms. The reaction is catalyzed by carbamate kinase, which may be termed as catabolic CK (cCK) in order to distinguish it from CP generating CK. CP is a thermally labile molecule, easily decomposed into phosphate and cyanate, or phosphate and carbamate depending on the pH of the solution, or the presence of enzyme. Biological systems have developed several mechanisms including channeling between enzymes, increased affinity of CP to enzymes, and keeping CP in a specific conformation to protect CP from decomposition. CP is highly important for our health as both a lack of, or decreased, CP production and CP accumulation results in many disease conditions.

New insight into the transcarbamylase family: the structure of putrescine transcarbamylase, a key catalyst for fermentative utilization of agmatine

Transcarbamylases reversibly transfer a carbamyl group from carbamylphosphate (CP) to an amine. Although aspartate transcarbamylase and ornithine transcarbamylase (OTC) are well characterized, little was known about putrescine transcarbamylase (PTC), the enzyme that generates CP for ATP production in the fermentative catabolism of agmatine. We demonstrate that PTC (from Enterococcus faecalis), in addition to using putrescine, can utilize L-ornithine as a poor substrate. Crystal structures at 2.5 Å and 2.0 Å resolutions of PTC bound to its respective bisubstrate analog inhibitors for putrescine and ornithine use, N-(phosphonoacetyl)-putrescine and δ-N-(phosphonoacetyl)-L-ornithine, shed light on PTC preference for putrescine. Except for a highly prominent C-terminal helix that projects away and embraces an adjacent subunit, PTC closely resembles OTCs, suggesting recent divergence of the two enzymes. Since differences between the respective 230 and SMG loops of PTC and OTC appeared to account for the differential preference of these enzymes for putrescine and ornithine, we engineered the 230-loop of PTC to make it to resemble the SMG loop of OTCs, increasing the activity with ornithine and greatly decreasing the activity with putrescine. We also examined the role of the C-terminal helix that appears a constant and exclusive PTC trait. The enzyme lacking this helix remained active but the PTC trimer stability appeared decreased, since some of the enzyme eluted as monomers from a gel filtration column. In addition, truncated PTC tended to aggregate to hexamers, as shown both chromatographically and by X-ray crystallography. Therefore, the extra C-terminal helix plays a dual role: it stabilizes the PTC trimer and, by shielding helix 1 of an adjacent subunit, it prevents the supratrimeric oligomerizations of obscure significance observed with some OTCs. Guided by the structural data we identify signature traits that permit easy and unambiguous annotation of PTC sequences.

Crystal structure and biochemical properties of putrescine carbamoyltransferase from Enterococcus faecalis: Assembly, active site, and allosteric regulation

Putrescine carbamoyltransferase (PTCase) catalyzes the conversion of carbamoylputrescine to putrescine and carbamoyl phosphate (CP), a substrate of carbamate kinase (CK). The crystal structure of PTCase has been determined and refined at 3.2 Å resolution. The trimeric molecular structure of PTCase is similar to other carbamoyltransferases, including the catalytic subunit of aspartate carbamoyltransferase (ATCase) and ornithine carbamoyltransferase (OTCase). However, in contrast to other trimeric carbamoyltransferases, PTCase binds both CP and putrescine with Hill coefficients at saturating concentrations of the other substrate of 1.53 ± 0.03 and 1.80 ± 0.06, respectively. PTCase also has a unique structural feature: a long C-terminal helix that interacts with the adjacent subunit to enhance intersubunit interactions in the molecular trimer. The C-terminal helix appears to be essential for both formation of the functional trimer and catalytic activity, since truncated PTCase without the C-terminal helix aggregates and has only 3% of native catalytic activity. The active sites of PTCase and OTCase are similar, with the exception of the 240's loop. PTCase lacks the proline-rich sequence found in knotted carbamoyltransferases and is unknotted. A Blast search of all available genomes indicates that 35 bacteria, most of which are Gram-positive, have an agcB gene encoding PTCase located near the genes that encode agmatine deiminase and CK, consistent with the catabolic role of PTCase in the agmatine degradation pathway. Sequence comparisons indicate that the C-terminal helix identified in this PTCase structure will be found in all other PTCases identified, suggesting that it is the signature feature of the PTCase family of enzymes.

Lmo0036, an ornithine and putrescine carbamoyltransferase in Listeria monocytogenes, participates in arginine deiminase and agmatine deiminase pathways and mediates acid tolerance

Listeria monocytogenes is a foodborne pathogen causing listeriosis. Acid is one of the stresses that foodborne pathogens encounter most frequently. The ability to survive and proliferate in acidic environments is a prerequisite for infection. However, there is limited knowledge about the molecular basis of adaptation of L. monocytogenes to acid. Arginine deiminase (ADI) and agmatine deiminase (AgDI) systems are implicated in bacterial tolerance to acidic environments. Homologues of ADI and AgDI systems have been found in L. monocytogenes lineages I and II strains. Sequence analysis indicated that lmo0036 encodes a putative carbamoyltransferase containing conserved motifs and residues important for substrate binding. Lmo0036 acted as an ornithine carbamoyltransferase and putrescine carbamoyltransferase, representing the first example, to our knowledge, that catalyses reversible ornithine and putrescine carbamoyltransfer reactions. Catabolic ornithine and putrescine carbamoyltransfer reactions constitute the second step of ADI and AgDI pathways. However, the equilibrium of in vitro carbamoyltransfer reactions was overwhelmingly towards the anabolic direction, suggesting that catabolic carbamoyltransferase was probably the limiting step of the pathways. lmo0036 was induced at the transcriptional level when L. monocytogenes was subjected to low-pH stress. Its expression product in Escherichia coli exhibited higher catabolic carbamoyltransfer activities under acidic conditions. Consistently, absence of this enzyme impaired the growth of Listeria under mild acidic conditions (pH 4.8) and reduced its survival in synthetic human gastric fluid (pH 2.5), and corresponded to a loss in ammonia production, indicating that Lmo0036 was responsible for acid tolerance at both sublethal and lethal pH levels. Furthermore, Lmo0036 played a possible role in Listeria virulence.

Biotechnological production of polyamines by bacteria: recent achievements and future perspectives

In Bacteria, the pathways of polyamine biosynthesis start with the amino acids L-lysine, L-ornithine, L-arginine, or L-aspartic acid. Some of these polyamines are of special interest due to their use in the production of engineering plastics (e.g., polyamides) or as curing agents in polymer applications. At present, the polyamines for industrial use are mainly synthesized on chemical routes. However, since a commercial market for polyamines as well as an industry for the fermentative production of amino acid exist, and since bacterial strains overproducing the polyamine precursors L-lysine, L-ornithine, and L-arginine are known, it was envisioned to engineer these amino acid-producing strains for polyamine production. Only recently, researchers have investigated the potential of amino acid-producing strains of Corynebacterium glutamicum and Escherichia coli for polyamine production. This mini-review illustrates the current knowledge of polyamine metabolism in Bacteria, including anabolism, catabolism, uptake, and excretion. The recent advances in engineering the industrial model bacteria C. glutamicum and E. coli for efficient production of the most promising polyamines, putrescine (1,4-diaminobutane), and cadaverine (1,5-diaminopentane), are discussed in more detail.

The gene cluster for agmatine catabolism of Enterococcus faecalis: study of recombinant putrescine transcarbamylase and agmatine deiminase and a snapshot of agmatine deiminase catalyzing its reaction

Enterococcus faecalis makes ATP from agmatine in three steps catalyzed by agmatine deiminase (AgDI), putrescine transcarbamylase (PTC), and carbamate kinase (CK). An antiporter exchanges putrescine for agmatine. We have cloned the E. faecalis ef0732 and ef0734 genes of the reported gene cluster for agmatine catabolism, overexpressed them in Escherichia coli, purified the products, characterized them functionally as PTC and AgDI, and crystallized and X-ray diffracted them. The 1.65-Angstroms-resolution structure of AgDI forming a covalent adduct with an agmatine-derived amidine reactional intermediate is described. We provide definitive identification of the gene cluster for agmatine catabolism and confirm that ornithine is a genuine but poor PTC substrate, suggesting that PTC (found here to be trimeric) evolved from ornithine transcarbamylase. N-(Phosphonoacetyl)-putrescine was prepared and shown to strongly (K(i) = 10 nM) and selectively inhibit PTC and to improve PTC crystallization. We find that E. faecalis AgDI, which is committed to ATP generation, closely resembles the AgDIs involved in making polyamines, suggesting the recruitment of a polyamine-synthesizing AgDI into the AgDI pathway. The arginine deiminase (ADI) pathway of arginine catabolism probably supplied the genes for PTC and CK but not those for the agmatine/putrescine antiporter, and thus the AgDI and ADI pathways are not related by a single "en bloc" duplication event. The AgDI crystal structure reveals a tetramer with a five-blade propeller subunit fold, proves that AgDI closely resembles ADI despite a lack of sequence identity, and explains substrate affinity, selectivity, and Cys357-mediated-covalent catalysis. A three-tongued agmatine-triggered gating opens or blocks access to the active center.

Acetylornithine transcarbamylase: a novel enzyme in arginine biosynthesis

Ornithine transcarbamylase is a highly conserved enzyme in arginine biosynthesis and the urea cycle. In Xanthomonas campestris, the protein annotated as ornithine transcarbamylase, and encoded by the argF gene, is unable to synthesize citrulline directly from ornithine. We cloned and overexpressed this X. campestris gene in Escherichia coli and show that it catalyzes the formation of N-acetyl-L-citrulline from N-acetyl-L-ornithine and carbamyl phosphate. We now designate this enzyme as an acetylornithine transcarbamylase. The K(m) values for N-acetylornithine and carbamyl phosphate were 1.05 mM and 0.01 mM, respectively. Additional putative transcarbamylases that might also be misannotated were found in the genomes of members of other xanthomonads, Cytophaga, and Bacteroidetes as well as in DNA sequences of bacteria from environmental isolates. It appears that these different paths for arginine biosynthesis arose very early in evolution and that the canonical ornithine transcarbamylase-dependent pathway became the prevalent form. A potent inhibitor, N(alpha)-acetyl-N(delta)-phosphonoacetyl-L-ornithine, was synthesized and showed a midpoint of inhibition at approximately 22 nM; this compound may prove to be a useful starting point for designing inhibitors specific to this novel family of transcarbamylases.

Retrieving sequences of enzymes experimentally characterized but erroneously annotated : the case of the putrescine carbamoyltransferase

Background

Annotating genomes remains an hazardous task. Mistakes or gaps in such a complex process may occur when relevant knowledge is ignored, whether lost, forgotten or overlooked. This paper exemplifies an approach which could help to ressucitate such meaningful data.

Results

We show that a set of closely related sequences which have been annotated as ornithine carbamoyltransferases are actually putrescine carbamoyltransferases. This demonstration is based on the following points : (i) use of enzymatic data which had been overlooked, (ii) rediscovery of a short NH₂-terminal sequence allowing to reannotate a wrongly annotated ornithine carbamoyltransferase as a putrescine carbamoyltransferase, (iii) identification of conserved motifs allowing to distinguish unambiguously between the two kinds of carbamoyltransferases, and (iv) comparative study of the gene context of these different sequences.

Conclusions

We explain why this specific case of misannotation had not yet been described and draw attention to the fact that analogous instances must be rather frequent. We urge to be especially cautious when high sequence similarity is coupled with an apparent lack of biochemical information. Moreover, from the point of view of genome annotation, proteins which have been studied experimentally but are not correlated with sequence data in current databases qualify as "orphans", just as unassigned genomic open reading frames do. The strategy we used in this paper to bridge such gaps in knowledge could work whenever it is possible to collect a body of facts about experimental data, homology, unnoticed sequence data, and accurate informations about gene context.

Analysis of an agmatine deiminase gene cluster in Streptococcus mutans UA159

An operon encoding enzymes of the agmatine deiminase system (AgDS) has been identified in the cariogenic bacterium Streptococcus mutans UA159. The AgDS is regulated by agmatine induction and carbohydrate catabolite repression. Ammonia is produced from agmatine at low pH, suggesting that the AgDS could augment acid tolerance.

Gene structure, organization, expression, and potential regulatory mechanisms of arginine catabolism in Enterococcus faecalis

Although Enteroccus faecalis is the paradigm for biochemical studies on the arginine deiminase (ADI) pathway of fermentative arginine catabolism, little genetic information exists on this pathway in this organism. We fill this important gap by characterizing, in an 8,228-bp region cloned from a lambdagt11 genomic library of E. faecalis, a five-gene cluster forming a transcriptional unit (revealed by Northern blots and primer extension in E. faecalis) that corresponds to the ADI operon. Four additional genes in the opposite DNA strand and one in the same DNA strand are also identified. Studies on the protein products, including heterologous expression and/or sequence comparisons, allow us to ascertain or propose functions for all but 1 of the 10 genes. The ADI operon genes, arcABCRD, encode, respectively, ADI, ornithine transcarbamylase, carbamate kinase, a putative Crp/Fnr-type regulator (ArcR), and a putative ornithine-arginine antiporter (ArcD). Arginine induces the expression of arcABCRD, most likely by means of two homologous ArgR/AhrC-type regulators encoded by two genes, argR1 and argR2, that precede arcABCRD in each DNA strand and that are transcribed monocistronically, their transcription being influenced differentially by glucose and arginine. Potential ArgR1/ArgR2 (double and single) binding sequences are found in the promoter regions of arcA and of argR1/argR2 themselves. In addition, putative binding sequences for ArcR and for CcpA are found, respectively, in the argR1/argR2 and arcA promoter regions. Of the three other genes identified, two form a transcriptional unit and encode a putative metal-sensitive transcriptional regulator (ArsR) and a cysteine protease.

Crystal structure of a transcarbamylase-like protein from the anaerobic bacterium Bacteroides fragilis at 2.0 A resolution

A transcarbamylase-like protein essential for arginine biosynthesis in the anaerobic bacterium Bacteroides fragilis has been purified and crystallized in space group P4(3)2(1)2 (a=b=153.4 A, c=94.8 A). The structure was solved using a single isomorphous replacement with anomalous scattering (SIRAS) and was refined at 2.0 A resolution to an R-factor of 20.6% (R-free=25.2%). The molecular model is trimeric and comprises 960 amino acid residues, two phosphate groups and 422 water molecules. The monomer has the consensus transcarbamylase fold with two structural domains linked by two long interdomain helices: the putative carbamoyl phosphate-binding domain and a binding domain for the second substrate. Each domain has a central parallel beta-sheet surrounded by alpha-helices and loops with alpha/beta topology. The putative carbamoyl phosphate-binding site is similar to those in ornithine transcarbamylases (OTCases) and aspartate transcarbamylases (ATCases); however, the second substrate-binding site is strikingly different. This site has several insertions and deletions, and residues critical to substrate binding and catalysis in other known transcarbamylases are not conserved. The three-dimensional structure and the fact that this protein is essential for arginine biosynthesis suggest strongly that it is a new member of the transcarbamylase family. A similar protein has been found in Xylella fastidiosa, a bacterium that infects grapes, citrus and other plants.

Human ornithine transcarbamylase: crystallographic insights into substrate recognition and conformational changes

Two crystal structures of human ornithine transcarbamylase (OTCase) complexed with the substrate carbamoyl phosphate (CP) have been solved. One structure, whose crystals were prepared by substituting N-phosphonacetyl-L-ornithine (PALO) liganded crystals with CP, has been refined at 2.4 A (1 A=0.1 nm) resolution to a crystallographic R factor of 18.4%. The second structure, whose crystals were prepared by co-crystallization with CP, has been refined at 2.6 A resolution to a crystallographic R factor of 20.2%. These structures provide important new insights into substrate recognition and ligand-induced conformational changes. Comparison of these structures with the structures of OTCase complexed with the bisubstrate analogue PALO or CP and L-norvaline reveals that binding of the first substrate, CP, induces a global conformational change involving relative domain movement, whereas the binding of the second substrate brings the flexible SMG loop, which is equivalent to the 240s loop in aspartate transcarbamylase, into the active site. The model reveals structural features that define the substrate specificity of the enzyme and that regulate the order of binding and release of products.

Bioenergetics and solute transport in lactococci

During the last few years the studies about the physiology and bioenergetics of lactic acid bacteria during growth and starvation have evolved from a descriptive level to an analysis of the molecular events in the regulation of various processes. Considerable progress has been made in the understanding of the modes of metabolic energy generation, the mechanism of homeostasis of the internal pH, and the mechanism and regulatory processes of transport systems for sugars, amino acids, peptides, and ions. Detailed studies of these transport processes have been performed in cytoplasmic membrane vesicles of these organisms in which a foreign proton pump has been introduced to generate a high proton motive force.

Transport of diamines by Enterococcus faecalis is mediated by an agmatine-putrescine antiporter

Enterococcus faecalis ATCC 11700 is able to use arginine and the diamine agmatine as a sole energy source. Via the highly homologous deiminase pathways, arginine and agmatine are converted into CO2, NH3, and the end products ornithine and putrescine, respectively. In the arginine deiminase pathway, uptake of arginine and excretion of ornithine are mediated by an arginine-ornithine antiport system. The translocation of agmatine was studied in whole cells grown in the presence of arginine, agmatine, or glucose. Rapid uncoupler-insensitive uptake of agmatine was observed only in agmatine-grown cells. A high intracellular putrescine pool was maintained by these cells, and this pool was rapidly released by external putrescine or agmatine but not by arginine or ornithine. Kinetic analysis revealed competitive inhibition for uptake between putrescine and agmatine. Agmatine uptake by membrane vesicles was observed only when the membrane vesicles were preloaded with putrescine. Uptake of agmatine was driven by the outwardly directed putrescine concentration gradient, which is continuously sustained by the metabolic process. Uptake of agmatine and extrusion of putrescine by agmatine-grown cells of E. faecalis appeared to be catalyzed by an agmatine-putrescine antiporter. This transport system functionally resembled the previously described arginine-ornithine antiport, which was exclusively induced when the cells were grown in the presence of arginine.

Control of enzyme synthesis in the oxalurate catabolic pathway of Streptococcus faecalis ATCC 11700: evidence for the existence of a third carbamate kinase

Streptococcus faecalis ATCC 11700 uses oxalurate as a sole energy source for growth. An oxamate carbamoyltransferase and a carbamate kinase, both induced by oxalurate, are involved in this process. The oxalurate-induced kinase is specific for the pathway. Its properties are different from those of the previously characterized agmatine and arginine-induced kinases. Glucose, but not arginine, nor agmatine, two other energy sources, represses the oxalurate pathway. In contrast, oxalurate was found to be at least as effective as glucose in repressing the arginine deiminase pathway in arginine grown cells, or the agmatine deiminase pathway during growth on agmatine.

Immunological and structural relatedness of catabolic ornithine carbamoyltransferases and the anabolic enzymes of enterobacteria

Purified catabolic ornithine carbamoyltransferase of Pseudomonas putida and anabolic ornithine carbamoyltransferase (argF product) of Escherichia coli K-12 were used to prepare antisera. The two specific antisera gave heterologous cross-reactions of various intensities with bacterial catabolic ornithine carbamoyltransferases formed by Pseudomonas and representative organisms of other bacterial genera. The immunological cross-reactivity observed only between the catabolic ornithine carbamoyltransferases and the anabolic enzymes of enterobacteria suggests that these proteins share some structural similarities. Indeed, the amino acid composition of the anabolic ornithine carbamoyltransferase of E. coli K-12 (argF and argI products) closely resembles the amino acid compositions of the catabolic enzymes of Pseudomonas putida, Aeromonas formicans, Streptococcus faecalis, and Bacillus licheniformis. Comparison of the N-terminal amino acid sequence of the E. coli anabolic ornithine carbamoyltransferase with that of the A. formicans and Pseudomonas putida catabolic enzymes shows, respectively, 45 and 28% identity between the compared positions; the A. formicans sequence reveals 53% identity with the Pseudomonas putida sequence. These results favor the conclusion that anabolic ornithine carbamoyltransferases of enterobacteria and catabolic ornithine carbamoyltransferases derive from a common ancestral gene.

Enzymes of agmatine degradation and the control of their synthesis in Streptococcus faecalis

Streptococcus faecalis ATCC 11700 uses agmatine as its sole energy source for growth. Agmatine deiminase and putrescine carbamoyltransferase are coinduced by growth on agmatine. Glucose and arginine were found to exert catabolite repression on the agmatine deiminase pathway. Four mutants unable to utilize agmatine as an energy source, isolated from the wild-type strain, exhibited three distinct phenotypes. Two of these strains showed essentially no agmatine deiminase, one mutant showed negligible activity of putrescine carbamoyltransferase, and one mutant was defective in both activities. Two carbamate kinases are present in S. faecalis, one belonging to the arginine deiminase pathway, the other being induced by growth on agmatine. These two enzymes have the same molecular weight, 82,000, and seem quite different in size from the kinases isolated from other streptococci.

Control of enzyme synthesis in the arginine deiminase pathway of Streptococcus faecalis

The formation of the arginine deiminase pathway enzymes in Streptococcus faecalis ATCC 11700 was investigated. The addition of arginine to growing cells resulted in the coinduction of arginine diminase (EC 3.5.3.6), ornithine carbamoyltransferase (EC 2.1.3.3), and carbamate kinase (EC 2.7.2.3). Growth on glucose-arginine or on glucose-fumarate-arginine produced a decrease in the specific activity of the arginine fermentation system. Aeration had a weak repressing effect on the arginine deiminase pathway enzymes in cells growing on arginine as the only added substrate. By contrast, depending on the growth phase, a marked repression of the pathway by oxygen was observed in cells growing on glucose-arginine. We hypothesize that, in S. faecalis, the ATP pool is an important signal in the regulation of the arginine deiminase pathway. Mutants unable to utilize arginine as an energy source, isolated from the wild type, exhibited four distinct phenotypes. In group I the three enzymes of the arginine deiminase pathway were present and probably affected in the arginine uptake system. Group II mutants had no detectable arginine deiminase, whereas group III mutants had low levels of ornithine carbamoyltransferase. Group IV mutants were defective for all three enzymes of the pathway.