Effect of arginine and canavanine on arginine messenger RNA synthesis

Biosynthesis of Arginine and Polyamines

Early investigations on arginine biosynthesis brought to light basic features of metabolic regulation. The most significant advances of the last 10 to 15 years concern the arginine repressor, its structure and mode of action in both E. coli and Salmonella typhimurium, the sequence analysis of all arg structural genes in E. coli and Salmonella typhimurium, the resulting evolutionary inferences, and the dual regulation of the carAB operon. This review provides an overall picture of the pathways, their interconnections, the regulatory circuits involved, and the resulting interferences between arginine and polyamine biosynthesis. Carbamoylphosphate is a precursor common to arginine and the pyrimidines. In both Escherichia coli and Salmonella enterica serovar Typhimurium, it is produced by a single synthetase, carbamoylphosphate synthetase (CPSase), with glutamine as the physiological amino group donor. This situation contrasts with the existence of separate enzymes specific for arginine and pyrimidine biosynthesis in Bacillus subtilis and fungi. Polyamine biosynthesis has been particularly well studied in E. coli, and the cognate genes have been identified in the Salmonella genome as well, including those involved in transport functions. The review summarizes what is known about the enzymes involved in the arginine pathway of E. coli and S. enterica serovar Typhimurium; homologous genes were identified in both organisms, except argF (encoding a supplementary OTCase), which is lacking in Salmonella. Several examples of putative enzyme recruitment (homologous enzymes performing analogous functions) are also presented.

Regulation and coupling of argECBH mRNA and enzyme synthesis in cell extracts of Escherichia coli

Cell extracts from Escherichia coli were used to study both transcription and coupled translation of the argECBH gene cluster. Argininosuccinase (the argH enzyme) and N-acetylornithinase (the argE enzyme) were synthesized for 90 to 120 min, and hybridizable argECBH mRNA was synthesized for 60 min after the addition of a lambda or phi 80 dargECBH DNA template. L-Arginine (2.5 mM) repressed synthesis by argR+ extracts of argECBH mRNA 2-, to 3-fold, argE enzyme 5- to 8-fold, and argH enzyme 20- to 60-fold. Repression was specific for L-arginine, and argR extracts were insensitive to added L-arginine. The argECBH mRNA made under conditions of restricted protein synthesis had reduced ability to function in the formation of the argE and argH enzymes and was found to be predominantly 6 to 8S in sucrose density gradients. When protein synthesis was allowed, the mRNA formed was functional, and large amounts of 14 to 23S argECBH mRNA appeared on sucrose gradients. An S-100 supernatant freed of ribosomes was capable of producing hybridizable arg mRNA, but significant functional message was only produced when ribosomes were present. When purified RNA polymerase was used, the formation of short 6 to 8S argECBH mRNA was dependent upon added rho protein. The data suggest that rho-dependent sites in the argECBH operon allow early termination of mRNA synthesis when transcription is not coupled to active enzyme synthesis.

Positive control of expression of the argECBH gene cluster in vitro by guanosine 5'-diphosphate 3'-diphosphate

By using a cell-free system derived from Escherichia coli, it was found that guanosine 5'-diphosphate 3'-diphosphate (ppGpp) was a positive effector for expression of both wings of bidirectionally transcribed argECBH gene cluster. A 7- to 20-fold increase in the synthesis of both argininosuccinase (the argH enzyme) and N-acetylornithinase (the argE enzyme) resulted with added ppGpp (0.2 mM optimum). Synthesis of hybridizable argECBH mRNA was enhanced only 30 to 100% by added ppGpp. Of the various guanosine nucleotides tested, only pppGpp mimicked ppGpp. Added ppGpp had no important effect upon (i) measurable argE or argH enzyme activity, (ii) total protein synthesis in the cell-free system, or (iii) the rate of decay of hybridizable argECBH mRNA. With extracts of an argR+ strain, added ppGpp had no effect on the repression of enzyme or mRNA synthesis by L-arginine. By using a two-stage system in which the bulk of argECBH mRNA was synthesized while protein synthesis was delayed, we showed that ppGpp acted at some point during transcription.

Cloning and endonuclease restriction analysis of argF and of the control region of the argECBH bipolar operon in Escherichia coli

A 1.8 kb DNA fragment, liberated by endonuclease HindIII, contains the control region of the argECBH bipolar operon near one end and the weak secondary promoter of argH at the other extremity; it has been cloned in plasmid pBR322. The same plasmid vector has been used to clone the argF gene liberated from the chromosome by endonuclease BamHI. Restriction patterns for the two hybrid plasmids have been determined, using enzymes AluI, BglI, EcoRI, HaeIII, HincII, HindIII, HpaI and II, PstI and SalI. Two AluI sites situated on either side of and close to a HincII target delineate two short fragments covering the whole of the argECBH control region. The argF control elements are located in a region accessible to further dissection by BamHI, EcoRI, PstI and HindIII. Carriers of the argF plasmid produce extremely high amounts of ornithine carbamoyltransferase, a feature useful for purification of this enzyme.

Isolation, cloning and characterization of argF gene DNA from Escherichia coli K-12

A 1650 base pair (BP) fragment carrying the entire argF structural gene with its associated control regions was isolated from an EcoRI/BamHI digest of phi80argFilambda cI857 DNA. This segment was cloned using the EcoRI and BamHI cleavage sites in the plasmid pBR322. A preliminary restriction map of the argF region was prepared. RNA polymerase binding studies indicated that the argF promoter is located approx. 30 base pairs from the EcoRI terminus of the cloned DNA segment.

Evidence for translational repression of arginine biosynthetic enzymes in Escherichia coli: altered regulation in a streptomycin-resistant mutant

The formation and repressibility of the arginine biosyntietic enzymes acetylornithine delta-aminotransferase (EC 2.6.1.11), acetylornithine deacetylase (EC 3.5.1.16), ornithine carbamoyltransferase (EC 2.1.3.3), and argininosuccinate lyase (EC 4.3.2.1) were studied in an Escherichia coli W derivative (strain 250-10) that carries (a) a mutant allele of the argR regulatory gene causing a diminished repression-derepression range and (b) a streptomycin resistance mutation. In comparison with the streptomycin-sensitive parent 250, all four enzymes (a) are formed as smaller proportions of the total protein (overall range, 12% to 71%), whether the conditions are repressive (arginine excess) or derepressive (arginine restriction), and (b) show increased repressibility ratios, the carbamoyltransferase giving the largest increase (from 5.7 to 25.0). These effects appear to depend on the concurrent expression of the regulatory-gene and streptomycin resistance mutations, as indicated by analogous experiments with canavanine-resistant mutants of 250-10 that have partial argR- character. The results provide evidence for translational repression in the arginine system, and are interpreted in terms of a functional interaction of a mutant arginine repressor with a mutant S12 ribosomal protein. The locale of translational repression may be near the site of S12, and this mode of regulation may involve initiational selectivity of groupwise recognizable arginine messenger RNA's.

Metabolism of arginine-specific messenger ribonucleic acid in Escherichia coli K-12

Ribonucleic acid-deoxyribonucleic acid (RNA-DNA) hybridization was employed for the determination of the level of messenger RNA (mRNA) transcribed from seven of the nine genes of the arginine regulon of Escherichia coli K-12. The quantity of RNA complexing with each of the separated DNA strands of the argA, argF, argE, and argCBH operons carried on specialized transducing phages was measured. The derepressed:repressed ratio of mRNA formed in vivo was found to vary between about 3 and 4 when measured by hybridization to DNA isolated from specialized transducing phages carrying the argA, argE, argCBH, argF, and argI operons.

Effect of arginine on the stability and size of argECBH messenger ribonucleic acid in Escherichia coli

The chemical stability of argECBH messenger ribonucleic acid (mRNA) produced by Escherichia coli was found to be unaltered during steady-state repression by arginine. During extreme arginine deprivation, the increase in argECBH mRNA stability was related to general effects of amino acid starvation on mRNA stability. Thus a mechanism whereby argECBH gene expression is regulated by altering the decay rate of this mRNA is not consistent with our data. Sucrose gradient analysis followed by hybridization revealed that both heavy (14S) and light (8S) components of argECBH mRNA were produced by cells of E. coli K-12 grown without arginine, whereas predominantly light (8S) mRNA was produced by cells grown with arginine. A functional argR gene and the EC portion of the argECBH cluster were found essential for the arginine restriction of heavy-mRNA production. Experiments suggest that light argECBH mRNA did not arise from heavy message, and 8u% of both light and heavy mRNA was found bound to ribosomes. The data appear most consistent with the notion that a second site of control by arginine regulates the amounts of light and heavy arginine mRNA in the cell either by early termination of transcription or by endonucleolytic processing. Consideration of these data in conjunction with those of the accompanying report (Krzyzek and Rogers, 1976) permits the tentative conclusion that light argECBH mRNA is not translated into active enzymes and is thus responsible for the discrepancy between the high content of hybridizable mRNA and low rates of enzyme synthesis found during arginine repression.

Dual regulation by arginine of the expression of the Escherichia coli argECBH operon

The correlation between the level of messenger ribonucleic acid (mRNA) specific for the argECBH gene cluster (argECBH mRNA) measured by ribonucleic acid-deoxyribonucleic acid (RNA-DNA) hybridization and the rates of synthesis of N-acetylornithine deacetylase (argE enzyme) and of argininosuccinate lyase (argH enzyme) of Escherichia coli strain K-12 were determined for steady-state growth with and without added L-arginine and during the transition periods between these two states. During the transient period after arginine removal (transient derepression), the synthesis of enzymes argE and argH was initially three to five times greater than the steady-state derepressed rate finally reached 50 min later. The level of argECHB mRNA correlated well both quantitatively and temporally with the rates of enzyme synthesis during this transition. The level of in vivo charged arginyl-transfer RNA (tRNAarg), monitored simultaneously, was initially only 5 to 10% and gradually increased to a final level of 80% after 45 min. During the transient period after arginine addition (transient repression), the rates of synthesis of enzymes argE and argH decreased to almost zero and gradually reached steady-state repressed rates after about 180 min. The argECBH mRNA level remained constant at the steady-state repressed level throughout transient repression, revealing a discontinuity between the level of this mRNA and rates of enzyme synthesis. A similar discrepancy was noted during the transition after ornithine addition. In vivo charged tRNAarg remained constant at 80% during this transition. After removal of arginine, the zero-level transient enzyme synthesis developed after only 7.5 min of arginine deprivation and was maximum after 30 min. The results suggest an accumulation of a molecule regulated by arginine that plays a role in transient repression. Our data indicate that arginyl-tRNA synthetase is not this molecule since its synthesis was unaffected by arginine. The ratios of steady-state argE and argH enzyme synthesis without arginine to that with arginine were 12 and 20, respectively, whereas the similar ratio for argECBH mRNA was 2 to 3. The repressed level of argECBH mRNA was not affected by attempts to repress or derepress the ppc+ gene (carried on the DNA used for hybridization), and the repressed level of argECBH mRNA was lowered about 50% in cells carrying an internal argBH deletion. These data taken together indicate the presence of an excess of untranslated argECBH mRNA during both transient and steady-state repression by arginine. Thus, a second regulatory mechanism, not yet defined, appears to play an important role in arginine regulation of enzyme synthesis.

Derepression and repression of the histidine operon: role of the feedback site of the first enzyme

Thiazolealanine, a false feedback inhibitor, causes transient repression of the his operon previously derepressed by a severe histidine limitation in strains with a wild-type or feedback-hypersensitive first enzyme but not in feedback-resistant mutants. Since experiments reported here clearly demonstrate that thiazolealanine is not transferred to tRNAHis, it is proposed that this "transient repression" is effected through the interaction of thiazolealanine with the feedback site of the enzyme. Experiments in the presence of rifampin indicate that this thiazolealanine-mediated effect is exerted at the level of translation. We conclude that histidine (free), in addition to forming co-repressor, also represses the operon at the level of translation through feedback interaction with the first enzyme of the pathway (adenosine 5'-triphosphate phosphoribosyltransferase). Rates of derepression in feedback-resistant strains are roughly half of those observed in controls, suggesting a positive role played by a first enzyme with a normal but unoccupied feedback site. Some feedback-resistant mutants, in contrast to the wild type, were unable to exhibit derepression under histidine limitation caused by aminotriazole.

Parameters of gene expression in the bipolar argECBH operon of E. coli K12. The question of translational control

The pattern of divergent transcription of the argEC BH cluster of genes previously demonstrated by the hybridization of RNA to the separated strand of a phi 80 darg transducing phage, is confirmed with the DNA of a set of different lambdadarg phages. The accurate determination of argE and argCBH m-RNA levels in different steady states of expression of the arg regulon supports the following conclusions: 1. The ratio between maximal (derepressed) and minimal (repressed) rates of expression is lower when it is expressed in terms of % hybridizable RNA than in terms of expression is lower when it is expressed in terms of % hybridizable RNA than in terms of enzyme specific activities. The discrepancy is about 3 fold. Thus in conditions of repression, the cell produces relatively more unused m-RNA than in derepression. Different interpretations of this phenomenon appear possible: a) the messenger RNA molecules synthesized in repressed cells could be degraded more rapidly or translated less efficiently than in derepressed cells. b) an untranslated segment of the RNA could account for a larger part of the RNA detected in repression than in derepression. These interpretations are not mutually exclusive. 2. The discrepancy observed between the amplitudes of variation of argE and argC BH expression, expressed in terms of enzyme specific activities, is, in fact, determined at the level of DNA transcription. This provides direct evidence for the occurrence of differential transcription effectiveness in a regulon exhibiting a correlative but not strictly coordinated pattern of enzyme synthesis. This also supports our earlier suggestion regarding the possible complexity of the internal operator region situated between argE and C.

Isoleucine and valine metabolism in Escherichia coli K-12: detection and measurement of ilv-specific messenger ribonucleic acid

Ribonucleic acid-deoxyribonucleic acid (RNA-DNA) hybridization was employed for the determination of messenger RNA transcribed from the ilv gene cluster of Escherichia coli K-12. Strains with derepressed levels of the isoleucine and valine biosynthetic enzymes owing to linked or unlinked genetic lesions were found to exhibit ilv messenger RNA levels from 1.5- to 4-fold higher than did their isogenic parents. When grown under conditions that specifically repressed the synthesis of isoleucine- and valine-forming enzymes, most strains exhibited drastically reduced ilv messenger RNA levels. Hybridization performed with the separated strands of ilv DNA showed that all the ilv genes are transcribed from the same strand, the "l strand" of lambdaphi80CI857St68dilv DNA. Sucrose gradient analyses of RNA extracted from cells starved for isoleucine, valine, or leucine resulted in the detection of at least two distinct types of ilv messenger RNA.

Participation of branched-chain amino acid analogues in multivalent repression

Two isoleucine analogues and two leucine analogues were examined for their ability to replace the natural amino acid preventing the accumulation of threonine deaminase-forming potential. The procedure used to study repression by the analogues distinguishes between true repression and the formation of inactive enzyme by the analogue in question. The leucine analogue 4-azaleucine was found to replace leucine in multivalent repression of threonine deaminase-forming potential in Escherichia coli but not in Salmonella typhimurium. Another leucine analogue, trifluoroleucine, was only partially effective in causing repression in either organism. The isoleucine analogue 4-azaisoleucine was ineffective in replacing isoleucine in repression. In contrast, 4-thiaisoleucine effectively replaced isoleucine in the repression of threonine deaminase-forming potential in S. typhimurium and E. coli.

Repression of enzymes of arginine biosynthesis by L-canavanine in arginyl-transfer ribonucleic acid synthetase mutants of Escherichia coli

We show that the arginine analogue, l-canavanine, repressed the accumulation of translatable messenger ribonucleic acid (RNA) for three arginine biosynthetic enzymes in Escherichia coli. The method used to determine the level of translatable messenger RNA depended upon measurement of a burst of enzyme synthesis as described previously. E. coli strains with defective arginyltransfer ribonucleic acid (tRNA) synthetase (argS mutants) were insensitive to canavanine repression. When deprived of leucine, a leu argS strain regained normal sensitivity to canavanine repression. The level of in vivo canavanyl-tRNA(arg) was determined for a normal strain and an argS mutant. After 20 min of growth with canavanine only 9% of tRNA(arg) from the argS strain was protected from periodate oxidation, while 42% of the tRNA(arg) from an argS(+) strain was charged. When deprived of leucine, leu argS or leu argS(+) strains grown with canavanine contained more than 60% charged tRNA(arg). Reverse phase column chromatography of periodate-oxidized tRNA from canavanine-grown argS and argS(+) strains showed no preferential charging of any isoaccepting species of tRNA(arg). Therefore, we failed to detect a specific arginyl-tRNA species that might be involved in repression by canavanine. However, the data suggest that canavanine repression of the arginine pathway occurs only when high levels of canavanyl-tRNA are present, and thus support the notion that arginyl-tRNA synthetase plays a role in generating a repression signal.

Arginine control of transcription of argECBH messenger ribonucleic acid in Escherichia coli

The level of messenger ribonucleic acid specific for the argECBH gene cluster (arg-mRNA) of Escherichia coli was measured by deoxyribonucleic acid-ribonucleic acid hybridization in a number of strains. During the first 10 min after removal of arginine (derepression), the rate of arg-mRNA accumulation was six to ten times greater than that found in arginine-repressed argR(+) cells. In the absence of arginine, l-canavanine (200 mug/ml) repressed arg-mRNA synthesis to a level only 20 to 30% lower than that found after arginine deprivation. High levels of arg-mRNA were produced by argR(-) strains with or without added arginine. Within about 2 min after arginine addition to argR(+) cells, the rate of synthesis of arg-mRNA reached the repressed level. Likewise, 2.5 min after rifampin addition, all transcription of arg-mRNA was completed. These data are consistent with the view that arginine signals repression by inhibiting the initiation of transcription of arg-mRNA mediated in some way by the argR gene. The kinetics of arg-mRNA accumulation and the kinetics of completion of transcription together with the profile of hybridizable arg-mRNA in sucrose density gradients (major component 16S) suggest that the argECBH gene cluster is transcribed in short pieces rather than as a single unit.