Repression of Arg mRNA synthesis by L-arginine in cell-free extracts of Escherichia coli

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.

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.

Transcription of the argF and argI genes of the arginine biosynthetic regulon of Escherichia coli K12, performed in vitro

The cell-free transcription of the argF and argI genes of the arginine biosynthetic regions is described using an S-30 system capable of coupled in vitro transcription-translation. Template DNA isolated from two independently isolated arginine transducing phages was used in this work. Steady state mRNA synthesis was observed which was attributed to RNAase degradation. Regulation of argF mRNA synthesis, directed by the argF gene carried on the specialized transducing phage phi80dargF is effected to the extent of at least 95% by the arginine holorepressor at the transcriptional stage and at least 80% of the regulation of the expression of the argI gene is mediated at the transcriptional stage. Evidence is presented which indicates that the arginine holorepressor prevents RNA polymerase from binding to the arginine promoter and suggests that the operator and promoter sites may overlap.

In vitro transcription of the Escherichia coli K-12 argA, argE, and argCBH operons

Deoxyribonucleic acid isolated from argA and argECBH transducing phages was utilized to study the in vitro synthesis of argA, argE, and argCBH messenger ribonucleic acid. The specific regulation of these operons by the arginine holorepressor was demonstrated, providing evidence that the majority, if not all, of the control of these operons is exercised at the transcriptional level. Data are presented which indicate that the arginine holorepressor functions by binding to the operator region and concomitantly prevents the binding of ribonucleic acid polymerase to the corresponding promoter region.

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.