Regulation of Escherichia coli ornithine transcarbamylase by orotate.

Ornithine transcarbamylase from Escherichia coli, strain W, exhibits negative cooperativity with respect to ornithine, and the enzymatic activity is further regulated by orotate. The effect of orotate on ornithine transcarbamylase is dependent not only upon the carbamylphosphate concentration, but also upon the concentration of ornithine. At high concentrations of carbamylphosphate (10 mM), a conversion from negative cooperativity to positive cooperativity is observed with 10 mM orotate. At 1 mM carbamylphosphate, however, 10 mM orotate activates the enzyme at low ornithine concentrations, but as the ornithine concentration is increased above 5 mM, inhibition is observed. Thus, a regulatory link has been established between the pathways of arginine biosynthesis and pyrimidine biosynthesis, each of which utilizes carbamylphosphate.     

Carbamoyl phosphate biosynthesis and partition in pyrimidine and arginine pathways of Escherichia coli. In situ properties of carbamoyl-phosphate synthase, ornithine transcarbamylase and aspartate transcarbamylase in permeabilized cells

A procedure for the permeabilization of Escherichia coli cells was adapted to the in situ determination of the catalytic and regulatory properties of the enzymes responsible for the biosynthesis of carbamoyl phosphate and its utilization in the pyrimidine and arginine pathways. Differences in enzyme sensitivity to effectors and changes in pH dependence were observed. Partition of carbamoyl phosphate in the two metabolic pathways could be measured under conditions of substrate saturation. The results obtained will allow to test experimentally the theoretical predictions made by A. Goldbeter (1973) PhD thesis, Université Libre de Bruxelles, on the distribution of carbamoyl phosphate and the oscillation of its intracellular concentration.     

Catalase synthesis in Escherichia coli is not controlled by catabolite repression

Catabolite repression is not involved in the regulation of catalase gene expression. The presence of glucose in minimal salts media and LB medium did not affect the basal levels of catalase but did enhance catalase synthesis following induction with either hydrogen peroxide or ascorbate. The cofactor for catabolite gene activator protein, cAMP, did not affect either the basal levels or the rate or extent of catalase synthesis. Catalase synthesis occurred normally in an adenylate cyclase mutant where β-galactosidase, a catabolite-sensitive enzyme was not synthesized.     

Synthesis of the Escherichia coli K12 isoenzymes of ornithine transcarbamylase, performed in vitro.

Summary The in vitro synthesis of enzymaticallyactive ornithine transcarbamylase (OTCase) directed by each of the E. coli K-12 OTCase genes (argF and argI) is described. The E. coli OTCase isoenzyme subunits are not identical, whether synthesized in vivo or in vitro, the argF-coded product being about 5% smaller. The OTCase protomers are enzymatically inactive but associate in vitro to an enzymatically active multimer. The rates of subunit association of argF and argI isoenzymes are considerably different. Utilizing the facile assay protocol presented, the regulation of in vitro OTCase synthesis by the specific holorepressor of the arginine regulon is demonstrated. Calculations based upon data presented indicate that there are about 65 molecules of argR gene product per bacterium, a substantially lower estimate than previously reported.This work is dedicated to Luigi Gorini without whom none of this would have been possible. His unbounded love of science and freedom will be remembered by so many, for so long.     

In vitro synthesis of ornithine transcarbamylase.

An in vitro system for the synthesis of ornithine transcarbamylase (OTCase) was established using iS-30 extract from E. coli MDS6-2(lambda) and DNA of a lambda transducing phage carrying argI and argF genes. This in vitro synthesis was completely dependent on the additon of DNA, and was sensitive to chloramphenicol and rifampicin. Radioisotopic analysis confirmed that the synthesized enzyme catalyzes the carbamylation of ornithine to citrulline. In the in vitro system the repression and derepression of OTCase synthesis could be observed by mixing iS-30 extracts prepared from argR+ and argR- cells. A remarkable maturation effect could be observed for the FFF enzyme, but not for the III enzyme. This system is considered to reflect the in vivo situation, and should therefore be useful for investigations on the regulation of OTCase synthesis in vivo.     

In vitro repression of n- -acetyl-L-ornithinase synthesis in Escherichia coli

Summary Development of a system for in vitro synthesis of N--acetyl-L-ornithinase of E. coli has made it possible to detect the argR gene product, i.e., the arginine repressor, in cell extracts.