Regulation of Galactokinase Synthesis by Cyclic Adenosine 3',5'-Monophosphate in Cell-free Extracts of Escherichia coli

Erythrocyte galactokinase assay with high performance liquid chromatography

An assay for erythtocyte galactokinase based on high performance liquid chromatographic determination of galactose 1-phosphate (Gal-1-P) is described. The determination of Gal-1-P was applied to a post-column fluorometric detection of reducing sugars using arginine. This method is as sensitive and accurate as conventional radioisotopic methods, but needs no radioisotopic facilities. It requires only a small blood sample and is suitable as a follow-up test in neonatal screening.

Prokaryotic gene expression in vitro: transcription-translation coupled systems

Transcription-translation coupled systems have been developed to study prokaryotic gene expression. Several types of expression system have been described. The original system consists of a crude unfractionated Escherichia coli extract, which supports protein synthesis directed by a template DNA. Control of gene expression at the transcriptional stage has been studied using this unfractionated system. In this respect, two examples of particular interest, lactose and tryptophan operons, are described. Other systems are either partially reconstituted or highly defined, containing up to 30 purified factors necessary for transcription (RNA polymerase) and translation (aminoacyl-tRNA synthetases, initiation, elongation and release factors). Additional differences between the various systems relate to the analysis of the gene products. Whereas most methods involve analysis of the totally synthesized protein, a particular system implies the formation of only the N-terminal di- or tripeptide of the gene product. Reconstituted systems have proved useful in studies on transcriptional, e.g., discovery and role of L factor, as well as translational regulation of gene expression, e.g., autogenous control of ribosomal protein synthesis.

[Nucleoside polyphosphates: occurrence, metabolism and function]

Procaryotes have regulatory systems allowing to vary the metabolism in response to nutritional variations, to reduce the growth, and to start development. Nucleoside polyphosphates are mediators of coordinated alterations of metabolism. In this review, after a brief recall of the characteristics of the stringent response, the occurrence, determinations, and the metabolism of the nucleoside polyphosphates are presented. The representation of the pleiotropic effects includes the regulation of the protein synthesis and of the protein synthesis apparatus, of the protein turnover, of the N- and carbohydrate metabolism, of the formation of cell membranes and cell walls as well as the possible function of the development.

Rapid detection of allelic recombination at the gall locus in yeast by assay of the recombinant gene product

A diploid heteroallelic at thegal1locus and producing little residual galactokinase activity was chosen. Following either γ- or UV-irradiation, recombination occurred leading to the formation of wild-typeGAL1 genes and an increase in detectable galactokinase. An enhanced level of enzyme was first detectable 2 h after irradiation and reached a maximum within 12–14 h. With increasing doses of irradiation, more enzyme was produced and this increase superficially resembled that obtained for plated recombinants. However, far more enzyme was actually synthesized than expected from the number of viable recombinants in the cells assayed. This suggests that recombination must have occurred in cells without colony-forming ability, indeed it may have occurred more frequently in these cells than in viable cells.

An equilibrium study of the cooperative binding of adenosine cyclic 3',5'-monophosphate and guanosine cyclic 3',5'-monophosphate to the adenosine cyclic 3',5'-monophosphate receptor protein from Escherichia coli

The binding of adenosine cyclic 3',5'-monophosphate (cAMP) and guanosine cyclic 3',5'-monophosphate (cGMP) to the adenosine cyclic 3',5'-monophosphate receptor protein (CRP) from Escherichia coli was investigated by equilibrium dialysis at pH 8.0 and 20 degrees C at different ionic strengths (0.05--0.60 M). Both cAMP and cGMP bind to CRP with a negative cooperativity that is progressively changed to positive as the ionic strength is increased. The binding data were analyzed with an interactive model for two identical sites and site/site interactions with the interaction free energy--RT ln alpha, and the intrinsic binding constant K and cooperativity parameter alpha were computed. Double-label experiments showed that cGMP is strictly competitive with cAMP, and its binding parameters K and alpha are not very different from that for cAMP. Since two binding sites exist for each of the cyclic nucleotides in dimeric CRP and no change in the quaternary structure of the protein is observed on binding the ligands, it is proposed that the cooperativity originates in ligand/ligand interactions. When bound to double-stranded deoxyribonucleic acid (dsDNA), CRP binds cAMP more efficiently, and the cooperativity is positive even in conditions of low ionic strength where it is negative for the free protein. By contrast, cGMP binding properties remained unperturbed in dsDNA-bound CRP. Neither the intrinsic binding constant K nor the cooperativity parameter alpha was found to be very sensitive to changes of pH between 6.0 and 8.0 at 0.2 M ionic strength and 20 degrees C. For these conditions, the intrinsic free energy and entropy of binding of cAMP are delta H degree = -1.7 kcal . mol-1 and delta S degree = 15.6 eu, respectively.

Stringent control and protein synthesis in bacteria

Most bacteria have evolved a number of regulatory mechanisms which allow them to maintain a balanced and rather constant cellular composition in response to nutritional variations. In particular, when the availability of any aminoacyl-tRNA species becomes limiting (namely through amino acid starvation or inactivation of an aminoacyl-tRNA synthetase), several biochemically distinct physiological processes are significantly modified. This coordinate adjustment of cellular activity is termed the "stringent response". Under such conditions of aminoacyl-tRNA limitation, protein synthesis still proceeds, but various quantitative as well as qualitative changes in polypeptide metabolism can be observed. In this review, after a brief recall of the main characteristics of the stringent response, several aspects concerning protein synthesis in deprived bacteria have been presented. First, the rates of residual protein formation, peptide chain growth and protein degradation, and the molecular weight distribution of proteins newly synthesized have been analyzed. Then, the data relative to the biosynthetic regulation of non-ribosomal and ribosomal proteins have been summarized and compared to the results obtained from in vitro experiments using transcription-translation coupled systems. Finally, the problem of translational fidelity during deprivation has been discussed in connection with the metabolic behavior of polysomal structures which are still maintained in cells. The stringent dependence of cellular activity on aminoacyl-tRNA supply is known to be abolished by single-site mutations which confer to bacteria a phenotype referred to as "relaxed". These mutant strains provide an useful analytical tool in the scope of understanding the stringency phenomenon. Therefore, their proteosynthetic activity under aminoacyl-tRNA deprivation has also been studied here, in comparison to that of normal wild-type strains.

Unusual location and function of the operator in the Escherichia coli galactose operon

The operator of the gal operon is located about 60 base pairs preceding the startpoints of the transcription of the two gal promoters. This location contrasts with the location of the operator in other phage or bacterial operons where the repressor binds more closely to the respective transcription initiation sites. Models explaining how the repressor-operator interactions may control the two gal promoters are presented.

Effect of guanosine 5'-diphosphate 3'-diphosphate and related nucleoside polyphosphates on induction of tryptophanase and beta-galactosidase in permeabilized cells of Escherichia coli

Exogenous addition of guanosine and adenosine 5'-(mono, di and tri) phosphate 3'-diphosphates (pppGpp, ppGpp, pGpp, pppApp, ppApp and pApp) stimulated the synthesis of tryptophanase and beta-galactosidase in permeabilized cells of Escherichia coli. From the results obtained with ppGpp and pppApp, this effect appeared to be at a transcriptional level and depended greatly on the growth condition; the largest effect was observed in cells under shiftdown or grown on poor enrgy source. ppGpp and pppApp, unlike cyclic AMP, did not act to overcome the inhibition of enzyme induction by glucose, but in combination with cyclic AMP caused a synergistic stimulation effect. In the shiftdown cells, ppGpp and pppApp gave 30% or more stimulation effect on tryptophanase induction while cyclic AMP did not stimulate induction. There was therefore a pronounced difference between cyclic AMP and ppGpp or pppApp in stimulatory function.

Interaction of super-repressible and dominant constitutive mutations for the synthesis of galactose pathway enzymes in Saccharomyces cerevisiae

Two dominant uninducible mutant alleles in the gal80 locus were identified. The GAL80s-1 and GAL80s-2 mutants showed novel phenotypes in response to the newly isolated GAL81-1 mutant allele, a dominant constitutive mutation linked to the gal4 locus; the GAL80s-1 GAL81-1 strain was inducible and the GAL80s-2 GAL81-1 strain was uninducible. Many galactose positive revertants from the GAL80s-2 GAL81-1 strain were isolated. It was proved that each revertant was due to a secondary mutation either in the gal80 or GAL81 locus, whereas revertants due to mutation at the supposed controlling site for the structural gene cluster of the galactose-pathway enzymes have not been isolated.

An adenosine 3',5'-monophosphate-requiring mutant of the luminous bacteria Beneckea harveyi

We have isolated a mutant of the luminous bacterium Beneckea harveyi, which requires exogenous adenosine 3',5'-monophosphate (cyclic AMP) to synthesize luciferase and emit light. The mutant was pleiotropic, lacking not only the ability to luminesce, but also the capacities to form flagella and the ability to utilize a variety of carbohydrates for growth. All these deficiencies could be corrected by added cyclic AMP. The cyclic AMP-induced de novo synthesis of luciferase was possible only after autoinduction had occurred. The induction time by cyclic AMP ranged between 6 and 10 min at 27 degrees C.

Regulation of galactose operon at the gal operator-promoter region in Escherichia coli K-12

The capR (lon) product controls expression of the gal operon independently of the galR repressor. Previously, mutations of the gal operon have been isolated that are semiconstitutive and alter response to the capR and/or capT product. Such mutants imply the existence of a distinct site in the operon that responds to capR (capT) control. This mutation could be either in a site near the operator-distal end of the galE gene, which signals rho factor termination of transcription in vitro or in a site in the operator-promoter region. Bacteriophage U3 was used to isolate galE mutations in HC2142 (a mutant exhibiting reduced response to capR control). P1 transduction was used to cross these mutants with a set of galE gene deletion. Analysis of the resulting Gal+ recombinants indicates that the regulatory site is in the operator-promoter region. Hence, it is unlikely that capR functions in control as an anti-rho factor at the operator-distal end of the galE gene, but more likely as previously suggested, at a second operator distinct from one responding to galR repressor control. Upon induction with D-fucose, a promoter mutant (UV211) isolated previously expressed 20 to 30% of the galactose enzymes that the wild type exhibited in the presence of the inducer D-fucose. The effects of various mutations in cya, capR, and galR on galactokinase synthesis in this mutant were determined. Galactokinase was derepressed by capR as well as galR, but the presence or absence of the cya gene product was unimportant.

Role of cyclic adenosine 3',5'-monophosphate in the in vivo expression of the galactose operon of Escherichia coli

Studies of levels of galactokinase in Escherichia coli with mutations affecting synthesis of, or response to, cyclic adenosine 3',5'-monophosphate show that this nucleotide does not play a major role in expression of the galactose operon, causing at most a twofold stimulation. The discrepancy between our in vivo results and the marked stimulation by cyclic adenosine 3',5'-monophosphate in in vitro systems indicates that current cell-free systems lack a factor which allows efficient expression of the galactose operon even in the absence of cyclic adenosine 3',5'-monophosphate or of the binding protein for this nucleotide.

Multiple regulator gene control of the galactose operon in Escherichia coli K-12

Previous studies showed that nonsense mutations in either of two genes (capR or capS) or an undefined mutation in a third gene (capT) led to pleiotropic effects: (i) increased capsular polysaccharide synthesis (mucoid phenotype); (ii) increased synthesis of enzymes specified by at least four spatially separated operons involved in synthesis of capsular polysaccharide including the product of the galE gene, UDP-galactose-4-epimerase (EC 5.1.3.2) in capR mutants. The present study demonstrated that the entire galactose (gal) operon (galE, galT, and galK) is derepressed by mutations in either the capR or the capT genes, but not by mutation in capS. Double mutants (capR9 capT) were no more derepressed than the capR9 mutant, indicating that capR9 and capT regulate the gal operon via a common pathway. Isogenic double mutants containing either galR(+), galR(-), galR(s), or galO(c) in combination with either capR(+) or capR9 were prepared and analyzed for enzymes of the gal operon. The results demonstrated that capR9 caused derepression as compared to capR(+) in all of the combinations. Strains with a galR(s) mutation are not induced, for the gal operon, by any galactose compound including d-fucose, and this was confirmed in the present study using d-fucose. Nevertheless, the derepression of galR(s) capR9 compared to galR(s) capR(+) was four- to sixfold. The same derepression was observed when galR(+)capR9 was compared to galR(+)capR(+). The data eliminate the explanation that internal induction of the gal operon by a galactose derivative was causing increased gal operon enzyme synthesis in capR or capT mutants. Furthermore, the same data suggest that the galR and capR genes are acting independently to derepress the gal operon. A modified model for the structure of the gal operon is proposed to explain these results. The new feature of the model is that two operator sites are suggested, one to combine with the galR repressor and one to combine with the capR repressor.