Phosphoglucose Isomerase Activity Research Articles

Characterization of the Biosynthetic Pathway of Cellulose from Glucose and Fructose inAcetobacter xylinum

For characterization of the biosynthetic pathway of cellulose in a cellulose-producing Acetobacter xylinum strain BPR2001, the activities of several enzymes were measured. The activity of phosphoglucose isomerase catalyzing the conversion of fructose-6-phosphate into glucose-6-phosphate was greatly increased by fructose in the medium. The UDP-glucose pyrophosphorylase activity catalyzing the synthesis of UDP-glucose was very high in strain BPR2001, consistent with the idea that this is the key enzyme in cellulose biosynthesis. Strain BPR2001 was found to have a fructose-specific phosphoenolpyruvate-dependent phosphotransferase system (PTS).

Enhanced accumulation of toxic compound in yeast cells having high glycolytic activity: a case study on the safety of genetically engineered yeast

SummaryThe cellular level of methylglyoxal (MG), a highly toxic 2‐oxoaldehyde, in Saccharomyces cerevisiae cells transformed with genes for some of the glycolytic enzymes was determined as an index of the safety of genetically engineered yeast and the level was compared with that in non‐transformed control cells. the phosphoglucose isomerase (PGI), phosphofructokinase (PFK) and triosephosphate isomerase (TPI) activities significantly increased in the transformants and were approximately five‐, three‐ and sevenfold higher, respectively, than those in the control. When these transformed cells were used for alcohol fermentation from glucose, they accumulated MG in cells at a level sufficient to induce mutagenicity. These results illustrate that careful thought should be given to the potential metabolic products and their safety when a genetically engineered yeast is applied to food‐related fermentation processes.

Differential control of the functional cell surface expression and content of hexose transporter GLUT-1 by glucose and glucose metabolism in murine fibroblasts

The present paper evaluates the contributions of glucose and its metabolites to the post-translational regulation of hexose transport and GLUT-1 content in murine fibroblasts. The effects of 3-O-methylglucose, a nearly non-metabolizable glucose analogue, on 2-deoxyglucose-uptake, cell-surface expression and content of GLUT-1, glucose 6-phosphate levels, and phosphoglucose isomerase (PGI) and hexokinase activities of murine fibroblasts were compared with those of glucose and fructose. Glucose (EC50 approximately 6 mM) or 3-O-methylglucose (EC50 approximately 12 mM), which are substrates of GLUT-1, but not fructose, which is not transported by GLUT-1, are able to prevent the glucose-deprivation-induced increases in both hexose transport and cell-surface expression of GLUT-1. In contrast, glucose (EC50 approximately 6 mM), but not 3-O-methylglucose or fructose, prevents the glucose-deprivation-induced accumulation of total GLUT-1 polypeptides. Glucose (> or = 5 mM), but not fructose or 3-O-methylglucose, leads to significant glucose 6-phosphate accumulation. Although 3-O-methylglucose is weakly phosphorylated by fibroblasts, accumulation of phosphorylated product does not correlate with hexose-transport regulation. The activities of hexokinase and PGI are not altered by glucose, fructose or 3-O-methylglucose. We suggest that, in murine fibroblasts: (i) hexose transport and GLUT-1 content are differentially regulated; (ii) substrates of GLUT-1 and/or their immediate metabolites regulate the cell-surface expression of functional GLUT-1; and (iii) glucose metabolism is required for the regulation of GLUT-1 content.

Evidence for Circadian Regulation of Starch and Sucrose Synthesis in Sugar Beet Leaves

Starch accumulation and sucrose synthesis and export were measured in leaves of sugar beet (Beta vulgaris L.) during a period of prolonged irradiance in which illumination was extended beyond the usual 14-hour day period. During much of the 14-hour day period, approximately 50% of the newly fixed carbon was distributed to sucrose, about 40% to starch, and less than 10% to hexose. Beginning about 2 hours before the end of the usual light period, the portion of newly fixed carbon allocated to sucrose gradually increased, and correspondingly less carbon went to starch. By the time the transition ended, about 4 hours into the extension of the light period, nearly 90% of newly fixed carbon was incorporated into sucrose and little or none into starch. Most of the additional sucrose was exported. Gradual cessation of starch accumulation was not the result of a futile cycle of simultaneous starch synthesis and degradation. Neither was it the result of a decrease in the extractable activity of adenosine diphosphoglucose pyrophosphorylase or phosphoglucose isomerase, enzymes important in starch synthesis. Nor was there a notable change in control metabolites considered to be important in regulating starch synthesis. Starch accumulation appeared to decrease markedly because of an endogenous circadian shift in carbon allocation, which occurred in preparation for the usual night period and which diverted carbon from the chloroplast to the cytosol and sucrose synthesis.

Cloning, characterization, and nucleotide sequence analysis of a Zymomonas mobilis phosphoglucose isomerase gene that is subject to carbon source-dependent regulation

The Zymomonas mobilis gene encoding phosphoglucose isomerase (pgi) was cloned by genetic complementation of an Escherichia coli pgi mutant. An enzyme assay and sodium dodecyl sulfate-polyacrylamide gel electrophoresis confirmed the presence of excess amounts of phosphoglucose isomerase in E. coli clones carrying the Z. mobilis pgi gene. The pgi gene is present in only one copy on the Z. mobilis genome. Nucleotide sequence analysis of the pgi region revealed an open reading frame of 1,524 bp preceded by a strong Shine-Dalgarno sequence. The pgi gene encodes a 507-amino-acid protein with a predicted molecular weight of 55,398. Z. mobilis phosphoglucose isomerase is between 38 and 43% identical to the enzyme from other species. Northern (RNA) blot analysis showed that the pgi transcript is 1.8 kb in length. The level of the pgi transcript was found to be influenced by the phase of growth and by the carbon and energy sources. Transcript levels increased with respect to total RNA during logarithmic growth and were threefold higher when grown on fructose than on glucose. These changes in transcript levels paralleled phosphoglucose isomerase activities in the cultures. Differential mRNA stability was not a factor, since the half-life of the pgi transcript was 6.3 min in glucose-grown cells and 6.0 min in fructose-grown cells. Thus, an increase in the rate of transcription appears to be at least partially responsible for the increased levels of phosphoglucose isomerase observed for Z. mobilis grown on fructose.

Subcellular distribution profile of glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, transketolase and phosphoglucose isomerase in rat brain

Abstract The activities of glucose 6-phosphate dehydrogenase (G6-PDH), 6-phosphogluconate dehydrogenase (6-PGDH), transketolase (TK), phosphoglucose isomerase (PGI), pyruvate dehydrogenase (PDH) and choline acetyltransferase (ChAT) were determined in freshly prepared brain homogenates and subcellular fractions derived therefrom. About 32–48% of the total activity of G6-PDH, 6-PGDH, TK and PGI in the brain homogenate was obtained in the supernatant, 34–39% in crude mitochondria, 9–13% in microsomes and 3–11% in nuclei; ChAT gave a distinct distribution profile of 22, 59, 17 and 7% respectively. PDH was found almost exclusively in the crude mitochondrial fraction. Synaptosomes isolated on subfractionation of the crude mitochondrial fraction comprised 17–29% of G6-PDH, 6-PGDH, TK and PGI, and 49% of ChAT; myelin and free mitochondria each contained 3–8% of their total activity. PDH was all distributed equally between synaptosomes (39%) and free mitochondria (46%). The significance of the presence in brain synaptosomes of substantial amounts of G6-PDH, 6-PGDH and TK has been discussed.

Decreased-activity mutants of phosphoglucose isomerase in the cytosol and chloroplast of Clarkia xantiana. Impact on mass-action ratios and fluxes to sucrose and starch, and estimation of Flux Control Coefficients and Elasticity Coefficients.

1. Subcellular-compartment-specific decreased-activity mutants of phosphoglucose isomerase in Clarkia xantiana were used to analyse the control of sucrose and starch synthesis during photosynthesis. Mutants were available in which the plastid phosphoglucose isomerase complement is decreased to 75% or 50% of the wild-type level, and the cytosol complement to 64%, 36% or 18% of the wild-type level. 2. The effects on the [product]/[substrate] ratio and on fluxes to sucrose or starch and the rate of photosynthesis were studied with the use of saturating or limiting light intensity to impose a high or low flux through these pathways. 3. Removal of a small fraction of either phosphoglucose isomerase leads to a significant shift of the [product]/[substrate] ratio away, from equilibrium. We conclude that there is no 'excess' of enzyme over that needed to maintain its reactants reasonably close to equilibrium. 4. Decreased phosphoglucose isomerase activity can also alter the fluxes to starch or sucrose. However, the effect on flux does not correlate with the extent of disequilibrium, and also varies depending on the subcellular compartment and on the conditions. 5. The results were used to estimate Flux Control Coefficients for the chloroplast and cytosolic phosphoglucose isomerases. The chloroplast isoenzyme exerts control on the rate of starch synthesis and on photosynthesis in saturating light intensity and CO2, but not at low light intensity. The cytosolic enzyme only exerts significant control when its complement is decreased 3-5-fold, and differs from the plastid isoenzyme in exerting more control in low light intensity. It has a positive Control Coefficient for sucrose synthesis, and a negative Control Coefficient for starch synthesis. 6. The Elasticity Coefficients in vivo of the cytosolic phosphoglucose isomerase were estimated to lie between 5 and 8 in the wild-type. They decrease in mutants with a lowered complement of cytosolic phosphoglucose isomerase. 7. The implications of these results for regulation and for evolution are discussed.

Genetics of allozyme variation in Cuphea lanceolata Ait.

Genetic markers are needed for mating systems and breeding experiments in Cuphea lanceolata Ait.; however, none have been described in this species. Allozyme variation was analyzed among 14 F2 populations assayed for aconitase (ACO), diaphorase (DIA), esterase (EST), fluorescent esterase transaminase (FEST), glutamine oxaloacetate transaminase (GOT), menadione reductase (MNR), phosphoglucomutase (PGM), phosphoglucose isomerase (PGI), and shikimate dehydrogenase (SKDH) enzyme activity. At least 23 loci were resolved in these enzyme systems: 6 monomorphic loci, 5 poorly resolved loci, and 12 clearly resolved polymorphic loci. Observed segregation ratios were generally not significantly different (P > 0.05) from expected segregation ratios; however, segregation distortion was observed at Skdh-1 and Mnr-1 (Dia-1) in some F2 populations. Skdh-1 and Pgm-2 and Est-1, Est-2, Fest-1, and Mnr-1 comprise putative linkage groups. Allozyme variation was observed between and within accessions. The expected average heterozygosity was 16.3%. There were one to eight polymorphic loci among the F2 populations analyzed. There were an average of 2.05 alleles per locus. Several useful codominant markers were identified and a partial allozyme linkage map was constructed. Additional work is needed to revise and complete the map.Key words: Cuphea, isozymes, goodness of fit test statistics, lauric acid, capric acid.

Reduced-activity mutants of phosphoglucose isomerase in the cytosol and chloroplast of Clarkia xantiana

(i) We have studied the influence of reduced phosphoglucose-isomerase (PGI) activity on photosynthetic carbon metabolism in mutants of Clarkia xantiana Gray (Onagraceae). The mutants had reduced plastid (75% or 50% of wildtype) or reduced cytosolic (64%, 36% or 18% of wildtype) PGI activity. (ii) Reduced plastid PGI had no significant effect on metabolism in low light. In high light, starch synthesis decreased by 50%. There was no corresponding increase of sucrose synthesis. Instead glycerate-3-phosphate, ribulose-1,5-bisphosphate, reduction of QA (the acceptor for photosystem II) and energy-dependent chlorophyll-fluorescence quenching increased, and O2 evolution was inhibited by 25%. (iii) Decreased cytosolic PGI led to lower rates of sucrose synthesis, increased fructose-2,6-bisphosphate, glycerate-3-phosphate and ribulose-1,5-bisphosphate, and a stimulation of starch synthesis, but without a significant inhibition of O2 evolution. Partitioning was most affected in low light, while the metabolite levels changed more at saturating irradiances. (iv) These results provide decisive evidence that fructose-2,6-bisphosphate can mediate a feedback inhibition of sucrose synthesis in response to accumulating hexose phosphates. They also provide evidence that the ensuing stimulation of starch synthesis is due to activation of ADP-glucose pyrophosphorylase by a rising glycerate-3-phosphate: inorganic phosphate ratio, and that this can occur without any loss of photosynthetic rate. However the effectiveness of these mechanisms varies, depending on the conditions. (v) These results are analysed using the approach of Kacser and Burns (1973, Trends Biochem. Sci. 7, 1149-1161) to provide estimates for the elasticities and flux-control coefficient of the cytosolic fructose-1,6-bisphosphatase, and to estimate the gain in the fructose-2,6-bisphosphate regulator cycle during feedback inhibition of sucrose synthesis.

The cdc30 mutation in Saccharomyces cerevisiae affects phosphoglucose isomerase, the cell cycle and sporulation.

Spontaneous revertants of the cdc30 mutation in Saccharomyces cerevisiae simultaneously regained the ability to grow and divide at 36.5 degrees C on glucose-containing media along with a more thermostable phosphoglucose isomerase (PGI). An independently isolated allele of cdc30 gave a similar phenotype to that previously described including temperature-sensitivity of PGI. Isoelectric focussing allowed the separation of two isoenzymes of PGI. These results all support the idea that two genes--PGI1 and CDC30--are responsible for PGI activity in yeast. Diploid strains homozygous for the cdc30 mutation sporulated poorly in potassium acetate irrespective of whether the cells had previously been cultured at a temperature that was permissive or restrictive for cell cycle progression. This was not surprising because a strain defective in PGI would not be expected to be able to complete the gluconeogenic events of sporulation.

Phosphofructokinase is responsible for the fructose 2,6-bisphosphate inhibition of hexokinase in tissue extracts

Mammalian and yeast hexokinases were reported to be reversibly inhibited by fructose 2,6-bisphosphate in the presence of cytosolic proteins ( H. Niemeyer, C. Cerpa, and E. Rabajille (1987) Arch. Biochem. Biophys. 257, 17–26 ). Reinvestigation of this finding using a radioassay with [ 14C]glucose as substrate showed no effect of fructose 2,6-bisphosphate on hexokinase activity of rat liver cytosols. Detailed reexamination of the spectrophotometric assay resulted in the observation that the fructose 2,6-bisphosphate-dependent inhibition was a function of the cytosolic phosphoglucose isomerase and phosphofructokinase activities compared to the amount of glucose-6-phosphate dehydrogenase used as auxiliary enzyme. The diminution or loss of the fructose 2,6-bisphosphate-dependent inhibition produced in aged cytosols was restored by addition of crystalline muscle phosphofructokinase, as well as by decreasing the amount of glucose-6-phosphate dehydrogenase in the assay. When phosphoglucose isomerase, phosphofructokinase, and hexokinase activities were separated by DEAE-chromatography of liver cytosol, no fructose 2,6-bisphosphate-dependent inhibition of hexokinase was found in any single fraction of the chromatogram. However, combination of fractions containing both phosphoglucose isomerase and phosphofructokinase displayed the fructose 2,6-bisphosphate-dependent inhibition on either endogenous hexokinase or added yeast hexokinase. From these results we conclude that the activation of phosphofructokinase elicited by fructose 2,6-bisphosphate is responsible for the hexokinase inhibition observed in the coupled spectrophotometric assay.

Study of a fructose-negative mutant ofRhizobium meliloti

Rhizobium meliloti grows on fructose as sole carbon source. Following nitrosoguanidine mutagenesis, a mutant of R. meliloti M5N1 was isolated as unable to grow on fructose. Enzyme assays with cell-free extracts showed it to lack significative phosphoglucose isomerase activity. Other enzymes were present at low levels. Both fructose and fructose 6-phosphate were accumulated within this mutant. The in vitro inhibition of fructokinase by fructose 6-phosphate was show. Symbiotic properties remained unaffected in the mutant strain.

Study of a fructose-negative mutant of Rhizobium meliloti

Rhizobium meliloti grows on fructose as sole carbon source. Following nitrosoguanidine mutagenesis, a mutant of R. meliloti M5N1 was isolated as unable to grow on fructose. Enzyme assays with cell-free extracts showed it to lack significative phosphoglucose isomerase activity. Other enzymes were present at low levels. Both fructose and fructose 6-phosphate were accumulated within this mutant. The in vitro inhibition of fructokinase by fructose 6-phosphate was show. Symbiotic properties remained unaffected in the mutant strain.

Comparative electrophoretic study of phosphoglucose isomerase, phosphoglucose mutase and tetrazolium oxidase in some hydatid cysts of Echinococcus granulosus collected from different mammal hosts

Hydatid cyst scolexes of lung and liver from sheep and water buffalo were electrophoreticaily examined for phosphoglucose isomerase (PGI), phosphoglucose mutase (PGM) and tetrazolium oxidase (Te) activity. One each of phosphoglucose isomerase, phosphoglucose mutase and tetrazolium oxidase activities were found in all of the hydatid cysts studied. PGI and PM in ail hydatid cysts studied appeared te be controlled by two codominant alleles. PGI and PGM proved to be a good taxonomic criterion to differentiate the hydatid cysts of the water buffalo from those from sheep. On the other hand, TO failed to differentiate between the sheep and water buffalo hydatid cysts. The hydatid cysts that parasitise the sheep's organs can be considered as arising from a different strain from the cysts that parasitise the water buffalo organs, while lung and liver hydatid cysts of the sheep appeared to be a similar strain.

Modulation of Sucrose Content by Fructose 2,6-bisphosphate during Photosynthesis in Rice Leaves Growing at Different Light Intensities

Reddy, A. R. and Das, V. S. R. 1987. Modulation of sucrose content by fructose 2,6-f>isphosphate during photosynthesis in rice leaves growing at different light intensities.—J. exp. Bot. 38: 828-833. The relationship between the rate of C02 fixation and sucrose concentration in the leaves of rice (Oryza sativa L.) grown at different light intensities was investigated. Maximum sucrose content coincided with maximum rates of C02 fixation, achieved at a photon flux density of 1600 /rmol m~2 s1. The levels of sucrose and fructose 2,6-feisphosphate were also compared in the leaves under different light intensities. Fructose 2,6-bisphosphate accumulated during growth at low light. The activity of fructose-6-phosphate 2-kinase was high in the leaves grown at low light while that of fructose-2,6-hisphosphatase was low. The activities of phosphoglucose isomerase and phospho glucomutase were slightly increased by growth at low light. The activities of UDP glucose pyrophosphorylase were adversely affected in vitro with increased concentrations of fructose 2,6-bisphosphate while those of sucrose phosphate synthase were moderately affected. Phospho glucose isomerase and phosphoglucomutase were activated by fructose 2,6-bisphosphate (8 0 mmol m~3) by 12-15%. The results suggested that low light intensities during growth result in an accumulation of fructose 2,6-Wsphosphate which modulates the key enzymes of sucrose biosynthesis thus regulating carbon flow under conditions of limited photosynthesis.

An improved purification of glycosomes from the procyclic trypomastigotes of Trypanosoma brucei

The glycosomes of in vitro grown procyclic trypomastigote forms of Trypanosoma brucei were purified by three different procedures and the results compared by electron microscopy, enzyme assays and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Centrifugation on a self-forming Percoll gradient followed by a sucrose gradient centrifugation resulted in the least enriched glycosomal preparation. Centrifugation on a pre-formed Nycodenz gradient gave an improved preparation but the most homogeneous preparation of intact glycosomes was obtained after centrifugation on two successive sucrose gradients. Glycosomes purified by both the Nycodenz and double sucrose gradient procedures appeared larger than in situ glycosomes presumably due to an osmotic effect resulting from disruption of the granular matrix of the organelles. Nevertheless, there appears to be no loss of cisternal contents due to the swelling of the organelles. The glycosomes of the bloodstream from trypomastigotes purified by the same procedures show, however, no sign of swelling. A comparison of glycosomes purified from procyclic trypomastigotes and bloodstream form trypomastigotes prepared by the same double sucrose procedure demonstrated that in the glycosome of procyclic trypomastigotes: (1) activities of hexokinase, phosphoglucose isomerase, phosphofructose kinase, aldolase and phosphoglycerate kinase and diminished by 80–100%; (2) activities of glyceraldehyde-3-phosphate dehydrogenase, triose phosphate isomerase and glycerol-3-phosphate dehydrogenase remain unchanged or are only slightly reduced; (3) there is an appearance of four major new proteins, among which could be phosphoenol pyruvate carboxykinase and malate dehydrogenase. These observations are in basic agreement with those by Hart et al. (Mol. Biochem. Parasitol. 12, 25–35, 1984).

Metabolic distinctiveness of ureaplasmas.

Enzymes of the Embden-Meyerhof-Parnas pathway and hexose monophosphate shunt were examined in cytoplasmic extracts of three serovars of Ureaplasma urealyticum. We found no glucose-6-phosphate or 6-phosphogluconate dehydrogenase, hexokinase, phosphoglucose isomerase, aldolase, or lactic dehydrogenase activities. We failed to find cytochrome pigments in extracts and found no significant production of 14CO2 from [U-14C]glucose, nor did we find oxygen-dependent reduced nicotinamide adenine dinucleotide oxidase activity. Lactic acid was found only at trace levels in spent culture fluids. Ureaplasmas are apparently nonfermentative and are unlike all other mollicutes in that they have no detectable oxygen-dependent reduced nicotinamide adenine dinucleotide oxidase activity.

Two genetically distinct pathways for transcriptional regulation of anaerobic gene expression in Salmonella typhimurium.

Expression of the tripeptide permease gene tppB is anaerobically induced. This induction is independent of the fnr (oxrA) gene product, which is known to be required for the anaerobic induction of several respiratory enzymes. We isolated, characterized, and mapped mutations in two genes, oxrC and tppR, which prevent the anaerobic induction of tppB expression. Mutations in oxrC were highly pleiotropic, preventing the anaerobic expression of the formate dehydrogenase component of formate hydrogen lyase (fhl), a tripeptidase (pepT), and two of the three known hydrogenase isoenzymes (hydrogenases 1 and 3). On the other hand, expression of nitrate reductase, fumarate reductase, and a number of other fnr (oxrA)-dependent enzymes was not affected by mutations in oxrC. Thus, there appeared to be at least two distinct classes of anaerobically induced genes, those which required fnr for their expression and those which required oxrC. It seems that fnr-dependent enzymes perform primarily respiratory functions, whereas oxrC-dependent enzymes served fermentative or biosynthetic roles. We found the primary defect of oxrC mutants to be a deficiency in phosphoglucose isomerase activity, implying that a product of glycolysis functions as an anaerobic regulatory signal. Mutations in tppR were specific for tppB and did not affect expression of other oxrC-dependent genes. However, tppR did exhibit phenotypes other than the regulation of tppB. Both oxrC and tppR mutants were hypersensitive to the toxic NAD analog 6-aminonicotinic acid. This suggests that oxrC and tppR may play a role in the regulation of NAD biosynthesis or, alternatively, that NAD or a related nucleotide serves as the anaerobic signal for oxrC-dependent enzymes.

Deletion of the phosphoglucose isomerase structural gene makes growth and sporulation glucose dependent in Saccharomyces cerevisiae.

The structural gene PGI1 coding for phosphoglucose isomerase was replaced by the LEU2 gene in the genome of Saccharomyces cerevisiae. Plasmids carrying the LEU2 gene between genomic regions flanking the PGI1 gene were constructed and used to transform a PGI1/pgi1 diploid strain. Stable transformants lacking the PGI1 allele were isolated. Southern analysis of their meiotic products showed that haploid strains with a deletion of 1.6 kb within the 2.2 kb PGI1 coding region were viable. Thus, the PGI1 gene is not essential in yeasts. However, unlike pgi1 mutants with residual phosphoglucose isomerase activity, no growth was detected in the pgi1 delta haploid strains when fructose was supplied as sole carbon source. The wild-type growth rate could be restored by adding 0.1% glucose to the medium. Furthermore, pgi1 mutants with residual enzymatic activity grew very slowly on fructose-supplemented media containing up to 2% glucose. Strains carrying the deletion allele, however, failed to grow at glucose concentrations higher than 0.5%. Also the pgi1 delta strains did not grow in glucose as sole carbon source. On the other hand pgi1 delta/pgi1 delta diploid strains did not sporulate on the usual acetate medium. This defect could be alleviated by the addition of 0.05% glucose to the sporulation medium. Under these conditions the pgi1 delta mutants sporulated with an efficiency of 25% compared with the wild type. These results suggest that the phosphoglucose isomerase reaction is the only step catalysing the interconversion of glucose-6-P and fructose-6-P, glucose-6-P is essential in yeasts, and the oxidation of glucose-6-P through the glucose-6-P dehydrogenase reaction is not sufficient to support growth in yeasts.

Reduced Enzyme Activity and Starch Level in an Induced Mutant of Chloroplast Phosphoglucose Isomerase

Ethyl methane sulfonate treatment was used to induce a mutation in the nuclear gene encoding the chloroplast isozyme of phosphoglucose isomerase in Clarkia xantiana. The mutation, which proved allelic to wild type activity, was backcrossed to wild type for five generations so that the two could be compared in a near isogenic background. An immunological analysis showed that the mutant, when homozygous, reduced the activity of the isozyme by about 50%. In contrast to wild type, the mutant showed little change in leaf starch level over a diurnal period or following a 72-hour continuous light treatment. By the end of the diurnal light period, the mutant accumulated only about 60% as much starch as wild type. However, mutant leaves had an increased sucrose level presumably because photosynthate was directly exported from the chloroplasts. The mutant also exhibited reduced leaf weight. These changes in metabolism and growth suggest that the wild type level of plastid phosphoglucose isomerase activity is necessary to achieve wild type carbohydrate status.