Investigations on the organization of genetic loci in Drosophila melanogaster: lethal mutations affecting 6-phosphogluconate dehydrogenase and their suppression

A transaldolase : An enzyme implicated in crab steroidogenesis

In arthropods, development is controlled by cholesterol-derived steroid hormones: the ecdysteroids. In vertebrates and insects, steroidogenesis is positively regulated and this is mediated by cAMP. In crustaceans, ecdysteroid biosynthesis by steroidogenic organs (Y-organs) is negatively regulated by a neuropeptide, the Molt Inhibiting Hormone (MIH). This neuropeptide-induced inhibition occurs via cyclic nucleotides and depends on protein synthesis. In the present work, we provide evidence that a major 36.2-kDa cytosolic protein (P36; pl: 6.8) from crab Y-organs is positively correlated with steroidogenic activity. On the basis of its amino acid sequence, P36 could be related to transaldolase, an enzyme of the pentose phosphate pathway which generates NADPH. In Y-organs, the enzymatic activity ofCarcinus transaldolase increases with steroidogenic activity, and MIH treatment decreases both synthesis and activity of transaldolase. Various transaldolases have been characterized in very distantly related groups, namely bacteria, yeasts, and humans. These enzymes are highly conserved and present strong structural homologies, interestingly the crab transaldolase is closest to that enzyme characterized in human cells.

Isolation and characterization of the glucose-6-phosphate dehydrogenase encoding gene (gsdA) from Aspergillus niger

Genomic and cDNA clones encoding glucose-6-phosphate dehydrogenase (G6PD) were isolated from the fungus Aspergillus niger. Sequence analysis of the glucose-6-phosphate dehydrogenase gene (gsdA) revealed an open reading frame of 1530 bp, encoding a protein of 58,951 kDa. The gsdA gene is interrupted by nine introns the most proximal of which is exceptionally large (348 bp). The region upstream of the ATG contains several C+T-rich stretches. The two major and one minor transcription start points are all located within these regions. In the upstream region several direct and inverted repeats, but no clear TATA or CCAAT boxes can be found. A. niger strains overproducing G6PD were constructed by cotransformation of gsdA subclones. Overexpression of G6PD was shown to be deleterious for the fungus, especially when cotransformants were grown on media containing ammonia. Attempts to construct a gsdA null mutant by gene disruption were unsuccessful.

A 6-phosphogluconate dehydrogenase gene from Trypanosoma brucei

A Trypanosoma brucei gene encoding 6-phosphogluconate dehydrogenase (6-PGDH) (EC 1.1.1.44) was identified and cloned by functional complementation of Escherichia coli gnd mutants with genomic trypanosome DNA. The T. brucei gnd gene is present as a single copy. In Northern blot experiments a probe derived from the gene hybridises to 2 transcripts (2.9 kb and 3.1 kb) which are found in both bloodstream and procyclic form organisms; the larger transcript is more abundant in bloodstream form organisms. The derived amino acid sequence of the protein is 479 amino acids in length, with a molecular weight of 52,000. It is homologous to 6-PGDHs from bacterial and mammalian sources, but diverges significantly from these other enzymes.

Comparison of in vitro and in vivo activities associated with the G6PD allozyme polymorphism in Drosophila melanogaster

Earlier studies of the A and B allozymes at the G6pd locus show a differential ability of the genotypes to suppress the loss of viability associated with a low activity 6-phosphogluconate dehydrogenase mutation, 6Pgdlo1. This observation indicates a relatively lower activity for the A allozyme genotype, but it is not known if this level of suppression required a large difference in in vivo activity. To clarify this difference an analysis of the biochemical properties of the purified allozymes was carried out, as well as an analysis of the activity level associated with an original low activity P element-derived allele which had partially reverted and lost its suppression ability. G6PD activity and protein level were studied in 47 X chromosome lines from North America. The A genotype averages a 9% lower Vmax. From analysis of the correlation between G6PD activity and protein level it remains unclear whether the allozyme Vmax difference results from dissimilarity in protein level or kcat. At 25 degrees and physiological pH, comparative studies of the steady-state kinetics show the two purified allozyme variants differ significantly in their KM values for glucose-6-phosphate and NADP, and the K1 for NADPH. In aggregate these parameters predict the A genotype possesses a 20% lower in vitro catalytic efficiency. A partial revertant of a P element-derived low activity B variant, was shown to lose the ability to suppress 6Pgdlo1 low viability after acquiring only 60% of normal B activity. This last comparison shows the A genotype activity must be reduced in vivo by at least 40%.

Molecular analysis of the structural gene for yeast transaldolase

We have cloned the structural gene for yeast transaldolase. Transformants carrying the TAL1 gene on a multicopy plasmid over-produced transaldolase. A deletion mutant which was constructed using the cloned gene did not show any detectable transaldolase activity in vitro. Furthermore, both transaldolase isoenzymes which were detected in wild-type crude extracts by immunoblotting were missing in the deletion mutants. Thus, TAL1 is the only transaldolase structural gene in yeast. TAL1 is not an essential gene. Deletion of the transaldolase gene did not affect growth on complete media with different carbon sources or on synthetic media. However, the transaldolase-deficient strains accumulated sedoheptulose 7-phosphate, an intermediate of the pentose-phosphate pathway. Mutants lacking both transaldolase and phosphoglucose isomerase grew more slowly than the single mutants. They accumulated more sedoheptulose 7-phosphate on medium containing fructose than on glucose medium. This shows that fructose 6-phosphate and glyceraldehyde 3-phosphate, metabolites of glycolysis, can enter the nonoxidative part of the pentose-phosphate pathway.

A genetic and molecular analysis of P-induced mutations at the glucose-6-phosphate dehydrogenase locus in Drosophila melanogaster

We examined P factor induced mutations of the Zw gene of Drosophila melanogaster in order to learn more about the site specificity of such mutations. Approximately 70,000 chromosomes were screened using a powerful positive selection scheme. As only two mutants were discovered, Zw is a "cold spot" for transposable element insertion. One mutation involved a complex P element associated chromosomal rearrangement which was used to define the orientation of the gene with respect to the centromere of the X chromosome. The second mutation was either a simple, non-dysgenically induced point mutation or a very unstable insertion.

6-Phosphogluconate dehydrogenase activity variants in Musca domestica L.: A further allele at the Pgd locus as proved by densitometric assay

A new electrophoretic variant of 6-phosphogluconate dehydrogenase (6PGD) has been detected in flies of a laboratory Musca domestica strain. This variant is to be added to the two already described, PGD-A and PGD-B, identified by a fast-weak and a slow-thick electrophoretic band, respectively. The new variant, PGD-C, has the same mobility as PGD-A but provides a more intensely stained band; therefore it can be described as a fast-thick phenotype. The staining intensity of PGD-C is slightly lower than that of PGD-B. Genetic and densitometric tests have shown that the different levels of enzymatic activity of the two fast variants A and C are inherited as alternative genetic units, and they have been interpreted as one aspect of the phenotypic expression of two Pgd alleles, namely, PgdA and PgdC. These alleles determine both the rates of electrophoretic mobility (fast in both cases) and the levels of activity (low for A, strong for C; shown by weak or thick stained electrophoretic bands). Similarly, the two distinctive features of PGD-B, namely, slow mobility and high activity level, are always jointly inherited and appear as two pleiotropic aspects of the phenotype coded for by the PgdB allele. The PgdB/PgdC heterozygous flies provide a slightly asymmetrical three-banded zymogram, while the PgdA/PgdC combination leads to a single-banded pattern, showing the same mobility as the parents and an intermediate staining intensity. The quantitative analysis of enzyme activity of 6PGD zymograms, performed through densitometric methods, has led to the recognition of three different activity levels coded for by Pgd alleles, one of which, namely, PgdC, would not have been detected using electrophoretic methods alone.

Pentose phosphate pathway mutants of yeast

A glucose-negative mutant of Saccharomyces cerevisiae lacking 6-phosphogluconate dehydrogenase, the second enzyme of the pentose phosphate pathway, has been obtained by inositol starvation. Suppression of this mutant for growth on glucose takes place by the loss of glucose 6-phosphate dehydrogenase. A lesion in the latter enzyme alone leaves growth practically unaffected. The mutations define the respective structural genes.

Polymorphism at the G6pd and 6Pgd loci in Drosophila melanogaster. IV. Genetic factors modifying enzyme activity

Different homozygous lines of similar genotype with respect to G6pd and 6Pgd were shown to have different enzyme activities for G6PD and 6PGD. Crosses between high and low lines suggested that there were modifying genes present on the autosomes, while others were probably located on the X chromosome. Allelic variation within each electrophoretic class of G6pd and 6Pgd might, however, also have contributed to this variation. An experiment on adaptation to sodium octanoate demonstrated that in adapted flies selection for lower enzyme activity had occurred, which provided further evidence for the existence of genetic differences in activity. Furthermore, a strong positive correlation between the activities of G6PD and 6PGD was found for each genotype. Since no correlation was found between MDH and the two enzymes G6PD and 6PGD, it could be concluded that this correlation was probably rather specific for G6PD and 6PGD. Interaction between genotypes with respect to activity was also found. It was shown that the variation at 6Pgd influenced the activity of G6PD within a genotype. The data are discussed in relation to fitness differences presented in foregoing articles.

6-Phosphogluconate dehydrogenase from Drosophila melanogaster. I. Purification and properties of the A isozyme

6-Phosphogluconate dehydrogenase is evident at all developmental stages of Drosophila melanogaster. The activity level is highest in early third instar larvae and declines to a lower, but relatively constant, level at all later stages of development. The enzyme is localized in the cytosolic portion of the cell. The A-isozymic form of 6-phosphogluconate dehydrogenase was purified to homogeneity and has a molecular weight of 105,000. The enzyme is a dimer consisting of subunits with molecular weights of 55,000 and 53,000. For the oxidative decarboxylation of 6-phosphogluconate the Km for substrate is 81 muM while that for NADP+ is 22.3 muM. The optimum pH for activity is 7.8 while the optimum temperature is 37 C.

Relationship of the oxidative pentose shunt pathway to lipid synthesis in Drosophila melanogaster

The tissue activities of the oxidative pentose shunt enzymes, glucose-6-phosphate dehydrogenase (E.C. 1.1.1.49) and 6-phosphogluconate dehydrogenase (E.C. 1.1.1.44), in the larvae of Drosophila melanogaster are not dependent on the amount of flux through the oxidative pentose shunt pathway. An oxidative pentose shunt deficiency effects about a 40% reduction in the NADPH concentration in early third instar larvae, resulting in a six-fold difference in the NADPH/NADP+ ratio between wild-type and pentose-shunt-deficient larvae. The capacity of pentose-shunt-deficient larvae to synthesize triglyceride in response to a high concentration of dietary sucrose is only 73% of the wild-type level. Environmental temperature influences on the fatty acid composition of larvae are not altered by an oxidative pentose shunt deficiency.

6-phosphogluconate dehydrogenase in the housefly, Musca domestica L.:evidence for inheritable 6PGD polymorphism

Two electrophoretic variants of the 6-phosphogluconate dehydrogenase (6 PGD) enzyme have been found in the WHO/IN/Musca domestica/l housefly laboratory strain. The patterns shown by Cellogel zone electrophoresis can be fully explained by the hypothesis of two codominant autosomal alleles. On this hypothesis, a specific Pgd locus has been postulated and the symbols PgdA and PgdB have been assigned to the two alleles causing the PGD-A and PGD-B phenotypes. The bands corresponding to the homozygous phenotypes PGD-A and PGD-B have different electrophoretic mobility and staining intensity; they can be described, respectively, as "fast-weak" and "slow-thick." The heterozygous phenotype PGD-AB gives a three-banded pattern, indicative of a dimeric structure for this enzyme; this pattern is asymmetrical. Heterozygous flies have been found both among wild-type strains of recent colonization and among old established laboratory colonies. Most strains are PgdB monomorphic; up to now only three strains have been PgdA monomorphic, all of them being multimarker strains. The Pgd locus has been traced to the housefly linkage group III.

Polymorphism at the G6PD and 6PGD loci in Drosophila melanogaster. II. Evidence for interaction in fitness

The influence of sodium octanoate on the polymorphism at the loci G6PD and 6PGD inDrosophila melanogasterwas investigated by studying its effect on egg hatchability, larval-to-adult survival and adult survival. The results demonstrate the existence of differences in fitness between the different genotypes of the two loci both for larval-to-adult survival and for adult survival. Furthermore, changes in enzyme activity of both enzymes, brought about by the sodium octanoate treatment, were observed. This makes it highly probable that the observed differences in fitness can be ascribed to the loci themselves and not to linked fitness genes. Finally, this study demonstrates the existence of strong interaction between the two loci with respect to fitness. Epistasis was demonstrated both in the larval-to-adult survival experiment and in the adult survival experiment.