Developmental changes of sepiapterin synthase activity associated with a variegated purple gene in Drosophila melanogaster

Mechanism of suppression inDrosophila melanogasterVIII. Comparison ofsu(s) alleles for ability to suppress the mutants purple, vermilion, and speck

The suppressor of sable [su(s)2] restores the function of vermilion (v), purple (pr) and speck (sp) as well as sable (s) inDrosophila melanogaster. In this report various alleles ofsu(s) are compared for their relative effectiveness on three target mutations,v,prandsp. Three criteria for suppression ofprandvwere employed: visible phenotype, eye pigment levels (drosopterins and xanthommatin) and enzyme levels (sepiapterin synthase and tryptophan oxygenase). Forsponly the visible phenotype was examined. By all three criteriaprwas found to be more easily suppressed thanv;vandspwere comparable. By use ofprwith various alleles ofsu(s) either homozygously or in heterozygous combination withsu(s)+, the extent of suppression ofprcan be best demonstrated by observing the levels of sepiapterin synthase; normal levels of drosopterins were found in females when sepiapterin synthase was only 20% of normal. On the other hand, the extent of suppression ofvis best demonstrated by the amount of xanthommatin eye pigment, because even the suppressed vermilion fly has < 10% of wild-type activity of tryptophan oxygenase when 1-day-old flies are examined; in older flies this enzyme can be as high as 50% of wild type. From these results we also demonstrated thatsu(s)2, and other alleles, are not recessive but, in heterozygous combination withsu(s)+, cause marked suppression ofprand slight, but reproducible, suppression ofv. The purple mutation, therefore, is particularly useful for studying the mechanism of suppression as well as for obtaining new mutant alleles ofsu(s).

Chromosome rearrangements induce both variegated and reduced, uniform expression of heterochromatic genes in a development-specific manner

In Drosophila melanogaster, chromosome rearrangements that juxtapose euchromatin and heterochromatin can result in position effect variegation (PEV), the variable expression of heterochromatic and euchromatic genes in the vicinity of the novel breakpoint. We examined PEV of the heterochromatic light (lt) and concertina (cta) genes in order to investigate potential tissue or developmental differences in chromosome structure that might be informative for comparing the mechanisms of PEV of heterochromatic and euchromatic genes. We employed tissue pigmentation and in situ hybridization to RNA to assess expression of lt in individual cells of multiple tissues during development. Variegation of lt was induced in the adult eye, larval salivary glands and larval Malpighian tubules for each of three different chromosome rearrangements. The relative severity of the effect in these tissues was not tissue-specific but rather was characteristic of each rearrangement. Surprisingly, larval imaginal discs did not exhibit variegated lt expression. Instead, a uniform reduction of the lt transcript was observed, which correlated in magnitude with the degree of variegation. The same results were obtained for cta expression. These two distinct effects of rearrangements on heterochromatic gene expression correlated with the developmental stage of the tissue. These results have implications for models of heterochromatin formation and the nuclear organization of chromosomes during development and differentiation.

Molecular hypotheses for position-effect variegation: anti-sense transcription and promoter occlusion

There is currently no comprehensive molecular hypothesis to account for position-effect variegation, the mosaic expression of a gene lying near a breakpoint of a chromosomal rearrangement. Here it is proposed that position-effect variegation arises from either anti-sense transcription or from promoter occlusion (transcription readthrough), the former mechanism operating for breakpoints on the 3' side of the affected gene and the latter for breakpoints on the 5' side. Anti-sense transcription will occur in rearrangements that place the anti-sense strand of genes next to a promoter. This anti-sense RNA hybridizes to, and thereby inactivates, sense mRNA transcripts (as anti-sense RNA is known to do). Promoter occlusion may occur in rearrangements that place the affected gene near an open upstream promoter. This promoter drives readthrough transcription that inhibits most normal transcripts. Occasional normal transcripts lead to phenotypic variegation. These hypotheses have three strengths: (i) they predict the major observed features of position-effect variegation including variegated phenotype, stable inheritance, the involvement of rearrangements, only some rearrangements causing variegation, the occurrence of both dominant and recessive variegation, the spreading effect of variegation to several loci, and the conditions required for expression of variegation; (ii) they can plausibly account for features of position-effect variegation that they do not specifically predict; (iii) they lead to a series of novel and testable predictions, including the presence of altered transcripts in rearrangements including position-effect variegation, the location of breakpoints required to cause variegation, and a correlation between the extent of the spreading effect and the length of the novel transcript. These mechanisms can account for several other cases of variegation in addition to classic position-effect variegation. Actual or putative examples of phenotypic variegation due to these mechanisms are known.

Mechanism of suppression in Drosophila: evidence for a macromolecule produced by the su(s)+ locus that inhibits sepiapterin synthase

Genetic suppression was studied in the purple mutant of Drosophila melanogaster and in suppressed purple by measurement of sepiapterin synthase activity. The addition of ammonium sulfate fractions from adult Drosophila that contain one, two, three or four doses of su(s)+ to the suppressed purple sepiapterin synthase resulted in an inhibition that increased progressively as the dosage of su(s)+ increased; the wild-type sepiapterin synthase was not inhibited. This inhibition is caused by a heat-labile macromolecule. We suggest that the mechanism of suppression is neither transcriptional nor translational but is the result of decreased amounts, or altered properties, of the normal product of the su(s)+ locus when su(s)+ is replaced by su(s)2 or su(s)e6.

Temperature mediated variation of DNA secondary structure in (A.T) clusters; evidence by use of the oligopeptide netropsin as a structural probe

The titration viscometric investigation of the multi-mode interaction of netropsin (Nt) with (A.T) clusters of NaDNA12 and NH4DNA10 has been extended to different temperatures. The position of two boundaries on the r-scale (r= [Nt]bound/[DNA-P]) with increasing temperature steadily (rI/II) or more abruptly (rO/I) shifts to lower values. For the most (A.T) rich Nt-binding sites of modes (O), (I) and (II) this observation suggests the existence of an equilibrium between different DNA secondary structures with a different translation per base pair. The mode specific changes delta L1Nt of DNA contour length as induced by one Nt molecule proved to be almost independent of temperature. Concomitant stiffening effects increase with decreasing temperature, contrary to initial expectation. Conformational variability of (A.T) clusters may represent an essential feature in specific or selective DNA-protein interaction.

Presence of queuine in Drosophila melanogaster: correlation of free pool with queuosine content of tRNA and effect of mutations in pteridine metabolism

Queuine, a modified form of 7-deazaguanine present in certain transfer RNAs, is shown to occur in Drosophila melanogaster adults in a free form and its concentration varies as a function of age, nutrition and genotype. In several, but not all mutant strains, the concentrations of queuine and the Q(+) (queuine-containing) form of tRNATyr are correlated. The bioassay employs L-M cells which respond to the presence of queuine by an increase in their Q(+)tRNAAsp that is accompanied by a decrease in the Q(-)tRNAAsp isoacceptors. The increase in Q(+)tRNATyr in Drosophila that occurs on a yeast diet is accompanied by an increase in queuine. Similarly the increase of Q(+)tRNAs with age also is accompanied by an increase in free queuine. In two mutants, brown and sepia, these correlations were either diminished or failed to occur. Indeed, the extract of both mutants inhibited the response of the L-M cells to authentic queuine. When the pteridines that occur at abnormally high levels in sepia were used at 1 x 10(-6)M, the inhibition of the L-M cell assay occurred in the order biopterin greater than pterin greater than sepiapterin. These pteridines were also inhibitory for the purified guanine:tRNA transglycosylase from rabbit but the relative effectiveness then was pterin greater than biopterin greater than sepiapterin. Pterin was competitive with guanine in the enzyme reaction with Ki = 0.9 x 10(-7)M. Also when an extract of sepia was chromatographed on Sephadex G-50, the pteridine-containing fractions only were inhibitory toward the L-M cell assay or the enzyme assay. These results indicate that free queuine occurs in Drosophila but also that certain pteridines may interfere with the incorporation of queuine into RNA.

Developmental regulation of phenylalanine hydroxylase activity in Drosophila melanogaster

Herein we demonstrate that Drosophila larvae possess a synthetic activity capable of converting phenylalanine to tyrosine. This system is readily extractable and displays many characteristics of phenylalanine hydroxylase systems described in other organisms, the most notable being that a tetrahydropteridine is required for full expression of activity. The level of phenylalanine hydroxylase activity present in the organism varies with the stage of development: from an undetected level of activity at the first larval instar, there is a rapid increase in phenylalanine hydroxylase activity which reaches a peak at the time of puparium formation, after which there is a rapid decrease again to an undetected level.