The scalloped {sd) gene of Drosophila melanogaster was initially characterized by mutants affecting structures on the wing of the adult fly. The sequence of a cDNA clone of the gene reveals a predicted protein sequence homologous to that of a human transcriptional enhancer factor, TEF-1 (68% identity). The homology includes a sequence motif, the TEA domain, that was shown previously to be a DNA-binding domain of TEF-1. An sd enhancer trap strain expresses the reporter gene in a subset of neuroblasts in the central nervous system and in the peripheral sense organs of the embryo. The reporter gene is later expressed in specific regions of the imaginal discs, including regions of the wing disc destined to become structures defective in viable sd mutants. Later still, expression in the adult brain is restricted to subsets of cells, some in regions involved in the processing of gustatory information. These observations indicate that the sd gene encodes a transcription factor that functions in the regulation of cell-specific gene expression during Drosophila development, particularly in the differentiation of the nervous system.
Several studies have suggested that P elements have rapidly spread through natural populations of Drosophila melanogaster within the last four decades. This observation, together with the observation that P elements are absent in the other species of the melanogaster subgroup, has lead to the suggestion that P elements may have entered the D. melanogaster genome by horizontal transmission from some more distantly related species. In an effort to identify the potential donor in the horizontal transfer event, we have undertaken an extensive survey of the genus Drosophila using Southern blot analysis. The results showed that P-homologous sequences are essentially confined to the subgenus Sophophora. The strongest P hybridization occurs in species from the closely related willistoni group. A wild-derived strain of D. willistoni was subsequently selected for a more comprehensive molecular examination. As part of the analysis, a complete P element was cloned and sequenced from this line. Its nucleotide sequence was found to be identical to the D. melanogaster canonical P, with the exception of a single base substitution at position 32. When the cloned element was injected into D. melanogaster embryos, it was able to both promote transposition of a coinjected marked transposon and induce singed-weak mutability, thus demonstrating its ability to function as an autonomous element. The results of this study suggest that D. willistoni may have served as the donor species in the horizontal transfer of P elements to D. melanogaster.
The usefulness in structure/function studies of molybdenum-containing hydroxylases in work with rosy mutant strains of Drosophila melanogaster has been investigated. At least 23 such strains are available, each corresponding to a single known amino acid change in the xanthine dehydrogenase sequence. Sequence comparisons permit identification, with some certainty, of regions associated with the iron-sulphur centres and the pterin molybdenum cofactor of the enzyme. Procedures have been developed and rigorously tested for the assay in gel-filtered extracts of the flies, of different catalytic activities of xanthine dehydrogenase by the use of various oxidizing and reducing substrates. These methods have been applied to 11 different rosy mutant strains that map to different regions of the sequence. All the mutations studied cause characteristic activity changes in the enzyme. In general these are consistent with the accepted assignment of the cofactors to the different domains and with the known reactivities of the molybdenum, flavin and iron-sulphur centres. Most results are interpretable in terms of the mutation affecting electron transfer to or from one redox centre only. The activity data provide evidence that FAD and the NAD+/NADH binding sites are retained in mutants mapping to the flavin domain. Therefore, despite some indications from sequence comparisons, it is concluded that the structure of this domain of xanthine dehydrogenase cannot be directly related to that of other flavoproteins for which structural data are available. The data also indicate that the artificial electron acceptor phenazine methosulphate acts at the iron-sulphur centres and suggest that these centres may not be essential for electron transfer between molybdenum and flavin. The work emphasizes the importance of combined genetic and biochemical study of rosy mutant xanthine dehydrogenase variants in probing the structure and function of enzymes of this class.
SUMMARYThis report examines several issues bearing upon intragenic recombination in higher eukaryotes. The fine structure data accumulated in our analysis of the genetic organization of the rosy locus in Drosophila melanogaster. Firstly, we confirm that a conversion event has a markedly less than 50% probability of resulting in flanking marker exchange, a finding consistent with more recent analyses of the available Saccharomyces data (e.g. Fogel et al. 1978). As reported earlier, co-conversion of recombinationally separable sites within the rosy locus occurs (McCarron, Gelbart & Chovnick, 1974). In this report, we demonstrate that the frequency of co-conversion is inversely proportional to the distance between co-converting sites. As in fungi, real conversion frequency differences are observed among rosy mutant alleles, and the data suggest that there may be a relationship between allele conversion frequency and map position. Unlike Neurospora and Saccharomyces, only one flanking marker exchange class is recovered from any given mutant heteroallele recombination experiment. In this respect, the Drosophila system resembles Aspergillus. As in Neurospora and Saccharomyces, rosy locus intragenic recombinants associated with flanking marker exchange exhibit interference with crossing over in adjacent regions, while no interference is seen among recombinants exhibiting parental flanking markers. Finally, experimental results are discussed which demonstrate the occurrence of postmeiotic segregation in Drosophila. These analogies between Drosophila and fungi provide further evidence in support of the notion that eukaryotes share common molecular mechanism(s) of meiotic recombination.
Prior studies of recombination which monitor exchange events in exceedingly short intervals (i.e., separable sites within a cistron) reveal that the basic event in recombination involves a non-reciprocal transfer of information, termed conversion. As a logical consequence of the model suggested by the work in Drosophila, the present investigation examined recombination between rosy mutant alleles (ry:3-52.0) in Drosophila melanogaster in a paracentric inversion (In(3R)P18) heterozygote, which placed the rosy region approximately at the center of the inverted region. Comparison of the results of this study with experiments carried out in standard chromosome homozygotes reveals a dramatic suppression of classical crossovers between the rosy mutant alleles in the inversion heterozygote. However, conversions continue to occur for all rosy mutant alleles in all heterozygous combinations in the inversion heterozygote. Moreover, the order of magnitude of conversion frequencies seen in the inversion heterozygote does not change from that seen in the standard chromosome homozygote study. The significance of these observations with reference to the role of rearrangements as barriers of information transfer is discussed. Particular attention is directed to the elaborate inversion polymorphisms seen in natural populations, and to notions concerning their role in the evolution of adaptive gene complexes.
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