Interactions between paired homologous genes can lead to changes in gene expression. Such trans-regulatory effects exemplify transvection and are displayed by many genes in Drosophila, in which homologous chromosomes are paired somatically. Transvection involving the yellow cuticle pigmentation gene can occur by at least two mechanisms, one involving the trans-action of enhancers on a paired promoter and a second involving pairingmediated bypass of a chromatin insulator. A system was developed to evaluate whether the action of the yellow enhancers in trans could be reconstituted outside of the natural near telomeric location of the yellow gene. To this end, transgenic flies were generated that carried a yellow gene modified by the inclusion of strategically placed recognition sites for the Cre and FLP recombinases. Independent action of the recombinases produced a pair of derivative alleles, one enhancerless and the other promoterless, at each transgene location. Transvection between the derivatives was assessed by the degree of interallelic complementation. Complementation was observed at all eight sites tested. These studies demonstrate that yellow transvection can occur at multiple genomic locations and indicate that the Drosophila genome generally is permissive to enhancer action in trans. G ene expression is controlled by regulatory elements that modulate transcription in appropriate temporal and spatial patterns. In eukaryotes, these control elements often reside in large, complex regions, requiring action over long distances to elicit changes in transcription of a target promoter. In some cases, the control elements of paired homologous genes can interact in trans to alter gene expression. Such trans-regulatory effects illustrate processes known as transvection. Transvection was described first in Drosophila (1), which has homologous chromosomes that are paired somatically. Transvection and related processes have been reported in many organisms besides Drosophila (for example, refs. 2-7, and reviewed in refs. 8-11). These observations suggest that transvection may be generally possible for eukaryotic control regions and that these processes may be important for the normal regulation of some genes. For example, chromosomal pairing is proposed to play a role in gene silencing associated with imprinting and X chromosome inactivation (4, 12).In Drosophila, interactions between homologous chromosomes can have either a positive or negative effect on transcription. Examples of negative effects include repression of white gene expression by certain alleles of zeste, trans silencing conferred by the insertion of a block of heterochromatin near the brown gene, and pairing-dependent silencing, as exemplified by the effects of Polycomb response elements (reviewed in refs. 8-11, 13, and 14). Examples of positive effects include events at the Ultrabithorax, Abdominal B, decapentaplegic, yellow, and eyes absent genes (1,(15)(16)(17)(18)(19)(20)(21)(22)(23)(24).A useful system for studying the mechanisms involved in posi...
Homologous chromosomes are paired in somatic cells of Drosophila melanogaster. This pairing can lead to transvection, which is a process by which the proximity of homologous genes can lead to a change in gene expression. At the yellow gene, transvection is the basis for several examples of intragenic complementation involving the enhancers of one allele acting in trans on the promoter of a paired second allele. Using complementation as our assay, we explored the chromosomal requirements for pairing and transvection at yellow. Following a protocol established by Ed Lewis, we generated and characterized chromosomal rearrangements to define a region in cis to yellow that must remain intact for complementation to occur. Our data indicate that homolog pairing at yellow is efficient, as complementation was disrupted only in the presence of chromosomal rearrangements that break #650 kbp from yellow. We also found that three telomerically placed chromosomal duplications, containing $700 or more kbp of the yellow genomic region, are able to alter complementation at yellow, presumably through competitive pairing interactions. These results provide a formal demonstration of the pairing-dependent nature of yellow transvection and suggest that yellow pairing, as measured by transvection, reflects the extent of contiguous homology flanking the locus. C YTOLOGICAL studies of a wide variety of systems are revealing the strategies by which a large amount of DNA can be organized into an extraordinarily small volume yet still be accurately expressed, replicated, and passed through cell divisions. In the somatic cells of Drosophila and other dipteran insects, a striking feature of nuclear organization is the extensive amount of pairing that occurs between homologous chromosomes. This pairing was first noted by Nettie Stevens (Stevens 1908) and Charles Metz (Metz 1916) through the examination of mitotic nuclei. Somatic pairing of homologous chromosomes has now been observed in Drosophila interphase nuclei using DNA as well as RNA in situ hybridization techniques (reviewed by McKee
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