Transvection

What is transvection?

There is something magical about transvection. It conveys the power and elegance of classical Drosophila genetics, and has attracted and perplexed many distinguished scientists since its discovery by Ed Lewis in 1954. Transvection refers to a special class of genetic complementation of mutant alleles on homologous chromosomes. The prevailing view is that regulatory DNAs located on one homolog can regulate the transcription unit on the other homolog in trans. In some cases, enhancers appear to trans-activate genes located on the other homolog, but transvection can also lead to trans-repression of gene expression across homologous chromosomes.

What is the evidence for transvection?

Lewis described genetic complementation between mutant alleles of the Hox gene Ultrabithorax (Ubx) in Drosophila. Both bx^34e and Ubx¹ mutants exhibit abnormalities in the patterning of the thorax, including partial transformations of halteres into wings: however, bx^34e/Ubx¹ trans-heterozygotes display less severe transformations than predicted from the phenotypes produced by the individual alleles. Only after the molecular cloning and mapping of Ubx mutant alleles did the mechanistic basis for this trans-complementation process come into focus.

Ubx is regulated by multiple enhancers, including the intronic enhancers abx and BRE (Figure 1A). The bx^34e mutant allele is caused by the insertion of a gypsy transposable element between these intronic enhancers and the Ubx promoter. The gypsy element contains twelve binding sites for the Zn-finger protein Su(Hw), which functions as an insulator to block enhancer–promoter interactions. Ubx¹ is a protein transposable element within the 5′ untranslated region (UTR) of the first exon. The favored interpretation of this partial genetic complementation is that the intronic abx and BRE enhancers in the Ubx¹ allele are able to activate transcription from the bx^34e allele in trans to produce functional Ubx transcripts (Figure 1A). Chromosome rearrangements disrupt this transvection effect, suggesting that trans enhancer–promoter interactions require physical association of the two alleles. Indeed, visualization of the endogenous Ubx locus by DNA fluorescence in situ hybridization (FISH) revealed substantial allele pairing during Drosophila development.

Figure 1. Transvection in Drosophila.

(A) Transvection in bx^34e/Ubx¹ trans-heterozygotes. Intronic abx and BRE enhancers on the Ubx¹al-lele activate transcription from the bx^34e allele in trans. (B) Loss of cis-linked promoter augments transvection. Ubx^Mx6 mutation consists of a 3.4 kb deletion including the Ubx promoter. (C) 5′ wing and body enhancers on the promoter-less allele activate transcription from the enhancer-less allele in trans. W, wing enhancer; B, body enhancer; Br, bristle enhancer; T, tarsal claw enhancer. (D) Insulator DNAs facilitate transvection at yellow.

How common is transvection?

Since the discovery of transvection at the Ubx locus, there have been numerous reports of trans-homolog interactions in Drosophila. For example, a mutant allele of yellow lacking 5′ enhancers can be complemented by a null mutation lacking the promoter region (Figure 1C). Synthetic promoter-less/enhancer-less yellow mutant alleles were integrated into different locations of the genome and found to recapitulate transvection. These results suggest that somatic pairing of homologous chromosomes may be pervasive in the Drosophila genome. Supporting this view, transvection has been observed at many different endogenous loci, including Abd-B, Scr, dpp, w, ap, eya and vg. DNA FISH assays have revealed frequent pairing events at many chromosomal locations. In mammalian systems, trans-homolog enhancer–promoter interactions have not been so clearly demonstrated, but homolog pairing is thought to take place during X-chromosome inactivation, B-cell maturation and ES cell differentiation.

What is promoter competition?

Transvection is often attenuated by the presence of a linked promoter in cis. The first evidence for such promoter competition came from studies of the yellow locus. Similar results have been reported at other loci, including Ubx. The promoter-less mutant Ubx^Mx6 produces more efficient suppression of the haltere-to-wing transformation than the promoter-containing Ubx¹ mutant when placed in trans to the bx^34e allele (Figure 1B). This observation suggests that the intronic enhancers located on the Ubx^Mx6 allele are now able to effectively trans-activate the functional Ubx transcription unit located on the bx^34e allele. Promoter competition has also been documented for the Hox gene Scr, suggesting that cis-preference may be a general property of transvection. This phenomenon might be related to prototypic examples of promoter competition seen at the β-globin locus, whereby the closest promoter displays preferential activation by a shared enhancer in cis.

What is the mechanism of transvection?

Insulator DNAs appear to be at the heart of trans-homolog enhancer–promoter interactions. As discussed above, enhancers preferentially activate neighboring promoters in cis. Trans-interactions are augmented by the loss of cis-promoters (Figure 1B and C). They are also augmented by insulator DNAs (Figure 1D). The analysis of the transvection mediating region of the Hox gene Abd-B suggests that insulators foster pairing of homologous chromosomes. Stable pairing might increase the opportunities for trans enhancer–promoter interactions. Importantly, insulators facilitate transvection even in the presence of a cis-linked promoter.

And future prospects?

Insulator DNAs are thought to be important agents of genome organization in vertebrates, leading to the formation of chromosomal loop domains, or TADs (topologically associating domains). It has been suggested that insulators also produce loop domains in Drosophila. For example, the depletion of CAP-H2, a key component of Condensin II, augments transvection at Ubx and yellow, possibly by antagonizing Cohesin. A major future goal is to determine the nature of putative chromosomal loop domains at transvecting loci. Do they serve as transcription ‘hubs’ for regulating homologous genes in cis and trans? With the advent of genome editing methods and quantitative live-imaging technologies, it won’t be long before these and other mysteries of transvection are revealed.