Visualization of trans-homolog enhancer-promoter interactions at the Abd-B Hox locus in the Drosophila embryo.

The Hox gene Abdominal-B (Abd-B) controls the morphogenesis of posterior abdominal segments in Drosophila. Expression is regulated by a series of 3' enhancers that are themselves transcribed. RNA FISH was used to visualize nascent transcripts associated with coding and noncoding regions of Abd-B in developing embryos. Confocal imaging suggests that distal enhancers often loop to the Abd-B promoter region. Surprisingly, enhancers located on one chromosome frequently associate with the Abd-B transcription unit located on the other homolog. These trans-homolog interactions can be interpreted as the direct visualization of a genetic phenomenon known as transvection, whereby certain mutations in Abd-B can be rescued in trans by the other copy of the gene. A 10 kb sequence in the 3' flanking region mediates tight pairing of Abd-B alleles, thereby facilitating trans looping of distal enhancers. Such trans-homolog interactions might be a common mechanism of gene regulation in higher metazoans.