Transvection (genetics)

Transvection is a phenomenon in genetics where the expression of a gene on one chromosome is affected by the presence of a sequence (usually an allele) on the homologous chromosome, even though that homologous sequence doesn't directly encode the protein produced. This interaction occurs only when the two chromosomes are physically close, typically in somatic pairing.

The classic understanding of gene regulation involves cis-acting elements (sequences that control gene expression on the same chromosome) and trans-acting factors (diffusible molecules that can regulate genes on any chromosome). Transvection provides a mechanism that blends these concepts, highlighting a regulatory effect achieved through the physical interaction between homologous chromosomes.

The mechanism of transvection is not fully understood but is thought to involve:

• Chromosome Pairing: For transvection to occur, the homologous chromosomes need to be in close proximity. This is often seen in organisms with strong somatic pairing, like Drosophila.
• Enhancer Sharing/Competition: A common hypothesis is that enhancers located on one chromosome can influence the promoter of a gene on the homologous chromosome, either by sharing the enhancer or by competing for access to it.
• Modification of Chromatin Structure: Interactions between the paired chromosomes might alter chromatin structure (e.g., through histone modifications) in a way that affects gene expression.
• Interchromosomal Communication: The physical pairing might facilitate direct communication or signal transduction between the two chromosomes.

Transvection can lead to both activation and repression of gene expression. For example, if one chromosome carries a defective enhancer, pairing with a chromosome containing a functional enhancer might rescue expression of the target gene. Conversely, a repressive element on one chromosome could silence the homologous allele.

Transvection is particularly relevant in understanding the complexities of gene regulation, especially in the context of allele-specific expression and the effects of chromosomal rearrangements. It highlights the importance of considering not only the sequence of a gene but also its chromosomal context and interactions with its homologous partner.