Question

The position effect variegation (PEV) phenotype described in this chapter can be used to identify new genes that regulate heterochromatin formation. For instance, strains of Drosophila melanogaster with the White variegation phenotype have been subjected to mutagenesis to screen for dominant mutations (in other genes) that either enhance or suppress PEV, meaning the mutations result in either lower or higher red pigment production, respectively. Which of the following mutations is expected to be an enhancer of variegation?

A. A mutation that results in the loss of function of the fly's HP1 (heterochromatin protein 1) gene.
B. A loss-of-function mutation in a gene encoding a histone deacetylase that deacetylates lysine 9 on histone H3.
C. A gain-of-function mutation in a gene encoding a histone methyl transferase that trimethylates lysine 9 on histone H3, resulting in a hyperactive form of the enzyme.
D. A gain-of-function mutation in a gene encoding a histone acetyl transferase that normally acetylates lysine 9 on histone H3, resulting in higher expression of the protein.

Answer 1

The expected enhancer of variegation is a gain-of-function mutation in a gene encoding a histone methyltransferase that trimethylates lysine 9 on histone H3.

Step-by-step explanation:

The expected enhancer of variegation in this scenario would be A gain-of-function mutation in a gene encoding a histone methyl transferase that trimethylates lysine 9 on histone H3, resulting in a hyperactive form of the enzyme.

Position effect variegation (PEV) is regulated by the modulation of heterochromatin formation, and histone modifications play a key role in this process. Trimethylation of lysine 9 on histone H3 (H3K9me3) by histone methyltransferases promotes heterochromatin formation and gene silencing.

A gain-of-function mutation in a gene encoding a histone methyltransferase that results in hyperactivity of the enzyme would lead to increased trimethylation of H3K9, which in turn would enhance the formation of heterochromatin and result in higher variegation.