The question stacks two filters on top of each other: the modification must be epigenetic (it must help control gene expression without changing the DNA sequence), and it must specifically be posttranslational, meaning it happens to a protein after that protein has already been made.
- Arginine methylation: histone tails carry arginine residues that get methylated by protein arginine methyltransferases once the histone protein already exists. This changes chromatin packing and gene activity, and because it modifies a finished protein, it counts as posttranslational. Correct.
- Lysine acetylation: histone acetyltransferases add acetyl groups to lysine residues on histone tails after the histones are made, which weakens the grip of histones on DNA and generally switches genes on. This is also a change to an already-made protein, so it too is posttranslational. Correct.
- Cytosine methylation: this adds a methyl group straight onto a cytosine base sitting in the DNA strand, most often at CpG sites. It is a real and major epigenetic mark (it usually silences genes), but the molecule being modified is DNA, not a protein, so it cannot be called a posttranslational modification.
- Cytosine deamination: this removes an amino group from cytosine, turning it into uracil (or turning 5-methylcytosine into thymine). It is a DNA-level chemical change too, and rather than being a controlled regulatory mark, it is usually a mutagenic event that DNA repair machinery has to fix. It is neither a protein modification nor a deliberate epigenetic control mechanism here.
The dividing line in this question is protein versus DNA: arginine methylation and lysine acetylation both act on histone proteins after those proteins are built, while cytosine methylation and deamination both act directly on the DNA base itself.
Let's summarize:
- Posttranslational modification means a change made to a protein after translation.
- Arginine methylation and lysine acetylation modify histone proteins and reshape chromatin, both posttranslational and epigenetic.
- Cytosine methylation and deamination modify the DNA base itself, so neither is a posttranslational modification, even though methylation is still epigenetic.
The correct choices are (A) arginine methylation and (B) lysine acetylation.