Final answer:
Lysine is the most frequent target for acetylation in histone tails, which plays a crucial role in gene expression by modifying the chromatin structure.
Step-by-step explanation:
The amino acid in histone tails that is the most frequent target for acetylation is lysine. This process is a key component in regulating gene expression, as it affects the chromatin structure and thus the accessibility of DNA to the transcription machinery. The positively charged side chains of basic amino acids such as lysine and arginine allow for the binding to the negatively charged DNA, but it is the acetylation of these lysine residues that reduces the positive charge and allows for a more relaxed DNA-histone interaction. This relaxation is crucial for the transcription of housekeeping genes and other DNA sequences as it influences the opening of chromatin structure, making certain regions more accessible for transcription.