Final answer:
The answer is D) It promotes gene expression by acetylating histones. Histone acetyltransferase (HAT) enzymes add acetyl groups to histones, leading to a looser, more open chromatin structure that facilitates transcription.
Step-by-step explanation:
The correct answer to the question 'Which of the following is true about histone acetyltransferase (HAT)?' is D) It promotes gene expression by acetylating histones. Histone acetyltransferase (HAT) is an enzyme that catalyzes the transfer of acetyl groups to the lysine residues of histone proteins. This acetylation of histones results in a more open chromatin structure, which in turn facilitates transcription because transcription factors can more easily access the DNA. Histone acetylation effectively reduces the positive charge on histones, thereby weakening the interaction between the positively charged histones and the negatively charged DNA. This process leads to the loose packing of nucleosomes, which is conducive to gene expression.