Final answer:
Histone acetylation most effectively activates transcription by causing the nucleosomes to pack loosely, thus making the DNA accessible for transcription machinery.
Step-by-step explanation:
The pattern of chromatin modification that would activate transcription to the greatest extent is acetylation of histones. Histone acetylation results in the loose packing of nucleosomes, which facilitates the opening of the underlying DNA for transcription. This process is catalyzed by histone acetyltransferases (HAT enzymes). When histones are acetylated, they lose their positive charge, leading to a less tight interaction with the negatively charged DNA, thereby making the DNA more accessible for transcription machinery. In contrast, other modifications such as methylation can have variable effects depending on the context, and phosphorylation and ubiquitination do not typically result in such an open chromatin state favorable for transcription.
In summary, by reducing the positive charge of histone proteins, acetylation diminishes the binding affinity between histones and DNA, creating an open chromatin conformation that promotes active transcription. This mechanism plays a crucial role in the regulation of gene expression, particularly in response to various signals that necessitate changes in gene activity.