Final answer:
Archaean histones lack acetylation, which makes gene expression possible.
Step-by-step explanation:
Archaean histones, if they exist, lack a specific modification known as acetylation. Acetylation of histones is an important process that regulates gene expression by making the DNA more accessible to transcription factors and RNA polymerase. In the absence of histone acetylation, the DNA remains tightly coiled and transcriptionally inactive. This means that the genes are not expressed and cannot be used to produce proteins.