List some ways that histones can be covalently modified?

Acetylation is the post-translational modifications of the core histones which include the Lys acetylation of side chains. Acetylation is catalyzed by the enzyme histone acetyl-transferases, which help in neutralization oftheir positive charge, which transfers the acetyl group from acetyl-coenzyme A to thelysine ε-NH⁺ group.

The post-transcriptional modification involves the methylation i.e., the addition of methyl group to specific Lys and Arg side chains with the help of enzyme histone methyltransferases.

The addition of phosphate group to specific Ser and Thr side chains, and the ubiquitination of specific Lys side chains modify the histone proteins.

These modifications neutralize the positive charge of the histone for nucleosome packaging.