Final answer:
Histone-lysine N-methyltransferases like EZH2 play a role in recruiting other enzymes and proteins to histones, spreading covalent modifications along DNA and promoting a heterochromatin state which is associated with suppressed gene expression.
Step-by-step explanation:
The question involves the role of certain enzymes in the spread of covalent modifications to histones along DNA, which is crucial for the regulation of the heterochromatin state. Different enzymes are responsible for chemical modifications that affect the chromatin structure, directly influencing gene expression. Histone modifications, like acetylation and methylation, are typically catalyzed by specific enzymes such as histone acetyltransferases (HAT enzymes) and histone-lysine N-methyltransferases. These modifications can either open up the chromatin structure to promote gene transcription or condense it to suppress gene expression. In the context of spreading heterochromatin, certain enzymes, which can include histone-lysine N-methyltransferases like EZH2, recruit other proteins and enzymes that further modify the histones, promoting a condensed and transcriptionally silent chromatin conformation.