Final answer:
Ubiquitylation of histone H2B at K123 plays a role in regulating gene expression by altering nucleosome dynamics and affecting transcriptional machinery accessibility.
Step-by-step explanation:
Ubiquitylation of histone H2B at K123 is a post-translational modification process that affects the chromatin architecture and has implications for gene transcription. Ubiquitylation at this site is associated with the regulation of transcriptional initiation and elongation, as such modifications can either signal for the recruitment of factors necessary for transcription or mark the histone for removal by the proteasome. This specific modification can lead to changes in nucleosome dynamics, affecting the accessibility of transcriptional machinery to DNA, thereby influencing gene expression.