Significance Polycomb Repressor Complex 2 (PRC2) is an epigenetic effector enzyme with key roles in gene silencing and establishment of facultative heterochromatin. Mutations of this histone lysine methyltransferase and its substrate H3K27 are linked to multiple human pathologies, and PRC2 has been identified as a therapeutic target in cancer treatment. The H3K27 methylation activity of PRC2 is sensitive to the chromatin environment, including the methylation state of H3K36 on the same histone tail. The mechanistic basis of this negative cross-talk is poorly understood. The present work sheds light on this question by identifying a regulatory feature on the catalytic subunit of PRC2, EZH2, which acts as a specific sensor for H3K36 methylation state. The work reveals an unusual mode of enzyme modulation by a substrate molecule.