1. Epigenetics and Histone Modifications: Histone modifying enzymes establish and maintain chromatin modifications states that play fundamental roles in governing transcription, DNA damage response, epigenetic gene regulation, and other genomic processes. Our laboratory has characterized the molecular determinants of the substrate specificities of several human histone modifying enzymes, including histone lysine methyltransferases and demethylases, employing structural and biochemical approaches. More recently, we have initiated a new research program aimed at understanding the functions of histone modifying enzymes expressed by bacterial pathogens. Theses enzymes are secreted by pathogens into host cells wherein they enter the nucleus and hijack host gene expression by altering histone modifications, promoting bacterial replication. Structural and functional characterization of these enzymes from bacterial pathogens such as Legionella pneumophila, the primary causative agent of Legionnaires’ Disease, is yielding novel insights into their unique histone substrate specificities and roles in molecular pathogenesis. Together, these studies are elucidating new aspects of host-pathogen interactions and represent an emerging area of research at the interface of chromatin biology and microbiology.

2. Mechanisms of AdoMet-dependent Methyltransferases: S-adenosylmethionine (AdoMet)-dependent methyltransferases methylate a diverse array of biological substrates, including proteins, nucleic acids, carbohydrates, lipids, cofactors, and hormones. Structural surveys of methyltransferases bound to AdoMet have revealed that the AdoMet methyl sulfonium cation engages in several types of unconventional non-bonded interactions, including methyl carbon-oxygen (CH---O) hydrogen bonding, sulfur chalcogen bonding, and methyl carbon tetrel bonding. The discovery of these unconventional bonds between the AdoMet methyl sulfonium cation and residues and ligands in methyltransferase active sites implicates these interactions in AdoMet binding and the SN2 reaction catalyzed by these enzymes. Using a model lysine methyltransferase and non-reactive lysine analogs, we are investigating the functional importance of these interactions in substrate recognition and catalysis using an interdisciplinary approach combining biochemistry, structural biology, spectroscopy, and computational chemistry. Collectively, these studies are elucidating the mechanisms by which CH---O hydrogen bonding, chalcogen bonding, and tetrel bonding promote AdoMet binding and catalysis in methyltransferases.