Role of histone tails in gene expression and cellular functions

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Significant effort from my research program in my laboratory has been aimed at studying how chromatin affects the process of gene regulation. More recently, my laboratory has been interested in understanding how H3 and H4 histone tails can regulate gene expression and other cellular processes. Remarkably, the non-structured N-terminal histone tails are extremely well conserved. This is best illustrated with the sequence of the histone H3 and H4 tails, which are practically 100% conserved from yeast to mammals. The histone H3 and H4 tails are redundant in their essential function, as either one can be deleted in the presence of the other, but they cannot be deleted together. Moreover, the H3 and H4 tails can be interchanged to reside on the core of H4 and H3 respectively. Work from several laboratories have shown that H3 and H4 tails are important for proper gene regulation and DNA repair and it is proposed that this mechanism requires the posttranslational modification (methylation, acetylation, phosphorylation, etc.) of several key residues (lysines, arginine, serines and threonines) located within the tail of histones. The importance of each of these residues for specific functionalities of H3 and H4 tails is particularly difficult to investigate because of the functional redundancy within and between tails, and thus the elucidation of the link between specific residues and histone H3 and H4 tail functions would require a tremendous number of combinations of point mutations. To address these complex issues, my laboratory has set up a genetic screen taking advantage of the fact that simultaneous deletion in the N-terminal tails of H3 and H4 is unviable. A library of H3 molecules bearing N-terminal tails was designed with random sequences and screened for sequences that could restore viability in H3-H4 mutant strains. We successfully identified synthetic tails that restored not only viability, but also increased the fitness of the original strain in certain stress conditions. The current research program will thus focus on the characterization of these synthetic tails both biochemically and genetically to uncover their mechanisms of action. Such screening methods have proven to be extremely useful in identifying protein-protein interactions and understanding complex mechanisms. In particular, we will focus on screening for H4 N-terminal tails as we have done with H3, and use proteomics and genetics approaches to find direct functional targets of these synthetic H3 and H4 histone tails.