Investigating the role of the Set1 histone methyltransferase in the G1/S checkpoint of the cell cycle

The Set1 histone methyltransferase is an enzyme complex that is well known for the methylation of histone H3 lysine 4 (H3K4), a conserved epigenetic regulator of gene expression. Impairment in the function of Set1 homolog in human leads to pathologies such as development disorders and malignancies. Recent studies have left much ambiguity regarding non-H3K4 methylation related functions of these complexes. To unravel the function of Set1 in Saccharomyces cerevisiae, preliminary yeast two hybrid screen identified Nrm1, a negative regulator of cell cycle, as a novel interactor with Set1. Nrm1 regulation forms an important step of the G1/S checkpoint of the cell cycle that is activated in response to DNA replication stress. This study investigated the functional implication of this interaction between Set1 and Nrm1. For this purpose, genetic analysis of Set1 and Nrm1 mutant yeast strains was utilised and revealed that Set1 is required for the same cellular process that implicate Nrm1, the response to DNA replication stress. Set1, in this novel role, required its catalytic activity while, Nrm1 acted epistatically to Set1. Following on from this, gene expression analysis of same mutants revealed a requirement for Set1 in promoting the transcriptional response of G1/S checkpoint. Surprisingly, this requirement was not linked to methylation of H3K4, suggesting that Set1 might be methylating a non-histone substrate during facilitation of this cellular process. Immunoblot result did not reveal an apparent impact for Set1 in determining Nrm1 protein levels, proposing for future studies to investigate the impact of Set1 on Nrm1 activity and or stability. Understanding the molecular interplay between chromatin modifiers such as Set1 and cell cycle regulators such as Nrm1, will advance our knowledge of novel important features in checkpoint regulation.