Investigation of histone exchange mechanisms and their roles in viral gene expression

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The regulated expression of genes is fundamental to growth and development. Although much progress has been made towards a comprehensive understanding of the mechanisms that regulate gene expression, many areas remain unknown.***Transcription and other processes that require access to the DNA are physically impeded by the assembly of DNA into chromatin with histones. DNA assembled in accessible chromatin is competent for transcription whereas that assembled in inaccessible chromatin is not. The mechanisms that regulate chromatin accessibility are not fully understood. It is also not clear how chromatin accessibility is modulated to enable transcription activation of DNA that is assembled into inaccessible chromatin. ***This proposed research program will use herpes simplex virus 1 (HSV) as a model system to investigate the processes that regulate chromatin accessibility and how they relate to transcription activation. Infecting HSV genomes are assembled into chromatin with cellular histones shortly after nuclear entry. Some HSV genomes are assembled into highly accessible, transcriptionally competent, chromatin, whereas the rest are assembled into highly inaccessible, transcriptionally incompetent, chromatin. The two populations of HSV chromatin within the same nucleus support a model whereby cellular antiviral defences attempt to silence HSV genomes through their assembly into inaccessible chromatin. HSV must counteract and disrupt silencing for viral genes to be expressed. Consistent with this model, the only essential HSV transcription activator, ICP4, enhances histone dynamics. Enhancing the dynamics of histones within the HSV chromatin would increase its accessibility and allow transcription. The enhancement of histone dynamics appears to be a novel mechanism of gene activation acquired by HSV to counteract cellular silencing attempts, although the mechanisms whereby ICP4 enhances histone dynamics are not known. ***Our hypothesis is that histone dynamics regulate HSV chromatin accessibility. Graduate (4 MSc, 2 PhD) and Undergraduate students (5) will be recruited and trained in virology, cell culture, DNA- and protein-centered techniques, and advanced cell imaging to: (1) Identify the minimal ICP4 domains necessary and sufficient to enhance histone dynamics, (2) Identify ICP4-interacting proteins functionally relevant to the enhancement of histone dynamics, and (3) Test the conservation of ICP4 enhancement of histone dynamics and chromatin accessibility among alphaherpesviruses. This research will support my long-term objective to characterize the mechanisms that regulate chromatin accessibility in order to increase our fundamental understanding of the processes that activate transcription of DNA assembled into inaccessible chromatin. Knowledge gained from this research program will be of importance to researchers in the fields of virology, epigenetics and chromatin biology, developmental biology, and oncology, among others.