Regulation of sub-telomeric chromatin structure and function in Saccharomyces cerevisiae



Rehman, Muhammad Attiq

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University of Guelph


This thesis explores some of the mechanistic links between replication factors and sub-telomeric sequences that confer the establishment and the inheritance of silencing at telomeres in 'Saccharomyces cerevisiae '. I hypothesized that DNA replication factors affect telomeric silencing through dormant sub-telomeric ACS. To test this hypothesis, I analyzed the effect of mutations in DNA replication factors ('mcm5-461, mcm5-1, orc2-1, orc5-1, cdc45-1, cdc6-1', and 'cdc7-1') on the silencing of a group of reporter constructs with "natural" sub-telomeric elements. I showed that the 'mcm5-461, mcm5-1', and ' orc2-1' mutations affect silencing through sub-telomeric ' ACS', while 'cdc6-1' affects silencing independently of 'ACS'. Both 'ACS'-dependent and ' ACS'-independent effects were seen for 'orc5-1, cdc45-1' and 'cdc7-1'. Previous studies on telomeric silencing found that mutation in 'ORC' (Origin Recognition Complex) genes have marginal effect on gene expression in the vicinity of 'ACS' whereas deletion of 'ACS' in the 'HMR-E' silencers enhances repression in 'orc' mutants. In an attempt to reslove this apparent inconsistancy on the role of 'ACS' in silencing, I revealed a novel feature of 'ACS'. Intriguingly, in many replication factor mutants the sub-telomeric 'ACS' alter their behavior and convert from a proto-silencer to an anti-silencer. This dynamic behaviour was typical for sub-telomeric 'ACS' but not for origin ' ACS'. Genes positioned proximal to telomeres are meta-stably repressed and infrequently convert their state. The cause of this epigenetic variegation is poorly understood. Here I demonstrate that the 'ACS'-independent effects of 'CDC7' are mediated through suppression of positional variegation. I show that mutations in 'CDC7' or deletion of the histone chaperone 'CAC1' dramatically enhance the heritability of transcriptional states thus producing "locked" repressed or de-repressed phenotypes. I describe a novel epigenetic phenomenon and provide the framework for further elucidation of the mechanisms by which cells establish, maintain, and transfer epigenetic information.



Saccharomyces cerevisiae, sub-telomeric chromatin structure, telomeric silencing, ACS, epigenetic