| dc.description.abstract | Chromatin is a dynamic structure which reorganizes to support numerous chromosomal processes. Various histone and non-histone proteins are involved in chromatin organization. One such group of non-histone proteins, known as SMC (Structural Maintenance of Chromosomes) proteins, plays a pivotal role in chromosome organization. SMC proteins are highly conserved in all three domains of life. The Saccharomyces cerevisiae genome codes for at least six SMC proteins, which along with non-SMC partners form three different complexes named as cohesin, condensin and the SMC5/6 complex. Cohesin is an evolutionary conserved multi-subunit protein complex involved in multiple chromosomal processes such as sister chromatid cohesion, chromosome condensation, regulation of gene expression, DNA replication and repair etc. Cohesin binds to the centromeres, at sites along the chromosome arms and subtelomeric regions. However, its role at the telomeres remains largely elusive. In budding yeast, telomeres exist in a heterochromatin-like structure and transcription within 20kb from telomeres is repressed, in part by the histone modifying SIR-complex, a phenomenon known as telomere position effect (TPE). Here, we report a role for cohesin in subtelomeric gene silencing that extends even beyond the zone of SIR binding. We find that TPE is impaired in the mutants defective in the cohesin complex. We also find that clusters of subtelomeric genes were preferentially de-repressed in the cohesin mutant, whereas SIR binding and associated histone modifications were largely unaffected. Interestingly, genetic interaction analysis revealed that cohesin mediated repression is independent of Sir proteins. Moreover, mutation in cohesin resulted in reduced telomere tethering to the nuclear envelope and increased telomere accessibility to ectopically expressed bacterial Dam methylase, indicating a defect in telomere organization. Cohesin undergoes various post-translational modifications such as acetylation, phosphorylation and sumoylation. Here, we have investigated the requirement of cohesin sumoylation for cohesin’s function in subtelomeric gene silencing. We created a sumoylation-deficient cohesin complex by fusing the catalytic domain of a SUMO protease, ULP1, to the C-terminus of Mcd1 (Mcd1-UD), a kleisin subunit of the complex. We have shown that cohesin sumoylation is required for repression of telomere proximal genes. In agreement with our earlier observations, Sir proteins remained bound to a de-repressed subtelomeric gene in the mutant. Genetic interaction analysis revealed that cohesin sumoylation mediated repression of subtelomeric genes is independent of Sir proteins. We also found that cohesin sumoylation is required for telomere tethering to the nuclear envelope. Together, these findings suggest a SIR-independent contribution of the cohesin complex in transcriptional repression of telomere proximal genes by maintaining telomere organization. Serendipitously, we found that a temperature sensitive mutant of the cohesin complex was resistant to zymolyase, indicating an altered cell wall structure in the cohesin mutant. We show that the mutant cells have higher amount of chitin in the lateral wall, a hallmark of cell wall stress. Indeed, cohesin mutants were hyper-sensitive to cell wall stress inducing agents and expression of stress induced genes was significantly increased in the cohesin mutant. Interestingly, temperature dependent lethality of cohesin mutants was partially osmoremedial, suggesting that temperature sensitive nature of these mutants was partly due to a defect in the cell wall. Together, these findings suggest a new role for the cohesin complex in cell wall maintenance. In conclusion, we show that cohesin is required for repression of telomere proximal genes independently of Sir proteins, possibly by virtue of its role in telomere tethering and compaction. We have also uncovered an unsuspected requirement of the function of the cohesin complex in cell wall maintenance | en_US |
