Structural maintenance of chromsomes : A complex tale of genomic integrity
Year: 2009
Type: dissertation
Abstract: Genomic integrity is an absolute requirement for cell survival. \nProgrammed events such as genome rearrangements and DNA replication can \ncause lesions in the DNA, as can exogenous agents such as radiation and \nchemicals. One of the most austere types of lesion is DNA double strand \nbreaks (DBSs). In Saccharomyces cerevisiae, budding yeast, they are \npreferentially repaired by the homologous recombination (HR) pathway \nusing a homologous DNA sequence as template. \n \n \n \nThe Structural maintenance of chromosomes (SMC) family proteins are \nessential for cell viability and have functions in chromosome \ncondensation, segregation and in DNA repair by HR. The cohesin complex is \nimportant for cohesion and correct segregation of sister chromatids. The \nSmc5/6 complex functions late in the HR process and it has another \nfunction, not yet entirely elucidated, that makes the complex essential. \n \n \n \nWe have investigated the chromosomal localization of the Smc5/6 complex \nand found that the complex associates with specific sites along the \nchromosome arms in a chromosome length-dependent manner. This association \nis dependent on the cohesin loading protein Scc2. The complex also \nlocalizes to chromosomal regions surrounding a DNA DSB in the G2/M phase. \nLocalization to DSBs is dependent on the damage-sensing HR protein Mre11, \nbut not on Scc2. Smc6 mutants exhibit a delay in chromosome segregation \nand a closer investigation suggests that this delay is caused by \npersisting replication forks. \n \n \n \nThe length-dependent distribution of the Smc5/6 complex on chromosomes \nwas found to reflect a function of the complex that is independent of its \nfunction in HR. A possible explanation for this length-dependency is the \naccumulation of replication-induced topological structures on longer \nchromosomes due to their inability to swivel off the torsional stress. A \ncircular short chromosome is therefore expected to generate more \nunresolved topological structures than a linear version of the same \nchromosome. Smc5/6 complex components showed an increase in binding \nregions on a circular chromosome compared to the linear version. Deletion \nof Top1, a protein required for release of replication-induced torsional \ntension in DNA, also shows a similar chromosome length-specific phenotype \nas the Smc5/6 complex components, indicating that topology is the \ninherent cause of the Smc5/6 complex association with chromosomes. \n \n \n \nThe main function of the cohesin complex is linking the sister chromatids \nfrom S phase until the metaphase-to-anaphase transition. To investigate a \nrole for cohesin in DSB repair, we examined its localization in response \nto a site-specific DSB. Cohesin is normally loaded onto DNA in late \nG1/early S phase, but when a DNA break has been induced in G2/M, cohesin \nlocalizes to the break area in a Scc2-dependent manner. In addition, we \nhave demonstrated that cohesin recruited in response to DSBs in G2/M \nphase can mediate cohesion, supporting the idea that cohesin and sister \nchromatid cohesion have a role in DNA repair. \n \n \n \nThe damage-induced cohesion can be distinguished from cohesion formed \nduring replication and the regulation and function of this damage-induced \ncohesion was found to be dependent on Mre11 and the Tel1 and Mec1 \nkinases. The HR protein Rad52 protein was not required, showing that \ncontrary to S phase-established cohesion, formation of damage-induced \ncohesion in G2/M phase is independent of DNA synthesis. \n \n \n \nA single DNA DSB is enough to generate cohesion throughout the entire \ngenome. Mec1, Scc2, Smc6 and the establishment of cohesion protein Eco1 \nare also required for genome-wide cohesion after DSB induction, and the \ndamage-induced cohesion is required for DNA repair.
Source: Figshare
Author: Hanna Betts Lindroos
Language: English
Topic: DNA Repair Mechanisms
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2169347739