Structural maintenance of chromsomes : a complex tale of genomic integrity

Abstract

Genomic integrity is an absolute requirement for cell survival. Programmed events such as genome rearrangements and DNA replication can cause lesions in the DNA, as can exogenous agents such as radiation and chemicals. One of the most austere types of lesion is DNA double strand breaks (DBSs). In Saccharomyces cerevisiae, budding yeast, they are preferentially repaired by the homologous recombination (HR) pathway using a homologous DNA sequence as template.

The Structural maintenance of chromosomes (SMC) family proteins are essential for cell viability and have functions in chromosome condensation, segregation and in DNA repair by HR. The cohesin complex is important for cohesion and correct segregation of sister chromatids. The Smc5/6 complex functions late in the HR process and it has another function, not yet entirely elucidated, that makes the complex essential.

We have investigated the chromosomal localization of the Smc5/6 complex and found that the complex associates with specific sites along the chromosome arms in a chromosome length-dependent manner. This association is dependent on the cohesin loading protein Scc2. The complex also localizes to chromosomal regions surrounding a DNA DSB in the G2/M phase. Localization to DSBs is dependent on the damage-sensing HR protein Mre11, but not on Scc2. Smc6 mutants exhibit a delay in chromosome segregation and a closer investigation suggests that this delay is caused by persisting replication forks.

The length-dependent distribution of the Smc5/6 complex on chromosomes was found to reflect a function of the complex that is independent of its function in HR. A possible explanation for this length-dependency is the accumulation of replication-induced topological structures on longer chromosomes due to their inability to swivel off the torsional stress. A circular short chromosome is therefore expected to generate more unresolved topological structures than a linear version of the same chromosome. Smc5/6 complex components showed an increase in binding regions on a circular chromosome compared to the linear version. Deletion of Top1, a protein required for release of replication-induced torsional tension in DNA, also shows a similar chromosome length-specific phenotype as the Smc5/6 complex components, indicating that topology is the inherent cause of the Smc5/6 complex association with chromosomes.

The main function of the cohesin complex is linking the sister chromatids from S phase until the metaphase-to-anaphase transition. To investigate a role for cohesin in DSB repair, we examined its localization in response to a site-specific DSB. Cohesin is normally loaded onto DNA in late G1/early S phase, but when a DNA break has been induced in G2/M, cohesin localizes to the break area in a Scc2-dependent manner. In addition, we have demonstrated that cohesin recruited in response to DSBs in G2/M phase can mediate cohesion, supporting the idea that cohesin and sister chromatid cohesion have a role in DNA repair.

The damage-induced cohesion can be distinguished from cohesion formed during replication and the regulation and function of this damage-induced cohesion was found to be dependent on Mre11 and the Tel1 and Mec1 kinases. The HR protein Rad52 protein was not required, showing that contrary to S phase-established cohesion, formation of damage-induced cohesion in G2/M phase is independent of DNA synthesis.

A single DNA DSB is enough to generate cohesion throughout the entire genome. Mec1, Scc2, Smc6 and the establishment of cohesion protein Eco1 are also required for genome-wide cohesion after DSB induction, and the damage-induced cohesion is required for DNA repair.