DNA replication represents a dangerous moment in the life of the cell as endogenous and exogenous events challenge genome integrity by interfering with the progression, stability and restart of the replication fork. Failure to protect stalled forks or to process the replication fork appropriately contribute to the pathological mechanisms giving rise to cancer, therefore an understanding of the intricate mechanisms that ensure fork integrity can provide targets for new chemotherapeutic assays. Smc5-Smc6 is a multi-subunit complex with a poorly understood function in DNA replication and repair. One of its subunits, Nse2, is able to promote the addition of a small ubiquitin-like protein modifier (SUMO) to specific target proteins. Recent work has revealed that the Smc5-Smc6 complex is required for the progression of replication forks through damaged DNA and is recruited de novo to forks that undergo collapse. In addition, Smc5-Smc6 mediate repair of DNA breaks by homologous recombination between sister-chromatids. Thus, Smc5-Smc6 is anticipated to promote recombinational repair at stalled/collapsed replication forks. My laboratory proposes to develop molecular techniques to study replication at the level of single replication forks. We will employ these assays to identify and dissect the function of factors involved in replication fork stability and repair. We will place an emphasis on the study of the Smc5-Smc6 complex in these processes because of its potential roles in recombination-dependent fork repair and restart. We also propose to identify novel Nse2 substrates involved in DNA repair using yeast model systems. Specifically, we will address the following points: (1) Development of assays for analysis of factors involved in stabilisation, collapse and re-start of single-forks, (2) Analysis of the roles of Smc5-Smc6 in fork biology using developed techniques, (3) Isolation and functional analysis of novel Nse2 substrates.