Studies of DNA structures linking sister chromatids, the proteins that regulate them, and their impact on segregation during anaphase

• Main Author: Sen, Nicholas
• Other Authors: Aragon, Luis
• Published: Imperial College London 2013
• Subjects: 610
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.650650

Type IIA topoisomerases have long been attributed the ability to simplify DNA topology - supercoils, knots and catenanes - far beyond their thermodynamic equilibrium. Such activity makes clear biological sense, however topology of a DNA molecule is a global attribute, and type IIA topoisomerases are capable only of recognizing local structures that constitute workable substrates. In the case of type IIA topoisomerases local DNA juxtapositions embody these substrates. To be able to reliably simplify DNA topology, without by chance increasing its complexity, these enzymes would be required to have the ability to discern some aspect of the global topology from the interactions with these local juxtaposed DNA substrates. Previous studies have generated computer simulations of juxtaposed chromatin segments whereby information regarding the global topological state is somehow communicated to the enzyme. These models vary in nature, from protein- centric models involving the forced introduction of 'kinks' into the DNA, to DNA-centric models, which suggest alterations in chirality of crosses are suggestive of the topological state of the molecule, and thus all local DNA juxtapositions encode details of higher order structure. For the system to function in this manner, information 'encoded' in one DNA juxtaposition would have to become altered when other distinct substrates are acted upon. This study aimed to investigate whether S. cerevisiae topoisomerase II is capable of enforcing directional alteration of the ratio of catenated plasmid dimers to monomers in vivo. Systems that artificially generate increased proximity between plasmids were tested and yielded inconclusive results pertaining to this question. Existing evidence has shown a shift in the supercoiling state from (-) to (+) occurs at the metaphase to anaphase transition, and that this forces Top2 to resolve catenated nodes in favour of relaxing supercoils, however this transition would gain credence in the case that prior to this event Top2 was able to concatenate DNA, and that the probability of this occurring was equal to the likelihood of catenated nodes becoming resolved. Further investigation of this transition has revealed the requirement for Cdc5, and Scc1. Additionally the localization of the Condensin complex was analyzed to gain a better understanding of how transition factors are regulated.