Dynamic Switch Between Assembly States of the Human Bloom's Syndrome Helicase during Homologous Recombination

Abstract

During normal cellular functioning, various endogenous and exogenous DNA-damaging effects arise, including the production of reactive oxygen species and UV irradiation. Double-stranded DNA breaks (DSB) represent one of the most toxic forms of DNA damage, which can be lethal for the cell or induce malignant transformation. Homologous recombination (HR) is an essential and evolutionarily conserved pathway for the error-free repair of DSBs. However, excessive HR can also lead to harmful large-scale genome rearrangements. HR events are therefore tightly regulated. A key player in HR is the human Bloom's syndrome helicase (BLM). During HR, BLM exerts various molecular activities. Based on ensemble biochemical and single-molecule AFM studies we show that the different actions of BLM during HR take place in different oligomeric forms of the enzyme. During single-stranded DNA translocation, which serves as a basis for quality control of HR via disruption of Rad51 nucleoprotein filaments, BLM functions as a monomer. Contrary, more complex DNA structures resembling later HR intermediates, including D-loops and Holliday junctions, induce dimerization and higher-order oligomerization of BLM. The results indicate that BLM exists in a dynamic equilibrium between different assembly states, which is modulated by the structure of DNA intermediates encountered during HR.