The Role of the RTR Complex in Somatic Genome Stability in Plants

The RTR complex consisting of a RecQ helicase, a type IA topoisomerase and the structural protein RMI1 is conserved throughout eukaryotes and involved in DNA repair and in the suppression of homologous recombination (HR) in somatic cells. In Arabidopsis it consists of the helicase RECQ4A, the homolog of which is mutated in the genetic disorder Bloom’s syndrome, the topoisomerase 3α and the structural protein RMI1, Interestingly RMI1 and topoisomerase 3α have also an essential role in the dissolution of meiotic recombination intermediates in plants. Through complementation studies, we now were able to determine which domains of the helicases as well as AtRMI1 are required for the respective functions. Both the N-terminal region and the helicase activity of RECQ4A are both essential for the crosslink repair. In contrast, loss of helicase activity or deletion of the N-terminus only partially complemented the mutant hyper-recombination phenotype. Furthermore, the helicase-deficient protein lacking its N-terminus did not complement the hyper-recombination phenotype at all. Therefore, RECQ4A seems to possess at least two different and independent sub-functions involved in the suppression of HR. AtRMI1 harbours an N-terminal DUF1767 domain and two OB-fold domains. Complementation experiments on Atrmi1 mutants demonstrated that the DUF1767 domain and the OB-fold domain 1 are both essential for the function of AtRMI1 in DNA cross-link repair as well as meiotic recombination, but partially dispensable for somatic HR suppression. The OB-fold domain 2 is not necessary for either somatic or meiotic HR, but it seems to have a minor function in DNA cross-link repair.