Metamorphism of catalytic domain controls transposition in Tn3 family transposases

Transposons account for a remarkable diversity of mobile genetic elements that play the role of genome architects in all domains of life. Tn3 is a family of widespread and among first identified bacterial transposons notorious for their contribution to dissemination of antibiotic resistance. Transposition within this family is mediated by a large TnpA transposase facilitating both transposition and target immunity. The structural framework for understanding the mechanism of the TnpA transposition is however absent. Here, we describe the cryo-EM structures of TnpA from Tn4430 in apo form and paired with transposon ends. We show that TnpA has a unique architecture and exhibits a family-specific regulation mechanism involving metamorphic refolding of the RNase H-like catalytic domain. The TnpA structure, constrained by a double dimerization interface, creates a unique topology that suggests a specific role for target DNA in transpososome assembly, activation, and target immunity.