Conformational isomerization in phage Mu transpososome assembly: effects of the transpositional enhancer and of MuB

Initiation of phage Mu DNA transposition requires assembly of higher order protein-DNA complexes called Mu transpososomes containing the two Mu DNA en ds and MuA transposase tetramer. Mu transpososome assembly is highly regula ted and involves multiple DNA sites for transposase binding, including a tr anspositional enhancer called the internal activation sequence (IAS). In ad dition, a number of protein cofactors participate, including the target DNA activator MuB ATPase. We investigated the impact of the assembly cofactors on the kinetics of transpososome assembly with the aim of deciphering the reaction steps that are influenced by the cofactors. The transpositional en hancer IAS appears to have little impact on the initial pairing of the two Mu end segments bound by MuA. Instead, it accelerates the post-synaptic con formational step(s) that converts the reversible complex to the stable tran spososome. The transpososome assembly stimulation by MuB does not require i ts stable DNA binding activity, which appears critical for directing transp osition to sites distant from the donor transposon.