COMMUNICATION Solution Structure of the Ig Subdomain of the Mu End DNA-binding Domain of

The MuA transposase of phase Mu is a large modular protein that plays a central role in transposition. We show that the Mu end DNA-binding domain, Ibg, which is responsible for binding the DNA attachment sites at each end of the Mu genome, comprises two subdomains, Ib and Ig, that are structurally autonomous and do not interact with each other in the absence of DNA. The solution structure of the Ig subdomain has been determined by multidimensional NMR spectroscopy. The structure of Ig comprises a four helix bundle and, despite the absence of any signi®cant sequence identity, the topology of the ®rst three helices is very similar to that of the homeodomain family of helix-turn-helix DNA-binding pro-teins. The helix-turn-helix motif of Ig, however, differs from that of the homeodomains in so far as the loop is longer and the second helix is shorter, reminiscent of that in the POU-speci®c domain