Identification of two topologically distinct Mu transpososomes: contribution of cis and trans elements to DNA topology
Abstract
Transposition of bacteriophage Mu takes place within nucleoprotein
complexes called transpososomes. Assembly of a transpososome requires transposase
binding to multiple sites within the L and R ends of Mu and an internal transpositional
enhancer E, present on supercoiled DNA. L, R and E interact such that five negative
DNA supercoils are trapped within the transpososome. We have carried out topological
studies aimed at understanding the contribution of all of these elements to
transpososome assembly. We have found that the transposase has an inherent capacity
to interwrap distant DNA sites. Under enhancer-independent reaction conditions, two
topologically distinct synapses were identified. These studies have revealed that the
enhancer as well as the ends, contribute to the topological selectivity of the Mu synapse.
We have identified specific end – enhancer interactions critical for assembly. Based on
our topological studies as well as other data, we propose a new model for the structure
of the 5-noded 3-site Mu complex.
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