The Mu enhancer is functionally asymmetric both in cis and in trans. Topological selectivity of Mu transposition is enhancer-independent.
Mu DNA transposition from a negatively supercoiled DNA substrate requires interaction of an enhancer element with the left (attL) and right (attR) ends of Mu. The orientation of the L and R ends with respect to each other (inverted) and with respect to the enhancer is normally inviolate. We show that when the enhancer is provided in trans as a linear fragment, the head to head orientation of the L/R ends is still required. Each functional half of the linear enhancer maintains the same "cross-wise" interaction with the subsites L1 and R1, when present in cis or in trans. In reactions catalyzed by an enhancer-independent variant of the Mu transposase, the need for negative supercoiling of the substrate and the inverted orientation of L and R ends is not relaxed. These results show that the orientation specificity of the enhancer is not determined by its topological linkage to the Mu ends. There is a functional asymmetry inherent to the enhancer. Furthermore, the enhancer does not directly impose topological constraints on the transposition reaction or specify the reactive orientation of the Mu ends.
Jiang
H
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Section of Molecular Genetics and Microbiology and Institute of Cellular and Molecular Biology, University of Texas, Austin, Texas 78712, USA.
Harshey
R M
RM
D010583:Bacteriophage mu / Q000235:genetics*; D004278:DNA, Superhelical / Q000235:genetics; D004279:DNA, Viral / Q000235:genetics; D004742:Enhancer Elements, Genetic; D011995:Recombination, Genetic
D004278:DNA, Superhelical; D004279:DNA, Viral