1887

Abstract

The ATP-dependent multimerization process undergone by the σ-dependent activator XylR of the TOL plasmid pWW0 of when bound to the upstream activating sequences (UAS) of the cognate promoter was examined by transmission electron microscopy (TEM). To this end, supercoiled DNA templates were combined with increasing concentrations of the constitutive XylR variant XylRΔA, with or without ATP or its non-hydrolysable analogue ATPγS, and the resulting complexes were visualized by TEM. The different types of DNA–protein association were analysed and a statistical study of the frequency of the various forms was made. ATP appeared to establish an equilibrium between different molecular associations, as well as major changes in the physical shape of the DNA–protein complexes. The formation of higher nucleoprotein structures frequently bearing DNA bends became manifest. Such complexes often engaged otherwise separated UAS-containing plasmids, indicating that the ATP-driven multimer included XylR molecules recruited . Whilst ATP caused the different types of XylR–DNA complex to occur at quite balanced frequencies, ATPγS appeared to displace the distribution predominantly towards the higher order forms. These data are compatible with the notion that each time ATP is hydrolysed the transcriptional activation complex is disassembled.

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2000-10-01
2020-04-08
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