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AFM Studies of λ Repressor Oligomers Securing DNA Loops

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Large, cooperative assemblies of proteins that wrap and/or loop genomic DNA may “epigenetically” shift configurational equilibria that determine developmental pathways. Such is the case of the λ bacteriophage which may exhibit virulent (lytic) or quiescent (lysogenic) growth. The lysogenic state of λ prophages is maintained by the λ repressor (CI), which binds to tripartite operator sites in each of the OL and OR control regions located about 2.3 kbp apart on the phage genome and represses lytic promoters. Dodd and collaborators have suggested that an initial loop formed by interaction between CI bound at OR and OL provides the proper scaffold for additional CI binding to attenuate the PRM promoter and avoid over production of CI. Recently, the looping equilibrium as a function of CI concentration was measured using tethered particle motion analysis, but the oligomerization of CI in looped states could not be determined. Scanning force microscopy has now been used to probe these details directly. An equilibrium distribution of looped and unlooped molecules confined to a plane was found to be commensurate to that for tethered molecules in solution, and the occupancies of specific operator sites for several looped and unlooped conformations were determined. Some loops appeared to be sealed by oligomers of 6-8, most by oligomers of 10-12, and a few by oligomers of 14-16.

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Keywords: Atomic force microscopy; DNA looping; Lambda repressor

Document Type: Research Article

Publication date: 01 August 2009

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  • Current Pharmaceutical Biotechnology aims to cover all the latest and outstanding developments in Pharmaceutical Biotechnology. Each issue of the journal contains a series of timely in-depth reviews written by leaders in the field covering a range of current topics in both pre-clinical and clinical areas of Pharmaceutical Biotechnology. Current Pharmaceutical Biotechnology is an essential journal for academic, clinical, government and pharmaceutical scientists who wish to be kept informed and up-to-date with the latest and most important developments.
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