Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.
Transcription of the ant gene during lytic growth of bacteriophage P22 (ref. 1) is regulated by the cooperative binding of two Arc repressor dimers to a 21-base-pair operator site. Here we report the co-crystal structure of this Arc tetramer-operator complex at 2.6 A resolution. As expected from genetic and structural studies and from the co-crystal structure of the homologous Escherichia coli MetJ repressor, each Arc dimer uses an antiparallel beta-sheet to recognize bases in the major groove. However, the Arc and MetJ complexes differ in several important ways: the beta-sheet-DNA interactions of Arc are far less symmetrical; DNA binding by Arc is accompanied by important conformational changes in the beta-sheet; and Arc uses a different part of its protein surface for dimer-dimer interactions.
Department of Biology, Massachusetts Institute of Technology, Cambridge 02139, USA.
By binding to adjacent subsites in its 21 base-pair operator, Arc represses transcription from two divergent promoters, Pant and Pmnt , in the immunity I operon of bacteriophage P22. Arc dimers bind to each subsite with nanomolar affinities and interact through protein - protein interactions to stabilize binding further. Here, we show that an Arc dimer bound to a single subsite reduces the rate of RNA polymerase open-complex formation and represses transcription from P ant and Pmnt promoter variants to varying degrees. Occupancy of the subsite proximal to the Pant - 35 region results in significantly greater repression than occupancy of the -10 proximal subsite. For repression of Pmnt, Arc bound at the -10 proximal subsite is more effective than Arc bound at the -35 proximal subsite. Because of the divergent orientations of the two promoters, the -35 proximal site in Pant is the same as the -10 proximal site in Pmnt. Thus, in both cases, the same operator subsite is primarily responsible for repression of transcription initiation.
Keywords: transcriptional regulation, multimerization, cooperative DNA binding, bacteriophage P22,
Journal of Molecular Biology, v 264, n 2, November 29, 1996, p233-242 (ID mb960637)
Copyright © 1996 Academic Press Limited
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