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Changing the Mechanism of Transcriptional Activation by Phage λ Repressor

Mei Li, W. R. McClure and Miriam M. Susskind
Proceedings of the National Academy of Sciences of the United States of America
Vol. 94, No. 8 (Apr. 15, 1997), pp. 3691-3696
Stable URL: http://www.jstor.org/stable/41891
Page Count: 6
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Changing the Mechanism of Transcriptional Activation by Phage λ  Repressor
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Abstract

The first steps of transcription initiation include binding of RNA polymerase to a promoter to form an inactive, unstable, closed complex (described by an equilibrium constant, KB) and isomerization of the closed complex to an active, stable, open complex (described by a forward rate constant, kf). λ cI protein activates the PRM promoter by specifically increasing kf. A positive control mutant, cI-pc2, is defective for activation because it fails to raise kf. An Arg to His change in the σ 70 subunit of RNA polymerase was previously obtained as an allele-specific suppressor of cI-pc2. To elucidate how the mutant polymerase restores the activation function of the mutant activator, abortive initiation assays were performed, using purified cI proteins and RNA polymerase holoenzymes. The change in σ does not significantly alter KB or kf in the absence of cI protein. As expected, cI-pc2 activates the mutant polymerase in the same way that wild-type cI activates the wild-type polymerase, by increasing kf. An unexpected and novel finding is that the wild-type activator stimulates the mutant polymerase, but not wild-type polymerase, by increasing KB.

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