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Related Experiment Videos

The activation defect of a lambda cI positive control mutant

F W Whipple1, M Ptashne, A Hochschild

  • 1Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115, USA.

Journal of Molecular Biology
|January 24, 1997
PubMed
Summary
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Bacteriophage lambda cI mutants bind DNA but do not activate transcription. This study refutes a competing hypothesis, showing the E34K mutant fails to activate transcription due to an intrinsic defect, not altered operator binding.

Area of Science:

  • Molecular Biology
  • Genetics
  • Biochemistry

Background:

  • The bacteriophage lambda cI protein regulates gene expression by binding to operators and interacting with RNA polymerase.
  • Certain 'positive control' mutants, like lambda cI E34K, bind DNA but fail to activate transcription, challenging existing models.
  • Previous interpretations suggested these mutants disrupt interactions with RNA polymerase or bind alternative operator sites.

Purpose of the Study:

  • To investigate the mechanism by which the lambda cI E34K mutant fails to activate transcription.
  • To test the hypothesis that the E34K mutant's failure is due to low-affinity binding to the O(R)3 operator site, mediating repression.
  • To clarify the role of specific residues, like glutamate 34, in transcriptional activation.

Main Methods:

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  • In vitro and in vivo transcription assays were performed using the P(RM) promoter.
  • Operator site occupancy was monitored using varying concentrations of wild-type and mutant lambda cI proteins.
  • Specific binding affinities to O(R)2 and O(R)3 operator sites were analyzed.

Main Results:

  • The lambda cI E34K mutant failed to activate transcription from P(RM) even when the O(R)2 operator was fully occupied.
  • Under conditions where O(R)2 was occupied and O(R)3 was vacant, wild-type lambda cI activated transcription, but the E34K mutant did not.
  • Results contradicted the hypothesis that E34K's failure is due to preferential binding to O(R)3.

Conclusions:

  • The study provides evidence against the model proposing that the lambda cI E34K mutant's transcriptional failure is caused by binding to the O(R)3 operator.
  • The findings support an intrinsic defect in the mutant protein's ability to activate transcription, rather than altered operator binding specificity.
  • Glutamate 34 is critical for the stimulatory interaction required for wild-type lambda cI-mediated transcriptional activation.