Lac repressor is a transient gene-activating protein.
We show, using a combination of methods, that contrary to the usual view, lac repressor increases, by more than 100-fold, the initial binding of RNA polymerase to E. coli lac UV5 promoter DNA. Kinetic studies revealed that the repressor acts to block the isomerization step in transcription initiation. When IPTG, a gratuitous inducer, is added, formation of open complex and productive transcription proceed. Because of the large increases in the binding constant, at low polymerase concentrations the presence of lac repressor (and then inducer) actually increases the rate of the first round of productive transcription, thus allowing the system to respond rapidly to the release of repression. This dual role of stabilization of a pretranscriptional complex coupled with blockage of transcription initiation may be a more general model for genetic regulation than that provided by the concept of simple repression.
Straney
S B
SB
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06511.
Crothers
D M
DM
D004269:DNA, Bacterial / Q000235:genetics; D012321:DNA-Directed RNA Polymerases / Q000235:genetics; D003851:Deoxyribonucleases / Q000235:genetics; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000235:genetics; D005090:Exodeoxyribonucleases / Q000235:genetics; D005786:Gene Expression Regulation; D007700:Kinetics; D009710:Nucleotide Mapping; D012097:Repressor Proteins / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics*; D014158:Transcription, Genetic
D004269:DNA, Bacterial; D012097:Repressor Proteins; D014157:Transcription Factors; D012321:DNA-Directed RNA Polymerases; D003851:Deoxyribonucleases; D005090:Exodeoxyribonucleases; C020133:exodeoxyribonuclease III