Tweaking a Switch

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Science  28 Mar 2014:
Vol. 343, Issue 6178, pp. 1407
DOI: 10.1126/science.343.6178.1407-b

Transcription factors regulate gene expression by binding to specific chromosomal operator sites. Many transcription factors are repressors, with transcription turned off when the repressor is bound. A simple operator occupancy model assumes that the level of repression is determined only by the equilibrium binding of the repressor to its operator. Hammar et al. have now used a single-molecule chase assay to directly test this in living cells. They measured the time that the lac repressor protein LacI remained bound to the natural lacO1 operator and to a stronger, artificial lacOsym operator. It is assumed that transcription is turned off during this time, so this is termed τoff. They also measured the average time that the operators remained unbound so that transcription can be on (τon). The repression ratio in the simple occupancy model would be given by RR = (τon + τoff)/τon. The calculated repression ratios were compared with repression ratios measured based on an enzymatic reporter assay, thus monitoring protein expression rather than repressor binding. There was agreement for the lacO1 operator, but for the lacOsym, more repression was seen than would be expected based on a simple occupancy model. This could be accounted for either by promoter-specific cooperative interactions between LacI and RNA polymerase or simply by transcription initiation driving the system out of equilibrium; fast transcription initiation could lead to the synthesis of transcripts before the repressor has equilibrated with DNA. Such effects need to be considered in examining mechanisms of gene regulation.

Nat. Genet. 10.1038/ng.2905 (2014).

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