I am performing a KI experiment using CRIPR/Cas. Our 5' homology arm is the 5'UTR of the gene of interest and therefore contains the endogenous promoter. I am looking for a repressor sequence to insert before the 5' arm that would prevent expression of the gene.
The thought being, when our construct in inserted on-target, this repressor would not be incorporated (as it is outside of the HA). And any instances of off-target integration would include the repressor and would not express our insert which includes a selection marker.
Is there a ubiquitous constitutive repressor, or would I need information on my specific endogenous promoter to know how to repress it?
Are you talking about eukaryotes or prokaryotes? For proks you could use any one of dozens of repressors, whether constitutive or not so long as you can easily maintain the repressed state. However in most cases you will need the repressor binding domain to be physically located very close to the promoter region so that protein binding to the operator site interferes with polymerase binding or transcription initiation. You could certainly look at lacI or trpR or lexA binding sites to use as operators, although there are dozens and dozens of others.
The situation gets more complicated in eukaryotes and I'm not sufficiently expert to make recommendations. But you might look at the Tet-off system.
Or the Tet-on system actually, which uses a repressor that is normally active, and is released from the DNA by tet. Good idea by Michael. Folks have engineered these kinds of systems, but making this work well is going to be a major research project. Good luck...
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