We chose to compare two proteins known to bind to the DNA in two different organisms: the tryptophan repressor of E. Coli and CRO protein of phage 434.
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These two proteins bind tightly to the DNA at a gene's promoter, thus preventing RNA polymerase from fixing and hence blocking the initiation of the transcription. These are repressors. They should thus have a common substructure that realizes this biological function.
There also exists proteins that bind next to the promoter of a gene and help the polymerase to bind, thus assisting the transcription of the gene. They are called activators.
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