Transcription factors are extraordinarily diverse, and any one factor represents only a tiny fraction of the protein molecules present in the cell. This page describes how one can isolate and purify such rare molecules.
Example: Isolating the lac Repressor
An E. coli cell contains only 10-20 copies of the lac repressor. This represents a ratio of only 1 molecule in 50,000 protein molecules in the cell. However, the specificity of the lac repressor for the DNA sequence of the operator provides a mechanism for fishing it out of the mixture.
Figure 9.9.1 Affinity Chromatography
The following procedure is carried out for an affinity chromatograph:
- Learn the sequence to which the repressor binds (by footprinting).
- Synthesize a segment of DNA containing the sequence.
- Attach this artificial molecule to beads of an inert, solid medium (the matrix).
- Pour an extract of E. coli cells over the beads.
- Only molecules specific for the DNA sequence — in this case, molecules of the lac repressor — will bind to the beads.
- After irrelevant protein molecules have passed through the column, wash the beads with a buffer that will release the lac repressor molecules so they can be studied.