In situ Detection of Proteins
Specific proteins can be detected directly from the bacterial cells in a gene library by using antibodies. The method has many similarities to the DNA homology method described above. This strategy would be used when the geneticist knows what protein is encoded by the gene they want to clone and the cloned gene can be expressed in the bacteria cells in the library.
Again it is important to visualize what is going on inside the cells of the bacteria in a gene library. The genes on the plasmid can function like the genes on the bacteria’s chromosome. All bacteria in the library will be able to make the same proteins with the exception of the protein encoded by the inserted DNA that differs among the colonies. But how can the inserted DNA instruct the bacteria cell to encode a protein? In thinking about this it is important to consider genomic and cDNA libraries separately. The two kinds of libraries will differ in how well the antibody screening method will work.
Fig.18: If bacteria in the library are expressing the gene of interest and able to make the protein encoded by the gene (bacteria from colony #298), an antibody specific for the protein will bind 'in situ' and emit a signal to indicate that this colony has the gene of interest. Bacteria from colony #187 make a different protein, the antibody will not bind and no signal will be produced. (image by P. Hain)