Recombinant DNA techniques: storage and screening of cDNA libraries with large numbers of individual colonies from initial transformations.

A typical cDNA library with a large number of initial transformants, plated in soft agarose, can be stored and shipped in 12% glycerol at -70 degrees C. To prepare the library for storage, the soft agarose layer is made into a paste and the agarose is removed by Sephadex G-25 filtration. This method of cDNA library storage does not alter the relative representation of the plasmids carried in the library. To achieve a very uniform distribution of colonies at high colony density, an aliquot of the cDNA library is diluted to 3000 to 10,000 colonies/ml. One milliliter of this suspension is evenly distributed on a nitrocellulose filter on an agar plate and air-dried. Filter copies are made and screened by published methods.