A procedure for DNA and RNA transfer to membrane filters avoiding weight-induced gel flattening.

We describe a technique of rapid (within 1-2 h) transfer of DNA and RNA from agarose gels to nitrocellulose or nylon membrane filters. It is characterized by nearly complete elimination of mechanical action on the gel (a thin layer of liquid is placed over the gel and, filtering through the gel into a stack of paper towels beneath, it transfers nucleic acids onto the filter under the gel). This "descending" transfer, as opposed to the widely used "ascending" Southern transfer, reduces the transfer time (to about 1 h) with equal or higher quality of the hybridization signal. The comparison of transfer kinetics by the both methods shows that (a) the Southern transfer of large size DNA fragments proceeds quicker than it has been thought so far and is almost complete within 4 h; (b) the descending transfer has an advantage over the ascending one in the rate of transfer (1-2 h) and its efficiency; and (c) the time of transfer may become a critical parameter upon using a filter with an apparently low retention capacity (Hybond N, Amersham) that is manifested by a decreased signal at longer than optimal transfer times.