A rapid non-radioactive procedure for plaque hybridization using biotinylated probes prepared by random primed labeling.

A simple, non-radioactive method has been developed for the rapid screening of phage libraries. In the present study, nanogram amounts of a small restriction fragment (135 bp) were biotinylated via random primed labeling and used to probe cDNA libraries using a modified plaque hybridization protocol. The high backgrounds that are seen typically with avidin/biotin-based methods for plaque hybridization were eliminated by incubation of filters with one of several different proteases prior to hybridization. A comparison of several detection systems indicated that streptavidin conjugated to calf intestinal alkaline phosphatase (AP) was the most sensitive, yielding signals comparable to those obtained with 32P-labeled probes. The times required for phage growth and pre-hybridization were reduced substantially, permitting a convenient one-day screening protocol. Nitrocellulose filters gave the best signal to noise ratio, although "streaking" of plaque DNA was observed occasionally; this problem can be overcome by using nylon-based membranes, which allows exact visualization of the positive plaques. The method was highly reliable; 29 out of 33 putative clones retested positive and the authenticity of these was confirmed by DNA sequence analysis. The random primed biotinylation procedure has been utilized successfully with several different cDNA fragments and has proven useful for other hybridization-based methods (Northern and Southern blots), without the problems associated with the use of radiolabeled probes.