Nonradioactive labeling of large DNA fragments for genome walking, RFLP and northern blot analysis.

In this report, we present a simple nonradioactive labeling procedure for DNA fragments of high specific labeling density that can be used for a variety of applications. The protocol is based on the universal mono-functional platinum reagent for chemical digoxigenin (DIG) labeling of nucleic acids. The labeling protocol was optimized for large DNA templates as complete bacterial artificial chromosomes (BAC). Variations of incubation time and temperature improved the labeling density such that about 30% of the nucleotides were DIG-modified within 30 min. Furthermore, the refined procedure generates in a single-tube reaction and without prior digestion-labeled DNA fragments of 0.5-4.0 kb from a 130-kb template. Hybridization experiments were performed on Southern and northern blots and allowed the detection of single copy genes in 2.5 micrograms genomic DNA from Arabidopsis thaliana, which has a haploid genome size of 0.13 pg (ca. 120 Mb) and medium expressed transcripts from 0.8 microgram poly(A)+ RNA, respectively. The extremely high specific labeling density, the stability and the universal application of the probe generated with the platinum reagent makes this method a useful alternative to classical radioactive nuclei acids labeling techniques.