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 D NA fragments of high specific labeling density that can be used for a varie ty of applications. The protocol is based on the universal mono-functional platinium reagent for chemical digoxigenin (DIG) labeling of nucleic acids. The labeling protocol was optimized for large DNA templates as complete ba cterial artificial chromosomes (BAC). Variations of incubation time and tem perature improved the labeling density such that about 30% of the nucleotid es were DIG-modified within 30 min. Furthermore, the refined procedure gene rates in a single-tube reaction and without prior digestion-labeled DNA fra gments 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 sing le copy genes in 2.5 mu g genomic DNA from Arabidopsis thaliana, which has a haploid genome size of 0.13 pg (ca. 120 Mb) and medium expressed transcri pts from 0.8 mu g poly(A)(+) RNA, respectively. The extremely high specific labeling density, the stability and the universal application of the probe generated with the platinium reagent makes this method a useful alternativ e to classical radioactive nuclei acids labeling techniques.