Isolation and regional localization of cosmid linking clones from human chromosome 12

We have developed a new method for constructing cosmid linking libraries. The method is based on the insertion of a selection gene, beta-lactamase, in genomic cosmid clones containing recognition sites for rare-cutting enzymes. The selection gene is maintained as a gene cassette in a plasmid and may be excised by the enzymes NotI, SacII, SplI, MluI, BssHII, and NarI or combinations of these enzymes. Using this gene cassette and a genomic cosmid library made from a human-hamster cell line containing the human chromosome 12 as its only human component, a chromosome 12-specific NotI linking library was constructed. The NotI linking clones contained recognition sites for other rate-cutting enzymes, SacII and BssHII, at high frequency, indicating the presence of CpG islands. Thirty cosmid linking clones were regionally localized by FISH and were found to be clustered to chromosome bands 12p13, 12q13, and 12q24.