AN INTEGRATED YAC-OVERLAP AND COSMID-POCKET MAP OF THE HUMAN-CHROMOSOME-21

We describe here the construction of an ordered done map of human chro mosome 21, based on the identification of ordered sets of YAC clones c overing > 90% of the chromosome, and their use to identify groups of c osmid clones (cosmid pockets) localised to subregions defined by the Y AC clone map. This is to our knowledge the highest resolution map of o ne human chromosome to date, localising 530 VAC clones covering both a rms of the chromosome, spanning > 36 Mbp, and localising more than 630 0 cosmids to 145 intervals on both arms of the chromosome. The YAC con tigs have been formed by hybridising a 6.1 equivalents chromosome 21 e nriched YAC collection displayed on arrayed nylon membranes to a serie s of 115 DNA markers and Alu-PCR products from YACs. Forty eight mega- YACs from the previously published CEPH-Genethon map of sequence tagge d sites (STS)have also been included in the contig building experiment s. A YAC tiling path was then size-measured and confirmed by gel-finge rprinting, A minimal tiling path of 70 YACs were then used as probes a gainst the 7.5 genome equivalents flow sorted chromosome 21 cosmid lib rary in order to identify the lists of cosmids mapping to alternating shared - non-shared intervals between overlapping YACs ('cosmid pocket s'). For approximately 1/5 of the minimal tiling path of YACs, locatio ns and non-chimaerism have been confirmed by fluorescence in situ hybr idisation (FISH), and approximately 1/5 of all cosmid pocket assignmen ts have independent, confirmatory marker hybridisations in the ICRF co smid reference library system. We also demonstrate that 'pockets' cont ain overlapping sets of cosmids (cosmid contigs). In addition to being an important logical intermediate step between the YAC maps published so far and a future map of completely ordered cosmids, this map provi des immediately available low-complexity cosmid material for high reso lution FISH mapping of chromosomal aberrations on interphase nuclei, a nd for rapid positional isolation of transcripts in the highly resolve d regions of genetic interest.