Characterization of four human YAC libraries for clone size, chimerism and X chromosome sequence representation.

Four collections of human X-specific YACs, derived from human cells containing supernumerary X chromosomes or from somatic cell hybrids containing only X human DNA were characterized. In each collection, 80-85% of YAC strains contained a single X YAC. Five thousand YACs from the various libraries were sized, and cocloning was assessed in subsets by the fraction of YAC insert-ends with non-X sequences. Cocloning was substantial, ranging up to 50% for different collections; and in agreement with previous indications, in all libraries the larger the YACs, the higher the level of cocloning. In libraries made from human-hamster hybrid cells, expected numbers of clones were recovered by STS-based screening; but unexpectedly, the two collections from cells with 4 or 5 X chromosomes yielded numbers of YACs corresponding to an apparent content of only about two X equivalents. Thus it is possible that the DNA of inactive X chromosomes is poorly cloned into YACs, speculatively perhaps because of its specialized chromatin structure.     

Integrated YAC/STS Physical and Genetic Map of 22.5 Mb of Human Xq24–q26 at 56-kb Inter-STS Resolution

A yeast artificial chromosome sequence-tagged site-based (YAC/STS) physical map of 22.5 Mb of the Xq24–q26 cytogenetic band region of the human X chromosome has been assembled. DNA coverage includes 857 large-insert clones formatted with 405 STSs to provide ninefold depth of DNA. At five points, no bridging clones have been recovered from 20 X-chromosome equivalents of human DNA in YACs or bacterial clones, but the placement of 25 (“CA”)_npolymorphic markers permits the ordering of contigs by comparison with the genetic linkage map and radiation hybrid data. The map localizes the X3000 translocation breakpoint and six genes (ANT2, NDUFA1, LAMP2, OCRL, IGSF1, and HDGF) at better than 100-kb resolution. The relatively gene-poor nature of the region is consistent with relatively low uniform 34–42% GC content in STSs across nearly all of the region.