CHARACTERIZATION OF 4 HUMAN YAC LIBRARIES FOR CLONE SIZE, CHIMERISM AND X-CHROMOSOME SEQUENCE REPRESENTATION

Four collections of human X-specific YACs, derived from human cells co ntaining supernumerary X chromosomes or from somatic cell hybrids cont aining only X human DNA were characterized. In each collection, 80 - 8 5% of YAC strains contained a single X YAC. Five thousand YACs from th e 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 agre ement with previous indications, in all libraries the larger the YACs, the higher the level of cocloning. In libraries made from human - ham ster hybrid cells, expected numbers of clones were recovered by STS-ba sed screening; but unexpectedly, the two collections from cells with 4 or 5 X chromosomes yielded numbers of YACs corresponding to an appare nt content of only about two X equivalents. Thus it is possible that t he DNA of inactive X chromosomes is poorly cloned into YACs, speculati vely perhaps because of its specialized chromatin structure.