Chromosome bands and giant G+C% mosaic structures of higher vertebrates genome

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We found human MHC (HLA) is a typical example of long-range G+C% mosaic structures (1-3). HLA is composed of classes I, III and II from telomere to centromere and contiguous classes I and III (a total of 3 Mb) correspond to evidently GC-rich domain and class II (1 Mb), to the domain with reduced G+C%. To precisely locate and characterize boundary of the Mb-level G+C% mosaic domains, cosmid walking from the most centromeric class III gene TN-X to class II and that from the most telomeric DRA to class III were conducted and contiguous clones totally covering the domain boundary were obtained bridging classes II and III.A sharp G+C% transition of the long-range G+C% mosaic domains corresponding to L <-> Hisochores (and presumably G <-> R bands) could be precisely defined, and the following organization was revealed.(L isochore) PAB-like - Dense L1 cluster - Dense Alu cluster (H isochore)Human sex chromosomes are divided into sex-specific and pseudoautosomal regions (PAR). Existence of PAR was deduced from observations of male meiosis, when sex chromosomes pair and form chiasmata, interface between sex-specific and PAR is the pseudoautosomal boundary (PAB, about 400 nt) that is the centromeric limit to recombination in PAR.Interestingly, complete form of PAB-like sequence (about 80% nucleotide identity) was found near the sharp G+C% transition point in HLA.A large number of PAB-like sequences could be detected in the human genome by hybridization using this HLA probe and the complete form of PAB-like sequence (about 600 nt) could be defined by sequencing several of them. (1) J.Mol.Biol.203,1 (1988). (2) Genomics 8,207 (1990). (3) HLA1991 (Oxford Univ.Press) vol.2, 125.