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

我们发现人类MHC（HLA）是长距离G+C%嵌合结构的典型例子（1-3）。HLA从端粒到着丝粒由I类、III类和II类组成，并且连续的I类和III类（总共3 Mb）对应于明显富含GC的结构域，而II类（1 Mb）对应于具有降低的G+C%的结构域。为了精确定位和表征Mb级G+C%嵌合结构域的边界，进行了从最着丝粒的III类基因TN-X到II类基因和从最端粒的III类基因到III类基因的粘粒步行，获得了完全覆盖II类和III类基因边界的连续克隆<-><->。（L等容线）PAB样致密L1簇-致密Alu簇（H等容线）人类性染色体分为性特异性区和假常染色体区（PAR）。PAR的存在是由雄性减数分裂的观察推断的，当性染色体配对并形成交叉时，性特异性与PAR之间的界面是假常染色体边界（PAB，约400 nt），这是PAR中重组的着丝粒限制。有趣的是，类PAB序列的完全形式（约80%的核苷酸同一性）被发现在HLA的尖锐G+C%转换点附近。通过使用这种HLA探针的杂交，可以在人类基因组中检测到类似序列，并且可以通过对其中几个序列进行测序来确定PAB样序列的完整形式（约600 nt）。(1)J.Mol.Biol.203，1（1988）. (2)Genomics 8，207（1990）. (3)HLA 1991（Oxford Univ.Press）vol.2，125.

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.