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

我们发现人类MHC （HLA）是一个典型的远距离G+C%镶嵌结构（1-3）。HLA由从端粒到着丝粒的I、III和II类组成，连续的I和III类（共3 Mb）对应于明显富含gc的结构域，II类（1 Mb）对应于G+C%降低的结构域。为了精确定位和表征mb级G+C%镶嵌结构域的边界，我们进行了从着丝粒最多的III类基因TN-X到II类和从端粒最多的DRA到III类的cosmid行走，获得了完全覆盖结构域边界的连续克隆，连接了II类和III类。L <-> hisochore（可能是G <-> R波段）对应的长程G+C%拼接域的急剧G+C%跃迁可以精确定义，并揭示了以下组织。人类的性染色体分为性别特异性区和假常染色体区（PAR）。从男性减数分裂的观察中推断出PAR的存在，当性染色体配对并形成交叉时，性别特异性和PAR之间的界面是假常染色体边界（PAB，约400 nt），这是PAR重组的着丝粒极限。有趣的是，在HLA中G+C%的突变点附近发现了完整的PAB样序列（核苷酸同源性约80%）。利用该HLA探针进行杂交，可以在人类基因组中检测到大量的pab样序列，通过对其中的几个序列进行测序，可以确定pab样序列的完整形式（约600 nt）。(1) J.Mol.Biol.203 1(1988)。(2)基因组学8,207（1990）。(3) HLA1991（牛津大学出版社）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.