Saccone S, Cacciò S, Kusuda J, Andreozzi L, Bernardi G
Laboratoire de Génétique Moléculaire, Institut Jacques Monod, Paris, France.
Gene. 1996 Sep 26;174(1):85-94. doi: 10.1016/0378-1119(96)00392-7.
The human genome is a mosaic of isochores, long DNA segments which are compositionally homogeneous and which can be partitioned into five families, L1, L2, H1, H2 and H3, characterized by increasing GC levels and by increasing gene concentrations. Previous investigations showed that in situ hybridization with a DNA fraction derived from the GC-richest and gene-richest isochores of the H3 family produced the highest concentration of signals on 25 R(everse) bands that include the 22 most thermal-denaturation-resistant T(elomeric) bands, a subset of R bands. Using an improved protocol for in situ hybridization and cloned H3 isochore DNA, we have now shown (i) that the number of bands which are characterized by strong hybridization signals, and which are here called T or H3+, is 28; (ii) that 31 additional R bands, here called T'or H3* bands, also contain H3 isochores, although at a lower concentration than H3+ bands; and (iii) that the remaining R bands (about 140 out of 200, at a resolution of 400 bands), here called R" or H3- bands, do not contain any detectable H3 isochores. H3+ and H3* bands contain all the gene-richest isochores of the human genome. The existence of three distinct sets of R bands is further supported (i) by the different compositional features of genes located in them; (ii) by the very low gene density of chromosomes 13 and 18, in which all R bands are H3- bands; (iii) by the compositional map of a H3* band, Xq28; (iv) by the overwhelming presence of GC-rich and GC-poor long (> 50 kb) DNA sequences in H3+/H3* and in H3-/G bands, respectively; and (v) by the large degree of coincidence of H3+ and H3* bands with CpG island-positive bands. These observations have implications for our understanding of the causes of chromosome banding and provide a classification of chromosomal bands that is related to GC level (and to gene concentration).
人类基因组是由等密度区带构成的镶嵌体，等密度区带是指组成上均一的长DNA片段，可分为L1、L2、H1、H2和H3五个家族，其特征是GC含量增加且基因浓度增加。先前的研究表明，用源自H3家族中GC含量最高且基因最丰富的等密度区带的DNA片段进行原位杂交，在25条R（反向）带（包括22条最耐热变性的T（端粒）带，R带的一个子集）上产生的信号浓度最高。使用改进的原位杂交方案和克隆的H3等密度区带DNA，我们现在已经表明：（i）以强杂交信号为特征且在此称为T或H3 +的带的数量为28条；（ii）另外31条R带，在此称为T'或H3 *带，也含有H3等密度区带，尽管其浓度低于H3 +带；（iii）其余的R带（在400条带的分辨率下，200条带中约140条），在此称为R"或H3 -带，不包含任何可检测到的H3等密度区带。H3 +和H3 *带包含人类基因组中所有基因最丰富的等密度区带。三组不同的R带的存在进一步得到以下支持：（i）位于其中的基因具有不同的组成特征；（ii）13号和18号染色体的基因密度非常低，其中所有R带都是H3 -带；（iii）H3 *带Xq28的组成图谱；（iv）H3 + / H3 *带和H3 - / G带中分别大量存在富含GC和贫GC的长（> 50 kb）DNA序列；（v）H3 +和H3 *带与CpG岛阳性带高度重合。这些观察结果对我们理解染色体带型形成的原因具有启示意义，并提供了一种与GC水平（以及基因浓度）相关的染色体带分类方法。
