Solovei I, Gaginskaya E R, Macgregor H C
Department of Zoology, University of Leicester, UK.
Chromosome Res. 1994 Nov;2(6):460-70. doi: 10.1007/BF01552869.
The arrangement of loops and chromomeres at the ends of lampbrush chromosomes in four species of bird is described with reference to chromomeres, loops and transcription units. Unlike the situation described in lampbrush chromosomes of amphibians, the lampbrush chromosomes of birds end in a terminal chromosome with conspicuous loops emerging from it. The fine-scale morphology of the ribonuclear protein matrix of these terminal loops is different from that of the majority of loops elsewhere on the chromosomes. In many cases the loops associated with the terminal chromomere are open ended, emerging from the chromomere but not returning to it at the other end. The distal ends of terminal open-ended loops therefore represent the true ends of the chromatids that make up a lampbrush half-bivalent. The pattern of binding of three telomeric DNA sequence probes to the terminal regions of bird lampbrush chromosomes, under conditions of DNA/DNA and DNA/RNA transcript in situ hybridization has been investigated by fluorescence in situ hybridization. All three probes gave the same results. With DNA/DNA and DNA/RNA transcript hybridization, three classes of structure were labelled: the terminal chromomere, a small number of interstitial chromomeres and the terminal transcription unit on telomere loops. Labelling of telomere loops, but not of terminal or interstitial chromomeres, was eliminated by ribonuclease treatment before in situ hybridization. The labelled regions of telomere loops were spaced away from the labelled terminal chromomere by an unlabelled sub telomeric transcription unit. After DNA/DNA in situ hybridization, no labelled loops were seen. DNA/RNA transcript in situ hybridization with single-stranded hexamers of each strand of telomeric DNA showed that the terminal transcription unit on telomere loops represents transcription exclusively from the C-rich strand of the repeat outwards towards the end of the chromosome. It is concluded that transcription specifically of the C-rich strand of strictly terminal clusters of telomere repeats is an obligatory event on the lampbrush chromosomes of birds and is unlikely to represent indiscriminate readthrough from proximally located gene elements.
参考染色粒、袢环和转录单位，描述了四种鸟类灯刷染色体末端的袢环和染色粒的排列情况。与两栖动物灯刷染色体中描述的情况不同，鸟类的灯刷染色体末端是一个末端染色体，从其上伸出明显的袢环。这些末端袢环的核糖核蛋白基质的精细形态与染色体上其他大多数袢环的形态不同。在许多情况下，与末端染色粒相关的袢环是开放末端的，从染色粒中伸出但在另一端不回到染色粒。因此，末端开放末端袢环的远端代表构成灯刷半二价体的染色单体的真正末端。通过荧光原位杂交研究了三种端粒DNA序列探针在DNA/DNA和DNA/RNA转录原位杂交条件下与鸟类灯刷染色体末端区域的结合模式。所有三种探针都给出了相同的结果。在DNA/DNA和DNA/RNA转录杂交中，标记了三类结构：末端染色粒、少数居间染色粒和端粒袢环上的末端转录单位。在原位杂交前用核糖核酸酶处理消除了端粒袢环的标记，但末端或居间染色粒的标记未被消除。端粒袢环的标记区域通过未标记的亚端粒转录单位与标记的末端染色粒隔开。在DNA/DNA原位杂交后，未观察到标记的袢环。用端粒DNA每条链的单链六聚体进行DNA/RNA转录原位杂交表明，端粒袢环上的末端转录单位仅代表从富含C的重复链向染色体末端向外的转录。得出的结论是，鸟类灯刷染色体上严格末端的端粒重复序列的富含C的链的特异性转录是一个必然事件，不太可能代表从近端基因元件的随意通读。
