Endo Masaki, Sugai Toshiro
Department of Biology, College of Science, lbaraki University, 2-1-1, Bunkyo, Mito 310-8512, Japan.
Zoolog Sci. 2011 Jul;28(7):482-90. doi: 10.2108/zsj.28.482.
The macronucleus of the ciliate Tetrahymena cell contains euchromatin and numerous heterochromatins called chromatin bodies. During cell division, a chromatin aggregate larger than chromatin body appears in the macronucleus. We observed chromatin aggregates in the dividing macronucleus in a living T. thermophila cell, and found that these were globular in morphology and homogeneous in size. To observe globular chromatin clearly, optimal conditions for making it compact were studied. Addition of Mg ion, benomyl and oryzalin, microtubule inhibitors, to cell suspension was effective. Globular chromatin appeared when the micronuclear anaphase began at the cell cortex, and disappeared long after cell separation. Using living cells with a small macronucleus at early log phase, we counted the number of globular chromatin per nucleus and measured the DNA content of globular chromatin in the macronucleus which was stained with Hoechst 33342 by using ImageJ. The number of globular chromatin per nucleus was reduced by half after division, indicating the globular chromatin is a distribution unit of DNA. A globular chromatin contained similar DNA content as that of the macronuclear genome. We developed methods for inducing and isolating a cell with an extremely small macronucleus with a DNA amount of one globular chromatin. These cells grew, divided, and give clones, suggesting that the macronuclear genome is not dispersed within the macronucleus and the globular chromatin may be a macronuclear genome. We named this globular chromatin "macronuclear genome unit" (MGU).
纤毛虫四膜虫细胞的大核包含常染色质和许多被称为染色质体的异染色质。在细胞分裂期间，大核中会出现一种比染色质体更大的染色质聚集体。我们在活的嗜热栖热菌细胞的分裂大核中观察到了染色质聚集体，发现它们形态呈球形且大小均匀。为了清晰地观察球形染色质，研究了使其致密化的最佳条件。向细胞悬液中添加镁离子、苯菌灵和oryzalin（微管抑制剂）是有效的。当微核后期在细胞皮层开始时，球形染色质出现，并在细胞分离很久后消失。使用处于对数早期、大核较小的活细胞，我们通过ImageJ软件对每个细胞核中球形染色质的数量进行计数，并测量用Hoechst 33342染色的大核中球形染色质的DNA含量。每个细胞核中球形染色质的数量在分裂后减少了一半，这表明球形染色质是DNA的一个分布单位。一个球形染色质所含的DNA量与大核基因组的相似。我们开发了诱导和分离具有极小大核且DNA量为一个球形染色质的细胞的方法。这些细胞生长、分裂并产生克隆，这表明大核基因组并非分散在大核内，并且球形染色质可能是大核基因组。我们将这种球形染色质命名为“大核基因组单位”（MGU）。
