Lowell Joanna E, Cross George A M
Laboratory of Molecular Parasitology, The Rockefeller University, 1230 York Avenue, New York, NY 10021, USA.
J Cell Sci. 2004 Nov 15;117(Pt 24):5937-47. doi: 10.1242/jcs.01515. Epub 2004 Nov 2.
Variant histones play critical roles in transcriptional activation and repression, DNA repair and chromosome segregation. We have identified HTV, a single-copy gene in Trypanosoma brucei encoding a variant form of histone H3 (H3V). H3V is present at discrete nuclear foci that shift over the course of the cell cycle and associate with the mitotic spindle, a pattern of localization reminiscent of that described previously for both mini-chromosomes and telomeres. By combining fluorescence in situ hybridization with indirect immunofluorescence, we confirmed that the H3V foci overlap with a 177-bp repetitive sequence element found predominantly in mini-chromosomes, as well as with the TTAGGG repeats that compose telomeres. Chromatin immunoprecipitation studies, however, reveal that only the telomeric repeat DNA is substantially enriched with H3V. HTV is not essential for viability, mini-chromosome segregation, telomere maintenance or transcriptional silencing at the telomere-proximal expression sites from which bloodstream-form T. brucei controls antigenic variation. We propose that H3V represents a novel class of histone H3 variant, a finding that has evolutionary implications.
变体组蛋白在转录激活与抑制、DNA修复以及染色体分离过程中发挥着关键作用。我们在布氏锥虫中鉴定出了HTV，它是一个单拷贝基因，编码组蛋白H3的一种变体形式（H3V）。H3V存在于离散的核灶中，这些核灶在细胞周期进程中会发生移动，并与有丝分裂纺锤体相关联，这种定位模式让人联想到之前针对微型染色体和端粒所描述的模式。通过将荧光原位杂交与间接免疫荧光相结合，我们证实H3V核灶与主要存在于微型染色体中的一个177碱基对的重复序列元件以及构成端粒的TTAGGG重复序列重叠。然而，染色质免疫沉淀研究表明，只有端粒重复DNA大量富集了H3V。HTV对于布氏锥虫血流形式控制抗原变异的端粒近端表达位点处的生存能力、微型染色体分离、端粒维持或转录沉默并非必不可少。我们提出H3V代表了一类新型的组蛋白H3变体，这一发现具有进化意义。
