生物学教科书需要修改，加州大学圣地亚哥分校的生物学家发现了从未注意到的遗传组分。

发现核小体的前体

根据教科书，染色质由核小体构成，在细胞中是DNA的自然存在形式。

高倍显微镜下观察核小体看起来像串珠，圣地亚哥分校的生物学家发现了界于DNA和核小体之间新的中间体颗粒。它虽然看起来像核小体，但是具有独特的结构。这一发现发表在8月19日的Molecular Cell期刊上。

圣地亚哥分校的生物学教授James Kadonaga 称，这一颗粒是核小体的前体，能引起我们对染色质的重新认识。在遗传物质的复制和表达中，它很可能是一个重要组分。

生物学家称，在显微镜下核小体的前体看起来像核小体，事实上，它界于DNA和核小体之间。

研究人员称，前体向核小体转变的过程中，动力蛋白利用ATP分子获得能量。

Kadonaga称，核小体前体的发现表明染色质可能由核小体和前体共同组成，改变了之前的错误认识—只有核小体组成。这一发现可能开启了我们对染色质的本质认识。

英文摘要

Identification of a Rapidly Formed Nonnucleosomal Histone-DNA Intermediate that Is Converted into Chromatin by ACF

Chromatin assembly involves the combined action of histone chaperones and ATP-dependent motor proteins. Here, we investigate the mechanism of nucleosome assembly with a purified chromatin assembly system containing the histone chaperone NAP1 and the ATP-dependent motor protein ACF. These studies revealed the rapid formation of a stable nonnucleosomal histone-DNA intermediate that is converted into canonical nucleosomes by ACF. The histone-DNA intermediate does not supercoil DNA like a canonical nucleosome, but has a nucleosome-like appearance by atomic force microscopy. This intermediate contains all four core histones, lacks NAP1, and is formed by the initial deposition of histones H3-H4. Conversion of the intermediate into histone H1-containing chromatin results in increased resistance to micrococcal nuclease digestion. These findings suggest that the histone-DNA intermediate corresponds to nascent nucleosome-like structures, such as those observed at DNA replication forks. Related complexes might be formed during other chromatin-directed processes such as transcription, DNA repair, and histone exchange.