Physicochemical studies of the folding of the 100 A nucleosome filament into the 300 A filament. Cation dependence.
Physicochemical studies of the folding of the 100 A nucleosome filament into the 300 A filament. Cation dependence.
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DOI：
10.1016/0022-2836(86)90012-4
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发表时间：
1986-08
影响因子：
5.6
通讯作者：
J. Widom
中科院分区：
文献类型：
--
作者：
J. Widom
在Na+、Mg ~（2+）、Co（NH_3）_（63+）和其它阳离子的浓度范围内，研究了阳离子诱导100 μ m核小体丝重折叠成300 μ m核小体丝的过程。X-射线衍射、电子显微镜和分析超离心法已被用来确定形成300微米细丝的条件。它表明，阳离子诱导染色质折叠作为一般的DNA抗衡离子。诱导重折叠所需的任何阳离子的浓度在很大程度上取决于该阳离子的价态。Na+（和，据推测，其他单价阳离子）具有双重作用：在高浓度（大于45至65 mm）它稳定了染色质的300 nm细丝状态;然而，在低浓度下，（小于1.45 mm），当存在更高价的阳离子并稳定300 μ m的细丝状态时，Na+具有相反的作用，与高价阳离子竞争结合染色质并使300 μ m的丝状态不稳定。结果表明，进一步添加阳离子染色质在300 μ m的细丝状态，导致进一步折叠的染色质中的沉降系数增加和X射线衍射谱带从核小体包装锐化。这可能反映了300 nm细丝内的细微结构变化，也可能反映了在100 nm和300 nm细丝状态之间波动的染色质平衡常数的变化。它还表明，与继续添加阳离子，300微米的细丝沉淀之前，任何“终点”是在这进一步折叠达到。300条微丝在体外聚集的趋势似乎是一种内在的特性，并可能反映了300条微丝在体内中期染色体中的堆积。
The cation-induced refolding of the 100 Å nucleosome filament into the 300 Å filament has been studied over a wide range of concentrations of Na+, Mg2+, Co(NH3)63+and other cations. X-ray diffraction, electron microscopy and analytical ultracentrifugation have been used to determine the conditions under which the 300 Å filament is formed. It is shown that cations induce chromatin refolding by acting as general DNA counterions. The concentration of any cation required to induce refolding is greatly dependent on the valence of that cation. Na+(and, presumably, other monovalent cations) has dual effects: at high concentrations (greater than 45 to 65 mm) it stabilizes the 300 Å filament state of chromatin; however, at low concentrations (less than ≈45mm), when cations of higher valence are present and stabilizing the 300 Å filament state, Na+has the opposite effect, competing with the higher-valence cation for binding to the chromatin and destabilizing the 300 Å filament state. It is shown that further addition of cations to chromatin in the 300 Å filament state causes a further folding of the chromatin in which the sedimentation coefficient increases and the X-ray diffraction bands resulting from nucleosomal packing sharpen. This may reflect subtle structural changes within the 300 Å filament, or it may reflect a shift in equilibrium constant for chromatin fluctuating between the 100 Å and 300 Å filament states. It is also shown that, with continued addition of cation, the 300 Å filaments precipitate before any “endpoint” is reached in this further folding. The tendency of 300 Å filaments to aggregatein vitroappears to be a built-in property, and may reflect the packing of 300 Å filaments within metaphase chromosomesin vivo.