Curriculum in Genetics and Molecular Biology, University of North Carolina, Chapel Hill, NC 27599.
Department of Biology, Emory University, Atlanta, GA 30322.
Mol Biol Cell. 2020 Jul 1;31(14):1525-1537. doi: 10.1091/mbc.E20-03-0176. Epub 2020 May 13.
The histone locus body (HLB) assembles at replication-dependent (RD) histone loci and concentrates factors required for RD histone mRNA biosynthesis. The genome has a single locus comprised of ∼100 copies of a tandemly arrayed 5-kB repeat unit containing one copy of each of the 5 RD histone genes. To determine sequence elements required for HLB formation and histone gene expression, we used transgenic gene arrays containing 12 copies of the histone repeat unit that functionally complement loss of the ∼200 endogenous RD histone genes. A 12x histone gene array in which all promoters were replaced with promoters (12x) does not form an HLB or express high levels of RD histone mRNA in the presence of the endogenous histone genes. In contrast, this same transgenic array is active in HLB assembly and RD histone gene expression in the absence of the endogenous RD histone genes and rescues the lethality caused by homozygous deletion of the RD histone locus. The HLB formed in the absence of endogenous RD histone genes on the mutant 12x array contains all known factors present in the wild-type HLB including CLAMP, which normally binds to GAGA repeats in the promoter. These data suggest that multiple protein-protein and/or protein-DNA interactions contribute to HLB formation, and that the large number of endogenous RD histone gene copies sequester available factor(s) from attenuated transgenic arrays, thereby preventing HLB formation and gene expression on these arrays.
组蛋白基因座体（HLB）在依赖复制的（RD）组蛋白基因座处组装，并浓缩 RD 组蛋白 mRNA 生物合成所需的因素。该基因组具有一个单一的基因座，由约 100 个串联排列的 5-kB 重复单元组成，每个重复单元包含每个 RD 组蛋白基因的一个拷贝。为了确定形成 HLB 和组蛋白基因表达所需的序列元件，我们使用了含有 12 个组蛋白重复单元的转基因基因阵列，该单元可在功能上补充约 200 个内源性 RD 组蛋白基因的缺失。一个将所有启动子替换为 启动子的 12x 组蛋白基因阵列（12x）在存在内源性组蛋白基因的情况下不会形成 HLB 或表达高水平的 RD 组蛋白 mRNA。相比之下，在缺乏内源性 RD 组蛋白基因的情况下，相同的转基因阵列在 HLB 组装和 RD 组蛋白基因表达中是活跃的，并挽救了 RD 组蛋白基因座纯合缺失引起的致死性。在突变的 12x 阵列上，在缺乏内源性 RD 组蛋白基因的情况下形成的 HLB 包含所有存在于野生型 HLB 中的已知因子，包括通常与 启动子中的 GAGA 重复结合的 CLAMP。这些数据表明，多个蛋白质-蛋白质和/或蛋白质-DNA 相互作用有助于 HLB 的形成，并且大量的内源性 RD 组蛋白基因拷贝会从减弱的转基因阵列中隔离可用的因子，从而防止 HLB 的形成和这些阵列上的基因表达。
