Keall Rebecca, Whitelaw Sandra, Pettitt Jonathan, Müller Berndt
Institute of Medical Sciences, School of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen, Scotland, UK.
BMC Mol Biol. 2007 Jun 14;8:51. doi: 10.1186/1471-2199-8-51.
Histone protein synthesis is essential for cell proliferation and required for the packaging of DNA into chromatin. In animals, histone proteins are provided by the expression of multicopy replication-dependent histone genes. Histone mRNAs that are processed by a histone-specific mechanism to end after a highly conserved RNA hairpin element, and lack a poly(A) tail. In vertebrates and Drosophila, their expression is dependent on HBP/SLBP that binds to the RNA hairpin element. We showed previously that these cis and trans acting regulators of histone gene expression are conserved in C. elegans. Here we report the results of an investigation of the histone mRNA 3' end structure and of histone gene expression during C. elegans development.
Sequence analysis of replication-dependent histone genes revealed the presence of several highly conserved sequence elements in the 3' untranslated region of histone pre-mRNAs, including an RNA hairpin element and a polyadenylation signal. To determine whether in C. elegans histone mRNA 3' end formation occurs at this polyadenylation signal and results in polyadenylated histone mRNA, we investigated the mRNA 3' end structure of histone mRNA. Using poly(A) selection, RNAse protection and sequencing of histone mRNA ends, we determined that a majority of C. elegans histone mRNAs lack a poly(A) tail and end three to six nucleotides after the hairpin structure, after an A or a U, and have a 3' OH group. RNAi knock down of CDL-1, the C. elegans HBP/SLBP, does not significantly affect histone mRNA levels but severely depletes histone protein levels. Histone gene expression varies during development and is reduced in L3 animals compared to L1 animals and adults. In adults, histone gene expression is restricted to the germ line, where cell division occurs.
Our findings indicate that the expression of C. elegans histone genes is subject to control mechanisms similar to the ones in other animals: the structure of C. elegans histone mRNA 3' ends is compatible with histone-specific mRNA 3' end processing; CDL-1 functions in post-transcriptional control of histone gene expression; and C. elegans histone mRNA levels are elevated at periods of active cell division, indicating that histone gene expression is linked to DNA replication.
组蛋白合成对于细胞增殖至关重要，并且是将DNA包装成染色质所必需的。在动物中，组蛋白由多拷贝复制依赖性组蛋白基因的表达提供。组蛋白mRNA通过组蛋白特异性机制加工，在高度保守的RNA发夹元件后终止，并且缺乏聚腺苷酸尾。在脊椎动物和果蝇中，它们的表达依赖于与RNA发夹元件结合的HBP/SLBP。我们之前表明，这些组蛋白基因表达的顺式和反式作用调节因子在秀丽隐杆线虫中是保守的。在此，我们报告了对秀丽隐杆线虫发育过程中组蛋白mRNA 3'末端结构和组蛋白基因表达的研究结果。
对复制依赖性组蛋白基因的序列分析揭示了组蛋白前体mRNA的3'非翻译区中存在几个高度保守的序列元件，包括一个RNA发夹元件和一个聚腺苷酸化信号。为了确定秀丽隐杆线虫中组蛋白mRNA 3'末端形成是否在这个聚腺苷酸化信号处发生并导致聚腺苷酸化的组蛋白mRNA，我们研究了组蛋白mRNA的mRNA 3'末端结构。使用聚腺苷酸选择、RNA酶保护和组蛋白mRNA末端测序，我们确定大多数秀丽隐杆线虫组蛋白mRNA缺乏聚腺苷酸尾，在发夹结构后三到六个核苷酸处终止，在A或U之后，并具有3'羟基。对秀丽隐杆线虫HBP/SLBP的CDL-1进行RNA干扰敲低不会显著影响组蛋白mRNA水平，但会严重消耗组蛋白水平。组蛋白基因表达在发育过程中有所变化，与L1动物和成虫相比，L3动物中的表达降低。在成虫中，组蛋白基因表达仅限于发生细胞分裂的生殖系。
我们的研究结果表明，秀丽隐杆线虫组蛋白基因的表达受到与其他动物类似的控制机制的调控：秀丽隐杆线虫组蛋白mRNA 3'末端的结构与组蛋白特异性mRNA 3'末端加工兼容；CDL-1在组蛋白基因表达的转录后控制中起作用；秀丽隐杆线虫组蛋白mRNA水平在活跃细胞分裂时期升高，表明组蛋白基因表达与DNA复制相关。
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