Sanz Cristina, Gorab Eduardo, Ruiz Maria Fernanda, Sogo José Manuel, Díez José Luís
Departamento de Biología Celular y del Desarrollo, Centro de Investigaciones Biológicas, CSIC, Ramiro de Maeztu 9, 28040, Madrid, Spain.
Chromosome Res. 2007;15(4):429-38. doi: 10.1007/s10577-007-1134-1. Epub 2007 May 15.
In eukaryotes the ribosomal gene population shows two different states in terms of chromatin structure. One subset is organized as nucleosomes (silent copies) while the other has a non-nucleosomal configuration (active copies). Insect cells are not the exception and this bimodal distribution of ribosomal chromatin also occurs in salivary gland cells, and cells of other larval tissues, of the midge Chironomus thummi. In run-on experiments on salivary glands cells we confirmed that transcribed rRNA genes show a non-nucleosomal configuration. The proportion of rRNA genes adopting an open, non-nucleosomal configuration was found to be tissue-dependent, suggesting that the population of unfolded ribosomal chromatin in C. thummi was established during cell differentiation. We propose that cell differentiation determines the fraction of non-nucleosomal rRNA gene copies and thus defines the range of possible rRNA synthesis rates in a particular cell type. In the salivary gland the fraction of unfolded chromatin was not significantly affected when transcription was repressed. However, transcription activation by pilocarpine led to a moderate increase in this fraction. These findings indicate that, in addition to a possible increase in the number of RNA-polymerases per transcribing rDNA unit, the proportion of transcribed ribosomal genes in differentiated cells can be modulated in response to an exceptional rRNA synthesis requirement.
在真核生物中，核糖体基因群体在染色质结构方面呈现出两种不同状态。其中一个亚群被组织成核小体（沉默拷贝），而另一个则具有非核小体构型（活性拷贝）。昆虫细胞也不例外，这种核糖体染色质的双峰分布也出现在摇蚊Chironomus thummi的唾液腺细胞和其他幼虫组织的细胞中。在对唾液腺细胞进行的连续转录实验中，我们证实转录的rRNA基因呈现非核小体构型。发现采用开放的、非核小体构型的rRNA基因比例是组织依赖性的，这表明Chironomus thummi中未折叠的核糖体染色质群体是在细胞分化过程中建立的。我们提出，细胞分化决定了非核小体rRNA基因拷贝的比例，从而定义了特定细胞类型中可能的rRNA合成速率范围。在唾液腺中，当转录被抑制时，未折叠染色质的比例没有受到显著影响。然而，毛果芸香碱诱导的转录激活导致这一比例适度增加。这些发现表明，除了每个转录的rDNA单元中RNA聚合酶数量可能增加外，分化细胞中转录的核糖体基因比例可以根据特殊的rRNA合成需求进行调节。
