Chromatin structure of ribosomal genes in Chironomus thummi (Diptera: Chironomidae): tissue specificity and behaviour under drug treatment

Abstract

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.

Chromatin Structure of Ribosomal RNA Genes in Dipterans and Its Relationship to the Location of Nucleolar Organizers

“…In contrast to the results observed in C. riparius [13] and in agreement with those obtained in R. americana , bimodal distribution of restriction fragments from the rDNA chromatin of S. coprophila was not observed after many attempts. A single band migrating as nucleosomal rDNA was detected even when massive amounts of DNA from the chromatin were used in Southern-blots (Figure 3B).…”

“…The same results were obtained when larvae from distinct developmental periods were used in the experiments. Again, controls done in parallel with chromatin DNA from the salivary gland of C. riparius , using the same equipments and reagents, showed two rDNA bands representing two different chromatin structures as demonstrated previously [13]. The controls thus allowed us to rule out any technical problem in the assays performed with this species.…”

“…On the other hand, use of the same method in C. riparius cells showed measurable, significantly different tissue-dependent proportions of rDNA free from nucleosomes and representing transcriptionally active rDNA chromatin [13].…”

Deletion of Rnt1p Alters the Proportion of Open versus Closed rRNA Gene Repeats in Yeast

“…In contrast, nucleosomes are present on inactive rRNA genes (44,61). These two forms of chromatin have been observed in a number of organisms, ranging from yeast to mammals, by a technique that is based on psoralen photo-cross-linking (58,65). Furthermore, electron micrographs of psoralen-cross-linked rRNA gene chromatin and biochemical studies monitoring DNA accessibility to nuclease-like DNase I, micrococcal nuclease, and restriction enzymes (8,13,42,43,48,49,60) have confirmed the absence of canonical nucleosomes from the coding regions of active rRNA genes in all of the organisms investigated.…”

Chromosomal organization of the ribosomal RNA genes in the genus Chironomus (Diptera, Chironomidae)

“…Along with transcriptionally active copies of ribosomal genes, free of nucleosomes, populations of these genes also contain transcriptionally inactive copies displaying nucleosome organization. The share of transcriptionally active copies in the population of ribosomal genes is tissue-specific, amounting to 80% in the fat body cells, to 50% in the salivary glands, and only 20% in the Malpighian tube cells (Sanz et al 2007). An analogous ratio is observed in the Chironomus tentans salivary gland cells, where 40% of the ribosomal genes are in a transcriptionally active state (Madalena et al 2012).…”