Different chromatin states of the intron? and type 1 intron+ rRNA genes of Calliphora erythrocephala

Abstract

In most species of dipteran fly examined, a fraction of the rDNA cistrons are interrupted by introns. These dipteran intron+ rRNA genes are unique in that they are transcriptionally inactive. Previous studies have investigated the mechanism underlying this transcriptional repression for rRNA genes carrying the best characterized sequence family of such introns, the so-called type 1 introns first identified in Drosophila melanogaster. These studies have established that cloned examples of both intron-free and type 1 intron+ rRNA genes will support transcription in a cell-free system and suggest therefore that a difference in the chromatin state of the two gene types must underlie their very different potential for in vivo transcription. We have examined this possibility for the type 1 intron+ rDNA cistrons of Calliphora erythrocephala by in situ hybridization studies using the polytene chromosome complement of the pupal bristle-forming (trichogen) cells. These studies show that the chromatin configuration of the two gene types is strikingly different. The intron-free genes are preferentially localized in the actively transcribed fibrillar center of the nucleolus. The intron+ genes are preferentially condensed in the blocks of heterochromatin attached to the nucleolus.

Ribosomal RNA genes in mosquitoes: localization by fluorescence in situ hybridization (FISH)

“…In Rhynchosciara and other Drosophila flies, in addition to the sex chromosomes, rRNA genes are also present on heterochromatic regions of other chromosomes (Gambarini & Lara, 1974;Hägele & Ranganath, 1983). A different pattern is found in species of Callyphora and Pseudodiamesia (Chironomidae) where rRNA genes are located on tiny dot chromosomes, largely heterochromatic (Beckingham & Rubacha, 1984;Zacharias, 1984). However, at least in Drosophila, these dot chromosomes show close affinity with the X chromosome suggesting a derivation of dot chromosomes from sex chromosomes or vice versa (Sinibaldi & Cummings, 1981).…”

Reduced DNA polytenization of a minichromosome region undergoing position-effect variegation in Drosophila

“…However, we strongly suspect that this was the case in at least some of the affected tissues. Bristles are produced by large polytene pupal trichogen cells that contain banded polytene chromosomes with reduced amounts of centromeric heterochromatin (Ribbert, 1979;Beckingham and Rubacha, 1984). It was not possible to correlate directly the copy number of functional yellow genes with the level of yellow expression in individual trichogen cells, because these cells constitute a small fraction of the developing epidermis and degenerate during late pupation prior to the completion of pigmentation.…”

Ribosomal RNA Genes of the Anopheles gambiae Species Complex

“…Most studies suggest that IS containing (IS+) rDNA genes are transcriptionally silent or produce only low levels of aborted transcripts, and some IS+ genes may be clustered in tandem blocks separate from the transcriptionally active, IS free (lS-) genes (4,5,44) . The IS+ genes can be quite abundant.…”