Nucleolar localization of a reverse transcriptase related to telomere maintenance in Chironomus (Diptera)

Abstract

A growing number of cellular processes originally thought not to involve the nucleolus now seem to be associated with this organelle. In recent years, a variety of RNAs and proteins with no apparent function in ribosome genesis have been discovered in this nuclear compartment. This paper reports the presence in the nucleolus of a reverse transcriptase (RT) previously found to be associated with telomeres in Chironomus. Immunofluorescence detection using a specific antibody against conserved domains shared by RTs showed a distinct pattern of staining in the giant nucleoli of polytenized cells. This nucleolar localization was confirmed in a number of larval tissues and embryonic cells of Chironomus thummi and C. pallidivitatus; its distribution showed a definite necklace pattern that did not completely colocalize with fibrillarin or nucleolin and appeared to be different to that of typical nucleolar components. There is evidence that both telomerase RT and RNA template subunits are present in the nucleoli of mammalian and yeast cells. However, chironomids do not have typical telomeres or telomerase. As in other Diptera, telomeres lack the short, simple repeats maintained by telomerase and instead have more complex sequences in the range of hundreds of nucleotides. It has been suggested that the RT associated with these telomeres might be involved in their maintenance, perhaps involving a mechanism similar to that of telomerase retrotranscription and retrotransposition in Drosophila. The present results indicate that the putative Chironomus telomere elongation machinery and telomerase share a nucleolar localization. This reinforces the idea that nucleoli are functionally linked to telomere maintenance irrespective of the differences in their molecular organization and therefore in the strategy adopted for their elongation.

Recent progress in histochemistry

“…They could establish a relationship between the increase in nucleolar activity, cell-cycle progression, nucleolar structure, the activity of NopA100, and the proteolysis of this nucleolin-like protein. The studies by Diez et al (2006) on a reverse transcriptase in the nucleolus indicated that the putative Chironomus telomere elongation machinery and telomerase share a nucleolar localization and reinforced the idea that nucleoli are functionally linked to telomere maintenance.…”

Transcription and activation under environmental stress of the complex telomeric repeats of Chironomus thummi

“…The possibility cannot be excluded that, besides a regulatory or structural role for Chironomus telomeres, these RNAs represent, at least in part, an intermediate in the retrotranscription processes for telomere reposition, because the lack of telomerase (containing its own internal template) requires a template in cis for the retrotranscriptase. It is worth mentioning that we have previously reported the existence of reverse transcriptase-like proteins associated with Chironomus telomeres (López et al 1999, Díez et al 2006, suggesting the possibility that an RNA-templated mechanism could also be involved in Chironomus telomere maintenance. In this hypothesis, retrotranscription-based mechanisms would become the universal model of telomere maintenance, although alternative recombination mechanisms have been found as back-up or emergency mechanisms, even in organisms with standard telomerase telomeres.…”

“…The complex repetitive sequences found in Chironomus telomeres must be functionally homologous to the other types of telomeric DNA and, most probably, are involved in the telomere-maintenance mechanism of these organisms. The localization of reverse transcriptase in Chironomid telomeres (López et al 1999, Gorab 2003, Díez et al 2006 suggests that retrotranscription processes could also be involved in the maintenance mechanism of these atypical telomeres. In addition, Ku and HSP90 proteins, related to telomerase telomeres, were also found associated with Chironomus telomeres (Gorab et al 1996, Morcillo et al 1993.…”

The histochemistry and cell biology vade mecum: a review of 2005–2006

“…A relationship between the increase in nucleolar activity, cell-cycle progression, nucleolar structure, the activity of NopA100, and the proteolysis of this nucleolin-like protein was found. Diez et al (2006) report the presence in the nucleolus of a reverse transcriptase previously found to be associated with telomeres in Chironomus. By immunoXuorescence, a distinct pattern of staining in the giant nucleoli of polytenized cells was observed, consisting of a deWnite necklace pattern that did not completely colocalize with Wbrillarin or nucleolin and appeared to be diVerent from that of typical nucleolar components.…”