Temperature-induced Balbiani rings in Chironomus thummi

CriTER-A: A Novel Temperature-Dependent Noncoding RNA Switch in the Telomeric Transcriptome of Chironomus riparius

The telomeric transcriptome of Chironomus riparius has been involved in thermal stress response. One of the telomeric transcripts, the so-called CriTER-A variant, is highly overexpressed upon heat shock. On the other hand, its homologous variant CriTER-B, which is the most frequently encoded noncoding RNA in the telomeres of C. riparius, is only slightly affected by thermal stress. Interestingly, both transcripts show high sequence homology, but less is known about their folding and how this could influence their differential behaviour. Our study suggests that CriTER-A folds as two different conformers, whose relative proportion is influenced by temperature conditions. Meanwhile, the CriTER-B variant shows only one dominant conformer. Thus, a temperature-dependent conformational equilibrium can be established for CriTER-A, suggesting a putative functional role of the telomeric transcriptome in relation to thermal stress that could rely on the structure-function relationship of the CriTER-A transcripts.

Telomeric transcriptome from Chironomus riparius (Diptera), a species with noncanonical telomeres

Although there are alternative telomere structures, most telomeres contain DNA arrays of short repeats (6-26 bp) maintained by telomerase. Like other diptera, Chironomus riparius has noncanonical telomeres and three subfamilies, TsA, TsB and TsC, of longer sequences (176 bp) are found at their chromosomal ends. Reverse transcription PCR was used to show that different RNAs are transcribed from these sequences. Only one strand from TsA sequences seems to render a noncoding RNA (named CriTER-A); transcripts from both TsB strands were found (CriTER-B and αCriTER-B) but no TsC transcripts were detected. Interestingly, these sequences showed a differential transcriptional response upon heat shock, and they were also differentially affected by inhibitors of RNA polymerase II and RNA polymerase III. A computer search for transcription factor binding sites revealed putative regulatory cis-elements within the transcribed sequence, reinforcing the experimental evidence which suggests that the telomeric repeat might function as a promoter. This work describes the telomeric transcriptome of an insect with non-telomerase telomeres, confirming the evolutionary conservation of telomere transcription. Our data reveal differences in the regulation of telomeric transcripts between control and stressful environmental conditions, supporting the idea that telomeric RNAs could have a relevant role in cellular metabolism in insect cells.

The effects of temperature shock on transcription and replication in Rhynchosciara americana (Diptera: Sciaridae)

The chromosomal response to temperature shock in Rhynchosciara americana is similar to that observed in other Diptera. After a 33 degrees C/90 min or a 36 degrees C/30 min shock the reaction for RNA polymerase II (RpII) is enhanced at five loci. The most prominent of these was identified by in situ hybridization as the site of the hsp70 gene. At 33 degrees C, an accumulation of heat shock factor (HSF) and an increase in the level of RpII was observed at some heat shock loci after 5 min and reached a maximum after 15 min at most loci. The pattern of accumulation of HSF and RpII at individual heat shock loci was similar and their increases were generally coordinated among the loci. RpII gradually decreased at sites active prior to shock, the rate of decrease varying with the site. The B2 DNA puff retained RpII for a significant length of time while the histone locus still contained RpII after a shock of 90 min. With a 36 degrees C/30 min shock, the size of the heat shock puffs and the intensities of HSF and RpII peaked at 1-4 h post stress. The level of HSF declined rapidly after 1 h while the level of RpII remained high for an additional 4 h. The reaction of the DNA puffs to heat shock varied. Usually they did not regress completely and retained traces of RpII. BrdU incorporation continued at both amplifying and non-amplifying bands after shock but on average it appeared depressed for about 24 h post stress.

Reverse transcriptase-related proteins in telomeres and in certain chromosomal loci of Rhynchosciara (Diptera: Sciaridae)

The localization of reverse transcriptase-related proteins in polytene chromosomes of dipterans was investigated using previously characterized antibodies to a recombinant polypeptide containing conserved motifs of insect reverse transcriptases. The immunoreactions were carried out with polytene chromosome squashes of eight sciarids, one chironomid and three Drosophila species. Telomeric staining was regularly observed on chromosomes of the sciarid Rhynchosciara americana under normal growth conditions. Five of eight chromosomal tips were labelled except for the heterochromatic ends that are occasionally found associated forming a chromocentre in the salivary gland. Reverse transcriptase-related proteins were detected at chromosomal tips of young larvae and remained bound to the telomeres throughout larval development. As in salivary gland chromosomes, five non-telocentric ends of the chromosomes from Malpighian tubules of R. americana appeared clearly stained with anti-reverse transcriptase. The occurrence of telomeric reverse transcriptase in R. americana correlates with the presence of RNA in addition to an unusual enrichment with homopolymeric dA/dT DNA associated with the telomeric heterochromatin. The antibodies also reacted with a few interstitial sites in chromosomes of four Rhynchosciara species, one band overlapping the histone gene locus of three species in the americana -like group. The results provide evidence for a reverse transcriptase-related protein as a constitutive component in telomeres of R. americana and also in certain interstitial loci of Rhynchosciara species in which RNA was immunologically detected in the form of RNA:DNA hybrids.

Extrachromosomal RNA-DNA complex containing long telomeric repeats in chironomids

We have analysed an extracted RNase sensitive fraction containing telomeric repeat sequences in the telomerase negative dipteran Chironomus tentans. It shows a slow and well-defined electophoretic migration corresponding to > 20 kb and is sensitive not only to RNase, but also to DNase. It hybridizes to both strands of the telomeric repeat with about equal intensities. DNA is probably the dominant component since the fraction is only slightly heavier than genomic DNA in isopycnic gradients but considerably lighter than RNA. It can, nevertheless, be shown to incorporate tritiated uridine. The material might represent another example of extrachromosomal telomeric repeats in telomerase negative cells.

Telomeres terminating with long complex tandem repeats

Telomeres of most investigated species terminate with short repeats and are elongated by telomerase. Short repeats have never been detected in dipteran species which have found other solutions to end a chromosome. Whereas in Drosophila melanogaster retroelements are added onto the termini, chironomids have long complex repeats at their chromosome ends. We review evidence that these units are terminal and probably have evolved from short telomeric repeats. In Chironomus pallidivittatus the units have been shown to belong to different subfamilies which have specific inter- and intrachromosomal distribution, the most terminal subfamily of repeats being characterized by pronounced secondary structures for the single strand. The complex repeats are efficiently homogenized both within and between different chromosome ends. Gene conversion is probably an important component in the coordinate evolution of the repeats but it is not known whether it is used for net synthesis of DNA. RNA is used as an intermediate in telomere elongation both by organisms having chromosomes terminating with short repeats and by D. melanogaster. It is therefore interesting that the terminal repeats in chironomids are transcribed.

Specific intranucleolar distribution of Hsp70 during heat shock in polytene cells

Hsp70, the most abundant and conserved heat shock protein, has been described as strongly concentrating in the nucleolus during heat shock. The important metabolic processes that take place in the nucleolus, rDNA transcription, processing, and assembling with ribosomal proteins, and the nucleolar architecture itself are very sensitive to temperature changes. In this work, we have analyzed in detail the nucleolar changes, in structure and activity, induced by temperature in Chironomus thummi salivary gland cells and the fine subnucleolar localization of Hsp70 during heat shock. The optimum temperature chosen to induce the heat shock response was 35 degrees C. Under these conditions transcription of heat shock genes, inactivation of previously active genes and maximum synthesis of Hsps take place, while survival of larvae and recovery were ensured. After 1 h at 35 degrees C, nucleoli change from a uniform control pattern to a segregated pattern of nucleolar components that can be observed even at the light microscopic level. The dense fibrillar component (DFC) and the granular component appeared perfectly differentiated and spatially separated, the former occupying mainly the central inner region surrounded by a rim of granular component. Hsp70 was specifically localized within the DFC upon heat shock as shown by immunolocalization by both light and electron microscopy. Pulse labeling with [3H]uridine proves that rRNA transcription continues during heat shock. The pattern of Hsp70 distribution within the nucleolus correlates with that of newly produced rRNA transcripts. Hsp70 also colocalizes with RNA polymerase I, both being restricted to the DFC. These data show that the DFC seems to be the intranucleolar target for Hsp70 in heat-shocked cells. We discuss these results in relation to the possible function of Hsp70 in the first steps of preribosome synthesis.

Ku-related antigens are associated with transcriptionally active loci in Chironomus polytene chromosomes

Antigens of Chironomus reactive with human sera containing anti-Ku antibodies and also with specific antibodies to each Ku subunit were characterized by immunoblot analysis. Three main antigen species were identified in nuclear-enriched extracts from salivary gland cells of Chironomus thummi, ranging in Mr from 55000 to 67000. The nuclear localization of Ku-related antigens in the dipteran Chironomus was studied by immunofluorescent labeling in polytene chromosomes of the salivary glands. Balbiani rings, loci highly active in transcription, were found to be strongly labeled by anti-Ku antibodies. Sugar-induced changes in the activity of the Balbiani ring genes were accompanied by the redistribution of Ku-related antigens as visualized by their absence in regressed Balbiani ring loci, and continued presence only in those that were transcriptionally active. A drastic change in the distribution of Ku-related antigens was also observed when C. thummi larvae underwent heat treatment as the immunofluorescent staining was restricted to previously described heat shock puffs. Anti-Ku sera reacted in addition with several chromosomal bands in which the presence of RNA polymerase II was also immunologically detected. The results show that Chironomus antigens reactive with anti-Ku antibodies are related to transcription in polytene chromosomes.

Transcription of heat shock gene loci versus non-heat shock loci in Chironomus polytene chromosomes: evidence for heat-induced formation of novel putative ribonucleoprotein particles (hsRNPs) in the major heat shock puffs

The heat shock response of Chironomus polytene chromosomes was reexamined. The in vivo effects of heat shock on chromosomal [3H]uridine labeling, RNA polymerase II distribution and ribonucleoprotein (RNP) formation were investigated. One primary result is a clarification of the number and location of chromosomal sites strongly induced by treatment at 37 degrees C for 60 min. In total, seven major heat shock loci were identified by transcription autoradiography in Chironomus tentans: I-20A, II-16B, II-10C, II-4B, II-1C, III-12B, and IV-5C. Secondly, combining immunofluorescence with transcription autoradiography, I find RNA polymerase II occurring after heat shock at multiple chromosomal sites that were also active under normal conditions (20 degrees C). Furthermore, the results demonstrate conclusively that the presence of RNA polymerase II at heat shock and non-heat shock loci is generally correlated with [3H]uridine labeling during heat shock. These latter results extend and corroborate previous findings. Thirdly, the most striking result of this study was revealed in ultrathin sections of puffs by electron microscopy: I discerned a site-specific ultrastructural difference in putative RNP particles between heat shock versus non-heat shock loci. At least three of the seven induced major heat shock puffs (I-20A, III-12B, IV-5C) were observed to contain globular particles that were different, i.e. significantly larger, 250-1,000 A in diameter with a prominent 500-750 A class, than RNP particles of other loci under non-heat shock conditions. These large heat shock puff particles presumably represent nascent or newly synthesized heat shock RNA associated with protein(s) to form heat shock RNPs (hsRNPs). This finding suggests the possible involvement of novel RNPs (hsRNPs) in transcriptional regulation or heat shock RNA turnover and may stimulate further molecular investigations on this subject in both cell physiological and structural terms. I conclude that the locus-specific putative hsRNPs are an intrinsic property of greatly increased heat shock gene transcription.

HSP90 associates with specific heat shock puffs (hsr omega) in polytene chromosomes of Drosophila and Chironomus

The heat shock protein HSP90, which is mainly cytoplasmic, has recently been reported to be present in the nucleus. We have found a specific chromosomal localization of HSP90 in different species of Drosophila and Chironomus using immunocytochemical techniques with different mono- and polyclonal antibodies for this hsp. HSP90 was found associated with heat shock-induced puffs at 93D and 48B in salivary gland chromosomes of Drosophila melanogaster and Drosophila hydei, respectively. The localization of HSP90 to locus 93D occurred rapidly after the onset of heat shock and disappeared during recovery, concomitant with puff regression. The association of HSP90 with the 93D locus was strictly heat shock dependent as shown by the absence of HSP90 in puff 93D induced by either benzamide or colchicine. No specific nuclear staining was observed in unstressed control cells. HSP90 was also found in the temperature-induced telomeric Balbiani ring puffs (T-BRs) in Chironomus thummi and in one heat shock puff at I-1C in Chironomus tentans. Other heat shock puffs also appeared lightly stained with the HSP90 polyclonal antibody in both species of Chironomus. HSP90 was absent from the T-BRs when RNA synthesis was inhibited with Actinomycin D suggesting that the localization of HSP90 is dependent on transcription. Inhibition of protein synthesis did not prevent association of this hsp with the T-BRs, indicating that pre-existing HSP90 can associate with this locus. HSP90 did not associate with any telomeric chromosomal regions of unstressed cells. The present observations suggest that heat shock gene products such as HSP90 may somehow be involved in the regulation at the chromosomal level of other members of the heat shock gene family. Puffs 93D (D. melanogaster) and 48B (D. hydei) are equivalent and correspond to homologous gene loci (hsr omega) that have unusual features that distinguish them from other heat shock puffs. The binding of HSP90 at T-BRs and at puff I-1C in the genus Chironomus is the first demonstration, albeit indirect, of the existence of hsr omega analogous loci in species other than Drosophila.

Heat-shock induction and cytoplasmic localization of transcripts from telomeric-associated sequences in Chironomus thummi

Transcription of telomeric-associated sequences has been detected in the salivary gland cells of the larvae Chironomus thummi. In this species, a heat shock induces puffing at some telomeres, especially at one of the telomeres of chromosome III. We found that this process was concomitant with an increase in the overall telomeric transcript levels. Transcription was also observed in all the telomeres under control conditions, by in situ hybridization, even when these telomeres appeared to be in a nonpuffed state. The telomeric transcripts were found in both, the nuclei and, at higher levels, in the cytoplasmic extracts of salivary gland cells. The heat-shock activation, however, appeared to be restricted to the nuclear level. Telomeric transcription and the peculiar behavior of C. thummi telomeres after a heat shock are discussed.

Subrepeats result from regional DNA sequence conservation in tandem repeats in Chironomus telomeres

Repeat units, widespread in eukaryotic genomes, are often partially or entirely built up of subrepeats. Homogenization between whole repeat units arranged in tandem usually can best be understood as a result of unequal crossing over. Such a mechanism is less plausible for maintaining similarities between subrepeats within a repeat unit when present in a regular array. In Chironomus telomeres, large blocks of tandemly repeated approximately 350 base-pair units contain two or three pairs of subrepeats with high mutual identities, embedded in linker DNA, non-repetitive within the repeat unit. Measurements of evolutionary base changes in two closely related species, Chironomus tentans and Chironomus pallidivittatus, permit us to conclude that the subrepeat arrangement is best explained as a consequence of regional sequence conservation after an earlier duplication of an ancestral half-unit.

Induction of the heat-shock response by carbon dioxide in Chironomus thummi

The effects of a set of stress treatments on gene expression of Chironomus thummi salivary gland cells have been analyzed. Among the treatments assayed, only during recovery from carbon dioxide have we observed a response similar to that previously described after heat-shock treatment: induction of the heat-shock puffs and synthesis of the heat-shock polypeptides. In these conditions, puffing and transcription of telomeric regions were observed, which led to the appearance of the temperature-inducible telomeric Balbiani ring T-BR-III. Other treatments failed to induce the heat-shock response, despite promoting real stress conditions to C. thummi larvae or salivary gland cells.

Telomeric DNA sequences differentially activated by heat shock in two Chironomus subspecies

The patterns of puffing, transcription and protein synthesis under heat shock were analysed in polytene nuclei of Chironomus thummi piger, in comparison with those obtained in the closely related subspecies C. th. thummi. Most chromosomal heat shock puffs, as well as heat shock induced polypeptides, in C. th. piger paralleled those previously reported for C. th. thummi. Nevertheless, we found a striking difference in behaviour in the induction of telomeric Balbiani rings by heat shock in the two subspecies. Although homologous sequences were present at all the telomeres in both subspecies, they were not always transcriptionally activated by heat shock. The most frequently puffed telomeres were that of chromosome III R in C. th. thummi and that of chromosome IV R in piger. Transcription of the same sequences from both telomeric Balbiani rings (T-BR-III and T-BR-IV) occurred under heat shock. The enigmatic behaviour of telomeres and the functional significance of T-BRs are discussed in relation to possible equivalents in other Diptera.