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In Drosophila melanogaster the COM locus directs the somatic silencing of two retrotransposons through both Piwi-dependent and -independent pathways.

Desset S, Buchon N, Meignin C, Coiffet M, Vaury C - PLoS ONE (2008)

Bottom Line: In the Drosophila germ line, repeat-associated small interfering RNAs (rasiRNAs) ensure genomic stability by silencing endogenous transposable elements.Piwi belongs to the subclass of the Argonaute family of RNA interference effector proteins, which are expressed in the germline and in surrounding somatic tissues of the reproductive apparatus.They demonstrate that different RNA silencing pathways are involved in ovarian versus other somatic tissues, since Piwi is necessary for silencing in the former tissues but is dispensable in the latter.

View Article: PubMed Central - PubMed

Affiliation: Centre National de la Recherche Scientifique (CNRS), UMR6247-GReD, Clermont Université; INSERM, Faculté de Médecine, BP38, Clermont-Ferrand, France.

ABSTRACT

Background: In the Drosophila germ line, repeat-associated small interfering RNAs (rasiRNAs) ensure genomic stability by silencing endogenous transposable elements. This RNA silencing involves small RNAs of 26-30 nucleotides that are mainly produced from the antisense strand and function through the Piwi protein. Piwi belongs to the subclass of the Argonaute family of RNA interference effector proteins, which are expressed in the germline and in surrounding somatic tissues of the reproductive apparatus. In addition to this germ-line expression, Piwi has also been implicated in diverse functions in somatic cells.

Principal findings: Here, we show that two LTR retrotransposons from Drosophila melanogaster, ZAM and Idefix, are silenced by an RNA silencing pathway that has characteristics of the rasiRNA pathway and that specifically recognizes and destroys the sense-strand RNAs of the retrotransposons. This silencing depends on Piwi in the follicle cells surrounding the oocyte. Interestingly, this silencing is active in all the somatic tissues examined from embryos to adult flies. In these somatic cells, while the silencing still involves the strict recognition of sense-strand transcripts, it displays the marked difference of being independent of the Piwi protein. Finally, we present evidence that in all the tissues examined, the repression is controlled by the heterochromatic COM locus.

Conclusion: Our data shed further light on the silencing mechanism that acts to target Drosophila LTR retrotransposons in somatic cells throughout fly development. They demonstrate that different RNA silencing pathways are involved in ovarian versus other somatic tissues, since Piwi is necessary for silencing in the former tissues but is dispensable in the latter. They further demonstrate that these pathways are controlled by the heterochromatic COM locus which ensures the overall protection of Drosophila against the detrimental effects of random retrotransposon mobilization.

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Related in: MedlinePlus

The U5, but not U3, region of the ZAM LTR is required for repression.The genomic structure of the ZAM retrotransposon is depicted at the top. Structures of the lacZ reporter trangenes used in this study are shown below on the left, and their expression in follicle cells from the S or U background are indicated at right. Transcripts initiated from the endogenous transcription initiation site of ZAM (black arrow) in transgenes pZ499 and pZ475 are homologous to ZAM over 173 and 149 bp, respectively. These transgenes are sensitive to the S or U status of the line as illustrated by the histochemical detection of β-galactosidase activity in the ovarioles. pZ310 contains the U3 sequence of the element and is expressed from a minimal heat shock promoter (white arrow) so that no sequence homologous to ZAM is present within the p310 transcript. Its expression is not under the control of the S or U status of the lines and is thus observed in the ovarioles from both the S and U backgrounds, as illustrated on the right.
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pone-0001526-g001: The U5, but not U3, region of the ZAM LTR is required for repression.The genomic structure of the ZAM retrotransposon is depicted at the top. Structures of the lacZ reporter trangenes used in this study are shown below on the left, and their expression in follicle cells from the S or U background are indicated at right. Transcripts initiated from the endogenous transcription initiation site of ZAM (black arrow) in transgenes pZ499 and pZ475 are homologous to ZAM over 173 and 149 bp, respectively. These transgenes are sensitive to the S or U status of the line as illustrated by the histochemical detection of β-galactosidase activity in the ovarioles. pZ310 contains the U3 sequence of the element and is expressed from a minimal heat shock promoter (white arrow) so that no sequence homologous to ZAM is present within the p310 transcript. Its expression is not under the control of the S or U status of the lines and is thus observed in the ovarioles from both the S and U backgrounds, as illustrated on the right.

Mentions: We have previously reported that the transgene denoted pZ499, which contains the full-length LTR of ZAM and the first 26 bp of its 5′UTR (499 bp) fused to a LacZ reporter gene, responds to the two types of control over ZAM expression: i) repression, which depends on the fly genotype (U or S); and ii) tissue-specific activation, which drives expression in a very specific group of cells located at the posterior pole of the follicle [19]. The LTR is composed of a U3 region spanning nucleotides 1 to 325, a central R region from nucleotides 326 to 347, and a U5 region from nucleotides 348 to 473. The transcription initiation site defines the boundary between the U3 and R regions, and the polyadenylation site corresponds to the boundary between the R and U5 regions. To investigate the specificity of ZAM transcription in the different lines and to localise the sequences involved in its regulation, we analysed the expression of two additional transgenes placed in an S or U genetic background. These transgenes, pZ310 and pZ475, contain ZAM fragments extending from nucleotides 1 to 310 or 1 to 475, respectively, fused to the LacZ reporter gene. We found that pZ475 responds to both the strain- and tissue-specific controls that have been previously described for the full-length LTR [19]: its expression is restricted to the follicle cells of the U line and is absent in the S line (Fig. 1). By contrast, pZ310 which is expressed from a minimal heat shock promoter responds to the tissue-specific control that restricts its expression to the posterior follicle cells, but is insensitive to the line-specific control, since it is expressed in both the U and S genetic backgrounds (Fig. 1).


In Drosophila melanogaster the COM locus directs the somatic silencing of two retrotransposons through both Piwi-dependent and -independent pathways.

Desset S, Buchon N, Meignin C, Coiffet M, Vaury C - PLoS ONE (2008)

The U5, but not U3, region of the ZAM LTR is required for repression.The genomic structure of the ZAM retrotransposon is depicted at the top. Structures of the lacZ reporter trangenes used in this study are shown below on the left, and their expression in follicle cells from the S or U background are indicated at right. Transcripts initiated from the endogenous transcription initiation site of ZAM (black arrow) in transgenes pZ499 and pZ475 are homologous to ZAM over 173 and 149 bp, respectively. These transgenes are sensitive to the S or U status of the line as illustrated by the histochemical detection of β-galactosidase activity in the ovarioles. pZ310 contains the U3 sequence of the element and is expressed from a minimal heat shock promoter (white arrow) so that no sequence homologous to ZAM is present within the p310 transcript. Its expression is not under the control of the S or U status of the lines and is thus observed in the ovarioles from both the S and U backgrounds, as illustrated on the right.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2211404&req=5

pone-0001526-g001: The U5, but not U3, region of the ZAM LTR is required for repression.The genomic structure of the ZAM retrotransposon is depicted at the top. Structures of the lacZ reporter trangenes used in this study are shown below on the left, and their expression in follicle cells from the S or U background are indicated at right. Transcripts initiated from the endogenous transcription initiation site of ZAM (black arrow) in transgenes pZ499 and pZ475 are homologous to ZAM over 173 and 149 bp, respectively. These transgenes are sensitive to the S or U status of the line as illustrated by the histochemical detection of β-galactosidase activity in the ovarioles. pZ310 contains the U3 sequence of the element and is expressed from a minimal heat shock promoter (white arrow) so that no sequence homologous to ZAM is present within the p310 transcript. Its expression is not under the control of the S or U status of the lines and is thus observed in the ovarioles from both the S and U backgrounds, as illustrated on the right.
Mentions: We have previously reported that the transgene denoted pZ499, which contains the full-length LTR of ZAM and the first 26 bp of its 5′UTR (499 bp) fused to a LacZ reporter gene, responds to the two types of control over ZAM expression: i) repression, which depends on the fly genotype (U or S); and ii) tissue-specific activation, which drives expression in a very specific group of cells located at the posterior pole of the follicle [19]. The LTR is composed of a U3 region spanning nucleotides 1 to 325, a central R region from nucleotides 326 to 347, and a U5 region from nucleotides 348 to 473. The transcription initiation site defines the boundary between the U3 and R regions, and the polyadenylation site corresponds to the boundary between the R and U5 regions. To investigate the specificity of ZAM transcription in the different lines and to localise the sequences involved in its regulation, we analysed the expression of two additional transgenes placed in an S or U genetic background. These transgenes, pZ310 and pZ475, contain ZAM fragments extending from nucleotides 1 to 310 or 1 to 475, respectively, fused to the LacZ reporter gene. We found that pZ475 responds to both the strain- and tissue-specific controls that have been previously described for the full-length LTR [19]: its expression is restricted to the follicle cells of the U line and is absent in the S line (Fig. 1). By contrast, pZ310 which is expressed from a minimal heat shock promoter responds to the tissue-specific control that restricts its expression to the posterior follicle cells, but is insensitive to the line-specific control, since it is expressed in both the U and S genetic backgrounds (Fig. 1).

Bottom Line: In the Drosophila germ line, repeat-associated small interfering RNAs (rasiRNAs) ensure genomic stability by silencing endogenous transposable elements.Piwi belongs to the subclass of the Argonaute family of RNA interference effector proteins, which are expressed in the germline and in surrounding somatic tissues of the reproductive apparatus.They demonstrate that different RNA silencing pathways are involved in ovarian versus other somatic tissues, since Piwi is necessary for silencing in the former tissues but is dispensable in the latter.

View Article: PubMed Central - PubMed

Affiliation: Centre National de la Recherche Scientifique (CNRS), UMR6247-GReD, Clermont Université; INSERM, Faculté de Médecine, BP38, Clermont-Ferrand, France.

ABSTRACT

Background: In the Drosophila germ line, repeat-associated small interfering RNAs (rasiRNAs) ensure genomic stability by silencing endogenous transposable elements. This RNA silencing involves small RNAs of 26-30 nucleotides that are mainly produced from the antisense strand and function through the Piwi protein. Piwi belongs to the subclass of the Argonaute family of RNA interference effector proteins, which are expressed in the germline and in surrounding somatic tissues of the reproductive apparatus. In addition to this germ-line expression, Piwi has also been implicated in diverse functions in somatic cells.

Principal findings: Here, we show that two LTR retrotransposons from Drosophila melanogaster, ZAM and Idefix, are silenced by an RNA silencing pathway that has characteristics of the rasiRNA pathway and that specifically recognizes and destroys the sense-strand RNAs of the retrotransposons. This silencing depends on Piwi in the follicle cells surrounding the oocyte. Interestingly, this silencing is active in all the somatic tissues examined from embryos to adult flies. In these somatic cells, while the silencing still involves the strict recognition of sense-strand transcripts, it displays the marked difference of being independent of the Piwi protein. Finally, we present evidence that in all the tissues examined, the repression is controlled by the heterochromatic COM locus.

Conclusion: Our data shed further light on the silencing mechanism that acts to target Drosophila LTR retrotransposons in somatic cells throughout fly development. They demonstrate that different RNA silencing pathways are involved in ovarian versus other somatic tissues, since Piwi is necessary for silencing in the former tissues but is dispensable in the latter. They further demonstrate that these pathways are controlled by the heterochromatic COM locus which ensures the overall protection of Drosophila against the detrimental effects of random retrotransposon mobilization.

Show MeSH
Related in: MedlinePlus