Regulation of chromatin structure during meiosis
Our goal is to understand the mechanisms by which unpaired DNA is recognized and silenced. Silencing of unpaired chromatin is a wide-spread phenomenon that appears to occur by distinct mechanisms in different species. In C. elegans, meiotic silencing occurs at the chromatin level. Unpaired chromosomes and chromosomal regions accumulate a different pattern of histone modifications than do paired regions. For example, unpaired regions accumulate a high level of histone H3 lysine 9 dimethylation (H3K9me2) during early meiosis. This widely conserved histone modification is associated with assembly of closed chromatin structure (heterochromatin) and subsequent transcriptional repression.
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In wildtype males, H3K9me2 foci (arrows), representing the X chromosome, are visible. H3K9me2 foci are absent in ego-1 mutant males, although the X chromosome appears to be condensed (DNA, arrows). Images from Maine et al. 2005. |
We have found that one C. elegans RNA-directed RNA polymerase (RdRP), EGO-1, is required specifically for H3K9me2 accumulation on unpaired chromatin during early meiotic prophase (Maine et al. 2005). A second RdRP, RRF-3, functions in the removal of H3K9me2 marks as germ cells begin to differentiate as sperm. We have identified a set of transcripts that are more abundant in the germ line in the absence of EGO-1 activity, which may be natural targets of this chromatin-level repression.
In other work, we showed that EGO-1 activity promotes many aspects of germline development and is required for fertility (Qiao et al. 1995; Smardon et al. 2000; Vought et al. 2005). Our working hypothesis is that EGO-1/RdRP participates in the biogenesis of double strand RNAs with functions in a variety of processes in the cell, including heterochromatin assembly and, perhaps, elimination of aberrant RNA. We suspect that the ego-1 developmental phenotype reflects the direct and indirect effects of failure to synthesize/ amplify these small RNAs.
The focus of our current work is to test alternative hypotheses for how H3K9me2 is targeted to unpaired DNA during meiosis, to identify target loci that receive those modifications, and to understand the biological function(s) of meiotic silencing. C. elegans provides an unparalleled system for studying the mechanism of meiotic silencing in the context of germlinedevelopment. The C. elegans germ line has proven to be an excellent model for studying genetic regulation and developmental questions. Extensive genetic tools are available in C. elegans that will allowus identify and study components of the meiotic silencing machinery as well as address larger questions about the role of meiotic silencing in germline development. By understanding the mechanism and targets of H3K9me2 accumulation, we will be in a position to investigate the developmental implications of this process. Moreover, our results will facilitate th study of meiotic silencing in more complex animals, including mammals.
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In wildtype males (left), H3K9me2 accumulates on the X chromosome (arrowheads). In csr-1mutant males (right), H3K9me2 accumulates on paired autosomal regions as well as on the X.
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We have identified four additional proteins whose activities influence meiotic heterochromatin assembly. These include: an Argonaute protein, CSR-1; a Tudor domain protein, EKL-1; a Dicer-related helicase, DRH-3; and SIN-3, a protein thought to function in assembly of histone deacetylase complexes. We are working to establish the relationships among these factors while also identifying additional regulators of heterochromatin assembly. Our data suggest that complementary mechanisms may promote the accumulation of H3K9me2 on unpaired chromatin and limits the accumulation of H3K9me2 on paired chromatin..
Selected Related Publications:
Maine,E.M., J. Hauth, T. Ratliff, V.E. Vought, X. She, and W.G. Kelly. (2005) EGO-1, a putative RNA-dependent RNA polymerase, is required for heterochromatin assembly on unpaired chromosomes during C. elegans meiosis. Current Biology, 15, 1972-1978. [PDF]
* This paper was cited by the Faculty of 1000.
V.E. Vought, M. Ohmachi, M-H. Lee, and E.M. Maine (2005) EGO-1, a putative RNA-directed RNA polymerase, promotes germline proliferation in parallel with GLP-1/Notch signaling and regulates the spatial organization of nuclear pore complexes and germline P granules in C. elegans. Genetics, 170, 1121-1132. [PDF]
Smardon, A., J. M. Spoerke, S.C. Stacey, M.E. Klein, N. Mackin, and E.M. Maine (2000) EGO-1 is related to RNA-directed RNA polymerase and functions in germline development and RNA interference in C. elegans. Current Biology 10, 169-178. [PDF] *This paper was featured in Science Editor's Choice and Current Biology Dispatch reviews.
Maine, E.M. (2000) A conserved mechanism for posttranscriptional gene silencing? Genome Biology 1, 1018.1-4. [PDF]
Qiao, L., J.L. Lissemore, P. Shu, A. Smardon, M. Gelber, and E.M. Maine (1995) Enhancers of glp-1, a gene required for cell-signaling in C. elegans, define a set of genes required for germline development. Genetics 141, 551-569. [PDF]
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