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First published online 10 January 2007
doi: 10.1242/dev.02771

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MRG-1, an autosome-associated protein, silences X-linked genes and protects germline immortality in Caenorhabditis elegans

Teruaki Takasaki^1,*, Zheng Liu^2,*, Yasuaki Habara^1,, Kiyoji Nishiwaki³, Jun-ichi Nakayama³, Kunio Inoue¹, Hiroshi Sakamoto¹ and Susan Strome^2,

¹ Department of Biology, Graduate School of Science and Technology, Kobe University, 1-1 Rokkodaicho, Nadaku, Kobe 657-8501, Japan.
² Department of Biology, Indiana University, Bloomington, IN 47405, USA.
³ RIKEN Center for Developmental Biology, Chuo-ku, Kobe 650-0047, Japan.

Author for correspondence (e-mail: sstrome{at}indiana.edu)

Accepted 27 November 2006

MRG15, a mammalian protein related to the mortality factor MORF4, is required for cell proliferation and embryo survival. Our genetic analysis has revealed that the Caenorhabditis elegans ortholog MRG-1 serves similar roles. Maternal MRG-1 promotes embryo survival and is required for proliferation and immortality of the primordial germ cells (PGCs). As expected of a chromodomain protein, MRG-1 associates with chromatin. Unexpectedly, it is concentrated on the autosomes and not detectable on the X chromosomes. This association is not dependent on the autosome-enriched protein MES-4. Focusing on possible roles of MRG-1 in regulating gene expression, we determined that MRG-1 is required to maintain repression in the maternal germ line of transgenes on extrachromosomal arrays, and of several X-linked genes previously shown to depend on MES-4 for repression. MRG-1 is not required for PGCs to acquire transcriptional competence or for the turn-on of expression of several PGC-expressed genes (pgl-1, glh-1, glh-4 and nos-1). By contrast to this result in PGCs, MRG-1 is required for ectopic expression of those germline genes in somatic cells lacking the NuRD complex component MEP-1. We discuss how an autosome-enriched protein might repress genes on the X chromosome, promote PGC proliferation and survival, and influence the germ versus soma distinction.

Key words: C. elegans, MRG-1, Germ line, X chromosome silencing

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