Academic journal article Genetics

Emb-4 Is a Conserved Gene Required for Efficient Germline-Specific Chromatin Remodeling during Caenorhabditis Elegans Embryogenesis

Academic journal article Genetics

Emb-4 Is a Conserved Gene Required for Efficient Germline-Specific Chromatin Remodeling during Caenorhabditis Elegans Embryogenesis

Article excerpt

ABSTRACT

In C. elegans, germline blastomeres are initially kept transcriptionally quiescent by the maternally loaded CCCH zinc-finger protein PIE-1. PIE-1 disappears upon the birth of the primordial germ cells Z2 and Z3, yet these cells appear to remain quiescent. We have previously demonstrated that there is a chromatin-based repression that succeeds PIE-1 degradation. The chromatin in Z2/Z3 loses certain histone modifications, including histone H3 lysine 4 dimethylation (H3K4me2), a conserved marker for transcriptionally competent chromatin. We find that mutations in the maternal-effect gene emb-4 cause defects in both PIE-1 degradation and germline-specific chromatin remodeling. emb-4 encodes a highly conserved protein with orthologs in fly, mouse, and human and has a subtle role in Notch signaling. The embryonic phenotype of emb-4 is consistent with a defect in the efficient and timely activation of developmental programs, including germline chromatin remodeling. We also find that, as in early somatic blastomeres, the degradation of PIE-1 in Z2/Z3 is facilitated by zinc-finger-interacting protein ZIF-1, and in the absence of either zif-1 or emb-4, PIE-1 is abnormally retained in Z2/Z3.

UNDERSTANDING the processes that establish and maintain the germ lineage has long been a fundamental goal of developmental biology research. Throughout embryogenesis, regulatory mechanisms are thought to safeguard the germline to maintain its pluripotent capacity and protect it from factors that promote somatic differentiation (SEYDOUX and STROME 1999). One such regulatory mechanism conserved in many organisms is the transcriptional inactivation of the embryonic germline (SEYDOUX and SCHEDL 2001). In both Caenorhabditis elegans and Drosophila melanogaster, for example, transcriptional quiescence has been shown to be an essential requirement for formation and survival of a functional germline (SEYDOUX and STROME 1999; LEATHERMAN and JONGENS 2003).

In the early C. elegans embryo, RNA polymerase II (RNAPII) transcription is repressed in the "P"-lineage germline blastomeres by the maternally synthesized protein PIE-1 (BATCHELDER et al. 1999). PIE-1 is a CCCH zinc-finger protein that segregates asymmetrically to remain concentrated within the posterior daughter cell at each P-blastomere division. In pie-1 mutant embryos, transcription is aberrantly activated in the P^sub 2^ blastomere, causing the P^sub 2^-lineage to undergo somatic transformation and to behave identically to the descendants of the EMS blastomere (the somatic sister of P^sub 2^) (MELLO et al. 1992, 1996; SEYDOUX et al. 1996). This strongly indicates that in C. elegans transcriptional repression is necessary for maintaining germ cell identity, and in the absence of transcriptional repression, the default cell fate of the germ lineage is somatic.

PIE-1 has been proposed to repress transcription by sequestering factors necessary for transcriptional elongation (ZHANG et al. 2003). Whereas the early somatic blastomeres are transcriptionally engaged at the four-cell stage and exhibit phospho-epitopes of RNAPII that correspond to both transcriptional initiation and elongation (phosSer5 and phosSer2, respectively), the early P-lineage expresses only phosSer5 (SEYDOUX and DUNN 1997). The phosSer2 epitope is observed in the P-lineage only after PIE-1 is degraded, which occurs after the division of P^sub 4^ and yields the "germline restricted" primordial germ cells Z2 and Z3 (Z2/Z3). The term "germline restricted" is used to describe P^sub 4^ and Z2/Z3 since they, unlike the other P blastomeres, do not contribute to any somatic lineages.

Although the phosSer2 epitope appears in Z2/Z3, there is little evidence for robust transcription in these cells, which remain mitotically inactive throughout embryogenesis (SEYDOUX and DUNN 1997). Only two zygotic transcripts have been identified as being produced in Z2/Z3, and both accumulate late in embryogenesis (SUBRAMANIAM and SEYDOUX 1999; KAWASAKI et al. …

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