Genetics, Vol. 178, 235-243, January 2008, Copyright © 2008
doi:10.1534/genetics.107.083212

Inhibition of Transcription by the Caenorhabditis elegans Germline Protein PIE-1: Genetic Evidence for Distinct Mechanisms Targeting Initiation and Elongation

Dolan Ghosh1 and Geraldine Seydoux2

Department of Molecular Biology and Genetics and Howard Hughes Medical Institute, Center for Cell Dynamics, Johns Hopkins School of Medicine, Baltimore, Maryland 21205

2 Corresponding author: Johns Hopkins School of Medicine, 725 N. Wolfe St., Baltimore, MD 21205-2185.
E-mail: gseydoux{at}jhmi.edu

In Caenorhabditis elegans embryos, specification of the germ lineage depends on PIE-1, a maternal protein that blocks mRNA transcription in germline blastomeres. Studies in mammalian cell culture have suggested that PIE-1 inhibits P-TEFb, a kinase that phosphorylates serine 2 in the carboxyl-terminal domain (CTD) repeats of RNA polymerase II during transcriptional elongation. We have tested this hypothesis using an in vivo complementation assay for PIE-1 function. Our results support the view that PIE-1 inhibits P-TEFb using the CTD-like motif YAPMAPT. This activity is required to block serine 2 phosphorylation in germline blastomeres, but unexpectedly is not essential for transcriptional repression or specification of the germline. We find that sequences outside of the YAPMAPT are required to inhibit serine 5 phosphorylation, and that this second inhibitory mechanism is essential for transcriptional repression and specification of the germ lineage. Our results suggest that PIE-1 uses partially redundant mechanisms to block transcription by targeting both the initiation and elongation phases of the transcription cycle.