The Puf RNA-binding proteins FBF-1 and FBF-2 inhibit the expression of synaptonemal complex proteins in germline stem cells

Abstract

SUMMARYFBF-1 and FBF-2 (collectively FBF) are two nearly identical Puf-domain RNA-binding proteins that regulate the switch from mitosis to meiosis in the C. elegans germline. In germline stem cells, FBF prevents premature meiotic entry by inhibiting the expression of meiotic regulators, such as the RNA-binding protein GLD-1. Here, we demonstrate that FBF also directly inhibits the expression of structural components of meiotic chromosomes. HIM-3, HTP-1, HTP-2, SYP-2 and SYP-3 are components of the synaptonemal complex (SC) that forms between homologous chromosomes during meiotic prophase. In wild-type germlines, the five SC proteins are expressed shortly before meiotic entry. This pattern depends on FBF binding sites in the 3Ј UTRs of the SC mRNAs. In the absence of FBF or the FBF binding sites, SC proteins are expressed precociously in germline stem cells and their precursors. SC proteins aggregate and SC formation fails at meiotic entry. Precocious SC protein expression is observed even when meiotic entry is delayed in fbf mutants by reducing GLD-1. We propose that parallel regulation by FBF ensures that in wild-type gonads, meiotic entry is coordinated with just-in-time synthesis of synaptonemal proteins.

Dynein light chain DLC-1 promotes localization and function of the PUF protein FBF-2 in germline progenitor cells

“…HTP-1 and HTP-2 are two highly homologous HORMA domain meiotic proteins that are silenced by the FBFs in the mitotic region (Merritt and Seydoux, 2010). We observed ectopic expression of endogenous HTP-1 and HTP-2 in 87% of dlc-1; fbf-1 hermaphrodites compared with 18% and 8% derepression in dlc-1 and dlc-1; fbf-2, respectively (Fig.…”

“…elegans germline stem cell maintenance depends on the activity of two conserved PUF family proteins, FBF-1 and FBF-2 (Zhang et al, 1997). In the absence of both FBF-1 and FBF-2, all cells in the mitotic zone precociously enter meiosis after the L4 stage of development when maintained at 20°C (Crittenden et al, 2002), but are maintained in a mitotic state if grown at 25°C (Merritt and Seydoux, 2010). FBF-1 and FBF-2 recognize the same motif present in the 3′UTR of their target mRNAs in vitro and form complexes with largely the same mRNAs in vivo Merritt and Seydoux, 2010;Prasad et al, 2016).…”

“…In the absence of both FBF-1 and FBF-2, all cells in the mitotic zone precociously enter meiosis after the L4 stage of development when maintained at 20°C (Crittenden et al, 2002), but are maintained in a mitotic state if grown at 25°C (Merritt and Seydoux, 2010). FBF-1 and FBF-2 recognize the same motif present in the 3′UTR of their target mRNAs in vitro and form complexes with largely the same mRNAs in vivo Merritt and Seydoux, 2010;Prasad et al, 2016). Despite high similarity between FBF-1 and FBF-2 (89% identity at the amino acid level) and apparent redundancy in their control of the switch from spermatogenesis to oogenesis, single fbf-1 and fbf-2 mutants have distinct phenotypes, suggesting that these genes play unique roles (Lamont et al, 2004).…”

The PUF binding landscape in metazoan germ cells

“…The in vitro FBF binding element (FBE) is UGUNNNAU, where N is any ribonucleotide Opperman et al 2005). Moreover, the in vivo relevance of the FBE has been confirmed (Zhang et al 1997;Merritt and Seydoux 2010). Second, 15 FBF target mRNAs are known to be regulated post-transcriptionally via FBEs (Supplemental Fig.…”

Translational Control in the Caenorhabditis elegans Germ Line

“…Concomitantly, the two translational activators, GLD-2 and GLD-3, enforce spatially regulated meiotic entry by presumably activating yet unknown meiotic genes (Eckmann et al 2004 ) . The bimodality of this cell fate switch is further assisted by FBF, which translationally represses GLD-1 (Crittenden et al 2002 ) , cyclin-dependent kinase inhibitor (CKI-1) accumulation (Kalchhauser et al 2011 ) , and the ectopic expression of meiotic proteins, such as the synaptonemal complex components HIM-3, HTP-1, SYP-1, and SYP-2 (Merritt and Seydoux 2010 ) (Table 8.5 ). Once female germ cells have entered meiosis, abundant GLD-1 levels ensure meiotic progression.…”