FBF, a PUF RNA-binding protein, is a key regulator of the mitosis/meiosis decision in the Caenorhabditis elegans germline. Genetically, FBF has a dual role in this decision: it maintains germ cells in mitosis, but it also facilitates entry into meiosis. In this article, we explore the molecular basis of that dual role. Previous work showed that FBF downregulates
gld-1 expression to promote mitosis and that the GLD-2 poly(A) polymerase upregulates
gld-1 expression to reinforce the decision to enter meiosis. Here we ask whether FBF can act as both a negative regulator and a positive regulator of
gld-1 expression and also investigate its molecular mechanisms of control. We first show that FBF co-immunoprecipitates with
gld-1 mRNA, a result that complements previous evidence that FBF directly controls
gld-1 mRNA. Then we show that FBF represses
gld-1 expression, that FBF physically interacts with the CCF-1/Pop2p deadenylase and can stimulate deadenylation in vitro, and that CCF-1 is partially responsible for maintaining low GLD-1 in the mitotic region. Finally, we show that FBF can elevate
gld-1 expression, that FBF physically interacts with the GLD-2 poly(A) polymerase, and that FBF can enhance GLD-2 poly(A) polymerase activity in vitro. We propose that FBF can affect polyadenylation either negatively by its CCF-1 interaction or positively by its GLD-2 interaction.