The Fission Yeast Stress-Responsive MAPK Pathway Promotes Meiosis via the Phosphorylation of Pol II CTD in Response to Environmental and Feedback Cues

Abstract

The RRM-type RNA-binding protein Mei2 is a master regulator of meiosis in fission yeast, in which it stabilizes meiosis-specific mRNAs by blocking their destruction. Artificial activation of Mei2 can provoke the entire meiotic process, and it is suspected that Mei2 may do more than the stabilization of meiosis-specific mRNAs. In our current study using a new screening system, we show that Mei2 genetically interacts with subunits of CTDK-I, which phosphorylates serine-2 residues on the C-terminal domain of RNA polymerase II (Pol II CTD). Phosphorylation of CTD Ser-2 is essential to enable the robust transcription of ste11, which encodes an HMG-type transcription factor that regulates the expression of mei2 and other genes necessary for sexual development. CTD Ser-2 phosphorylation increases under nitrogen starvation, and the stress-responsive MAP kinase pathway, mediated by Wis1 MAPKK and Sty1 MAPK, is critical for this stress response. Sty1 phosphorylates Lsk1, the catalytic subunit of CTDK-I. Furthermore, a feedback loop stemming from activated Mei2 to Win1 and Wis4 MAPKKKs operates in this pathway and eventually enhances CTD Ser-2 phosphorylation and ste11 transcription. Hence, in addition to starting meiosis, Mei2 functions to reinforce the commitment to it, once cells have entered this process. This study also demonstrates clearly that the stress-responsive MAP kinase pathway can modulates gene expression through phosphorylation of Pol II CTD.

Individual letters of the RNA polymerase II CTD code govern distinct gene expression programs in fission yeast

“…The latter scenario applies to the Ser2 coding letter, insofar as a heterothallic haploid fission yeast rpb1-S2A mutant can grow vegetatively, but is unable to mate with heterothallic WT cells of either mating type, i.e., S2A is sterile. Sterility results from the inability of S2A cells to induce expression of the Ste11 transcription factor that triggers the meiotic sexual differentiation pathway (5,6,8). Microarray analyses of the transcriptome of S2A cells vs. WT highlighted ≥1.5-fold changes in the expression level of many meiotically regulated genes (5,9).…”

Signaling pathways for fission yeast sexual differentiation at a glance

“…They include the cyclic adenosine monophosphate (cAMP) pathway, the target of rapamycin (TOR) complex 1 (TORC1) pathway and the two mitogen-activated protein kinase (MAPK) pathways (Yamamoto et al, 1997;Otsubo and Yamamoto, 2010). Recently, it has become apparent that phosphorylation of the C-terminal domain (CTD) of RNA polymerase II (Pol II) by CTD kinase I (Ctdk1) specifically affects the expression of ste11 (Coudreuse et al, 2010;Sukegawa et al, 2011). It has also been shown that the meiotic regulator Mei2, the expression of which is governed by Ste11, enhances expression of ste11 through a positivefeedback loop (Sukegawa et al, 2011).…”

“…Recently, it has become apparent that phosphorylation of the C-terminal domain (CTD) of RNA polymerase II (Pol II) by CTD kinase I (Ctdk1) specifically affects the expression of ste11 (Coudreuse et al, 2010;Sukegawa et al, 2011). It has also been shown that the meiotic regulator Mei2, the expression of which is governed by Ste11, enhances expression of ste11 through a positivefeedback loop (Sukegawa et al, 2011). These new discoveries, together with previous findings, illustrate that a number of signal transduction pathways orchestrate the regulation of ste11 expression in response to intricate environmental changes.…”

“…The P-specific genes include map2, which encodes the mating pheromone P-factor, and map3, which encodes the M-factor receptor. M-specific genes include mfm1, mfm2 and mfm3, which all encode the mating pheromone M-factor, and mam2, which encodes the P-factor receptor (Tanaka et al, 1993;Kitamura and Shimoda, 1991;Davey, 1992;Imai and Yamamoto, 1994;Kjaerulff et al, 1994). Subsequently, Pfactor, a simple peptide of 23 amino acids is secreted from h + cells, and M-factor, a peptide of nine amino acids that is farnesylated and carboxymethylated at the C-terminus, is secreted from h 2 cells (Imai and Yamamoto, 1994;Davey, 1992).…”

“…Subsequently, Pfactor, a simple peptide of 23 amino acids is secreted from h + cells, and M-factor, a peptide of nine amino acids that is farnesylated and carboxymethylated at the C-terminus, is secreted from h 2 cells (Imai and Yamamoto, 1994;Davey, 1992). P-factor binds to the P-factor receptor Mam2 on the surface of h 2 cells, whereas M-factor binds to the M-factor receptor Map3 on the surface of h + cells (Tanaka et al, 1993;Kitamura and Shimoda, 1991;Toda et al, 1991). Upon pheromone binding, the two receptors dock to the same Ga protein, Gpa1, and activate it.…”