Potential Role For Nucleolar Protein Dnt1 In RDNA Silencing In Fission Yeast

In all eukaryotes,rDNA is organized as one or more arrays containing anywhere from＜100 to＞10,000 repeating units,which can form one or more nucleoli where rRNA synthesis, processing and assembly into ribosomes occur.The integrity of rDNA is important for cell growth and survival.As such a large and repetitive locus,it's thought to be unstable,unequal recombination events occurring and might affect cell survival.However,cells have therefore evolved mechanisms to protect the stability and integrity of rDNA which might through inhibiting homologous recombination and repressing transcription from foreign promoters.Recently,a new nucleolar protein Dnt1 was found participateing in rDNA silencing,a phenomenon that RNA PolⅡtranscribed genes are repressed when integrated into rDNA regions.Dntl was also required for nucleolar localization of Pcs1.Their homologs in budding yeast Csm1 and Lrs4 were required for rDNA silencing.Csm1,Lrs4 and RENT complex, together with Tof2 formed a protein bridge to regulate rDNA silencing and homologous recombination in budding yeast,Dnt1 happened to be homologous to Tof2 to some extent.So we predicted that there might be a similar regulating complex in fission yeast.Dnt1 was also found to be required for maintaining the exclusive nucleolar localization of the largest subunit of RNA polymerase I named Nucl and might also required for an upstream activating factor(UAF) for RNA polymerase I involving in transcription of ribosomal RNA (rRNA) called Rrn5,vice versa.A new paper reported that Nucl and Rrn5 were required for condensin accumulation at rDNAs in mitotic metaphase.Interestingly,there has ever been reported that condensin was involved in rDNA silencing in budding yeast.Therefore Dnt1 might participate in rDNA silencing through condensin. The most interestingly,unlike budding yeast,there was RNAi in fission yeast as in higher eukaryotes.People have known that RNAi regulated rDNA silencing directly or indirectly, protected the stability and integrity of rDNA.In this paper,taking the advantage of genetic,cell and molecular biological methods,we found that unlike their homologs in budding yeast,Pcs1 and Mde4 were not required for rDNA silencing.As expected,condensin subunits were involved in rDNA silencing, moreover,condensin accumulation at rDNAs in mitotic metaphase required Dnt1,mutants of other proteins which were required for condensin accumulation at rDNAs in mitotic metaphase also leaded to disruption of rDNA silencing.Therefore we concluded that Dnt1 might participated in rDNA silencing through condensin.We also found that Dnt1 regulated rDNA silencing in the pathway independent of Ago1 and Dcr1,showing the complexity and diversity of rDNA regulation in fission yeast.