Studies On The Regulation Of Gene Silencing At RDNA Region By Condensins And Nucleolar Proteins In Fission Yeast

The nucleolus is the site of ribosomal RNA transcription and assembly of ribosomal subunits.This subnuclear compartment forms around an array of repeated rDNA genes.In model organism fission yeast,Schizosaccharomyces pombe,100-200 rDNA gene copies are located on both left and right ends of chromosomeâ…¢.The integrity of rDNA is critical for cell growth and survival.As such a large and repetitive locus,it's thought to be unstable as recombination events may cause shortened rDNA arrays.However,cells have evolved mechanisms to maintain the stability and integrity of rDNA,which might be through inhibiting homologous recombination and repressing transcription from foreign promoters.In this study,we have employed the genetic approaches and tools to investigate the potential functions of condensins and nucleolar proteins in gene silencing at rDNA region in fission yeast.Our results showed that besides its functions in chromosome condensation and segregation,condensin,a five subunits protein complex,is essential for rDNA silencing but is not involved in the centromere silencing.Consistently,factors required for condensin accumulation at the rDNA in mitotic metaphase are also involved in rDNA silencing.We also found that nucleolar protein Dnt1,Rrn5 and Nuc1 are involved in gene silencing at rDNA region,likely through regulating condensin's localization in nucleolus.Contrary to our expectation,disruption of nucleolar structure by mutations in nuc1+ and rrn5+ does not lead to the enhancement of centromeric silencing,although these mutants have enhanced enrichment of condensin at centromeres.Budding yeast nucleolar proteins Csm1 and Lrs4 have been identified as factors influencing rDNA silencing,our genetic analysis showed that homologs of these two proteins in fission yeast(called Pcs1 and Mde4 respectively) do not have conserved functions in rDNA silencing.Our genetic analysis also suggested that condensin may fulfill its function in rDNA silencing through collaboration with RNAi machinery in fission yeast.