RNAi-directed assembly of heterochromatin in fission yeast

Heterochromatin is an epigenetically heritable and conserved feature of eukaryotic chromosomes with important roles in chromosome segregation, genome stability, and gene regulation. The formation of heterochromatin involves an ordered array of chromatin changes, including histone deacetylation, histone H3-lysine 9 methylation, and recruitment of histone binding proteins such as Swi6/HP1. Recent discoveries have uncovered a role for the RNA interference (RNAi) pathway in heterochromatin assembly in the fission yeast Schizosaccharomyces pombe and other eukaryotes. Purification of two RNAi complexes, RITS and RDRC, from fission yeast has provided further insight into the mechanism of RNAi-mediated heterochromatin assembly. These discoveries have given rise to a model in which small interfering RNA molecules act as specificity factors that initiate epigenetic chromatin modifications and double strand RNA synthesis at specific chromosome regions.     

RNA silencing in Aspergillus nidulans is independent of RNA-dependent RNA polymerases

The versatility of RNA-dependent RNA polymerases (RDRPs) in eukaryotic gene silencing is perhaps best illustrated in the kingdom Fungi. Biochemical and genetic studies of Schizosaccharomyces pombe and Neurospora crassa show that these types of enzymes are involved in a number of fundamental gene-silencing processes, including heterochromatin regulation and RNA silencing in S. pombe and meiotic silencing and RNA silencing in N. crassa. Here we show that Aspergillus nidulans, another model fungus, does not require an RDRP for inverted repeat transgene (IRT)-induced RNA silencing. However, RDRP requirements may vary within the Aspergillus genus as genomic analysis indicates that A. nidulans, but not A. fumigatus or A. oryzae, has lost a QDE-1 ortholog, an RDRP associated with RNA silencing in N. crassa. We also provide evidence suggesting that 5' --> 3' transitive RNA silencing is not a significant aspect of A. nidulans IRT-RNA silencing. These results indicate a lack of conserved kingdom-wide requirements for RDRPs in fungal RNA silencing.