3 RNAi Function and Diversity in Fungi

Abstract

The RNA interference (RNAi) phenomenon is taxonomically widely distributed among eukaryotes and has been found in fungi, plants, and animals, suggesting that it was already present very early in eukaryotic evolution. Genes for the three core components Dicer, Argonaute, and RNA-dependent RNA polymerase (RdRP) have been found in most fungal genomes analyzed to date, with a few exceptions. It has been suggested that the ancestral RNAi primarily had a function in defense against viruses and transposons, but a role in gene regulation is a possibility. In this chapter, RNAi functions in three main processes; genome defense, heterochromatin formation, and gene regulation are discussed. Dicer, Argonaute, and RdRP control the biogenesis of the various regulatory small interfering RNAs (siRNAs). The Dicer, an RNase III endonuclease, processes the pre-RNA to form the siRNA. The mature siRNA is then incorporated into a multiprotein complex referred to as the RNA-induced Silencing Complex (RISC) in which Argonaute is the core protein. Argonaute functions as a siRNA-guided endonuclease in the RISC. RISC then recognizes mRNAs with complementary sequence to the engaged siRNA. An RdRP then generates dsRNA from single-stranded transcripts either by de novo, primer-independent second-strand synthesis or by the use of siRNA as primers to synthesize RNA complementary to the target mRNA resulting in the production and amplification of dsRNA as well as silencing of the target gene. However, it seems to be an unlimited number of variation and species-specific differences of the mechanisms, and the more systems that are explored the more mechanisms are revealed of which a few are presented in detail here.