Fidelity in RNA-based recognition of transposable elements

Identification of an active RNAi pathway in Candida albicans

RNA interference (RNAi) is a fundamental regulatory pathway with a wide range of functions, including regulation of gene expression and maintenance of genome stability. Although RNAi is widespread in the fungal kingdom, well-known species, such as the model yeast Saccharomyces cerevisiae, have lost the RNAi pathway. Until now evidence has been lacking for a fully functional RNAi pathway in Candida albicans, a human fungal pathogen considered critically important by the World Health Organization. Here, we demonstrated that the widely used C. albicans reference strain (SC5314) contains an inactivating missense mutation in the gene encoding for the central RNAi component Argonaute. In contrast, most other C. albicans isolates contain a canonical Argonaute protein predicted to be functional and RNAi-active. Indeed, using high-throughput small and long RNA sequencing combined with seamless CRISPR/Cas9-based gene editing, we demonstrate that an active C. albicans RNAi machinery represses expression of subtelomeric gene families. Thus, an intact and functional RNAi pathway exists in C. albicans, highlighting the importance of using multiple reference strains when studying this dangerous pathogen.

The origin of RNA interference: Adaptive or neutral evolution?

The origin of RNA interference (RNAi) is usually explained by a defense-based hypothesis, in which RNAi evolved as a defense against transposable elements (TEs) and RNA viruses and was already present in the last eukaryotic common ancestor (LECA). However, since RNA antisense regulation and double-stranded RNAs (dsRNAs) are ancient and widespread phenomena, the origin of defensive RNAi should have occurred in parallel with its regulative functions to avoid imbalances in gene regulation. Thus, we propose a neutral evolutionary hypothesis for the origin of RNAi in which qualitative system drift from a prokaryotic antisense RNA gene regulation mechanism leads to the formation of RNAi through constructive neutral evolution (CNE). We argue that RNAi was already present in the ancestor of LECA before the need for a new defense system arose and that its presence helped to shape eukaryotic genomic architecture and stability.

Where does RNA interference come from? This Essay describes a new step-by-step evolutionary model of how RNA interference might have originated in early eukaryotes through neutral events from the molecular machinery present in prokaryotes.

Ccr4-Not complex reduces transcription efficiency in heterochromatin

Heterochromatic silencing is thought to occur through a combination of transcriptional silencing and RNA degradation, but the relative contribution of each pathway is not known. In this study, we analyzed RNA Polymerase II (RNA Pol II) occupancy and levels of nascent and steady-state RNA in different mutants of Schizosaccharomyces pombe, in order to quantify the contribution of each pathway to heterochromatic silencing. We found that transcriptional silencing consists of two components, reduced RNA Pol II accessibility and, unexpectedly, reduced transcriptional efficiency. Heterochromatic loci showed lower transcriptional output compared to euchromatic loci, even when comparable amounts of RNA Pol II were present in both types of regions. We determined that the Ccr4-Not complex and H3K9 methylation are required for reduced transcriptional efficiency in heterochromatin and that a subset of heterochromatic RNA is degraded more rapidly than euchromatic RNA. Finally, we quantified the contribution of different chromatin modifiers, RNAi and RNA degradation to each silencing pathway. Our data show that several pathways contribute to heterochromatic silencing in a locus-specific manner and reveal transcriptional efficiency as a new mechanism of silencing.

RNA Interference (RNAi ) as a Tool for High-Resolution Phenotypic Screening of the Pathogenic Yeast Candida glabrata

After its discovery RNA interference (RNAi) has become a powerful tool to study gene functions in different organisms. RNAi has been applied at genome-wide scale and can be nowadays performed using high-throughput automated systems (robotics). The simplest RNAi process requires the expression of two genes (Dicer and Argonaute) to function. To initiate the silencing, constructs generating either double-strand RNA or antisense RNA are required. Recently, RNAi was reconstituted by expressing Saccharomyces castellii genes in the human pathogenic yeast Candida glabrata and was used to identify new genes related to the virulence of this pathogen.In this chapter, we describe a method to make the C. glabrata pathogenic yeast competent for RNAi and to use RNA silencing as a tool for low- or high-resolution phenotypic screening in this species.

Bioinformatics Analysis of Evolution and Human Disease Related Transposable Element-Derived microRNAs

Transposable element (TE) has the ability to insert into certain parts of the genome, and due to this event, it is possible for TEs to generate new factors and one of these factors are microRNAs (miRNA). miRNAs are non-coding RNAs made up of 19 to 24 nucleotides and numerous miRNAs are derived from TE. In this study, to support general knowledge on TE and miRNAs derived from TE, several bioinformatics tools and databases were used to analyze miRNAs derived from TE in two aspects: evolution and human disease. The distribution of TEs in diverse species presents that almost half of the genome is covered with TE in mammalians and less than a half in other vertebrates and invertebrates. Based on selected evolution-related miRNAs studies, a total of 51 miRNAs derived from TE were found and analyzed. For the human disease-related miRNAs, total of 34 miRNAs derived from TE were organized from the previous studies. In summary, abundant miRNAs derived from TE are found, however, the function of miRNAs derived from TE is not informed either. Therefore, this study provides theoretical understanding of miRNAs derived from TE by using various bioinformatics tools.

5' and 3' modifications controlling RNA degradation: from safeguards to executioners

RNA degradation is a key process in the regulation of gene expression. In all organisms, RNA degradation participates in controlling coding and non-coding RNA levels in response to developmental and environmental cues. RNA degradation is also crucial for the elimination of defective RNAs. Those defective RNAs are mostly produced by 'mistakes' made by the RNA processing machinery during the maturation of functional transcripts from their precursors. The constant control of RNA quality prevents potential deleterious effects caused by the accumulation of aberrant non-coding transcripts or by the translation of defective messenger RNAs (mRNAs). Prokaryotic and eukaryotic organisms are also under the constant threat of attacks from pathogens, mostly viruses, and one common line of defence involves the ribonucleolytic digestion of the invader's RNA. Finally, mutations in components involved in RNA degradation are associated with numerous diseases in humans, and this together with the multiplicity of its roles illustrates the biological importance of RNA degradation. RNA degradation is mostly viewed as a default pathway: any functional RNA (including a successful pathogenic RNA) must be protected from the scavenging RNA degradation machinery. Yet, this protection must be temporary, and it will be overcome at one point because the ultimate fate of any cellular RNA is to be eliminated. This special issue focuses on modifications deposited at the 5' or the 3' extremities of RNA, and how these modifications control RNA stability or degradation.This article is part of the theme issue '5' and 3' modifications controlling RNA degradation'.