"The discovery of RNA interference in plant and Caenorhabditis elegans, and all related RNA silencing processes, was one of the major progresses in modern biology and has added a higher degree of complexity to our understanding of the regulation processes in cell. The most abundant and studied eukaryotic regulatory RNAs are miRNAs. Although the mode of action of miRNAs is well known, the principles governing their expression and activity are only beginning to emerge. Approximately 50% of mammalian miRNA loci are found in close proximity to other miRNAs. These clustered miRNAs are transcribed from a single polycistronic transcription unit leading to a primary transcript which is successively matured by a nuclear (Drosha) and a cytoplasmic (Dicer) RNase III-type enzyme to yield small RNA duplexes.Expression of many miRNAs is spatially and temporally highly controlled. More attributed to transcriptional control, this phenomenon is also now related to differential processing of precursors, or degradation of mature miRNAs.Recently, viruses infecting mammalian cells (mostly of the herpes family) have been shown to express their own miRNAs. These miRNAs can act both in cis, to ensure accurate expression of viral genomes, and in trans, to modify the expression of host transcripts. They regulate fundamental processes in the cell such as immunity, apoptosis, or key steps for optimal infection environment establishment. The project will address how viral miRNAs are processed and how this processing is regulated. We intend to approach the question in a structural way, using the large miRNA cluster found in Kaposi’s Sarcoma-Associated Herpesvirus (KSHV) as a model to uncover mechanisms explaining the sequential steps leading from a long RNA molecule containing numerous hairpins to short RNA duplexes. Interestingly, although all 12 KSHV miRNAs derive from a single pri-miRNA in latently cells, dramatic differences in their levels of expression were observed."