Role of short RNAs in regulating the expression of genes and mobile elements in germ cells

Two main types of short RNAs, 21 to 25 nucleotides long, are involved in the negative regulation of gene expression in eukaryotes: microRNAs and small interfering RNAs (siRNAs) of the RNA interference system. MicroRNAs predominantly suppress the translation of mRNA targets, while siRNAs not only prevent mRNA translation and/or lead to mRNA degradation, but are also involved in the regulation of gene expression at the transcriptional level. In germ cells, translational regulation of gene expression plays a significant role and its mechanism has been extensively studied in oogenesis of Drosophila. The role of heterochromatization and chromatin compaction, which can repress the expression of mobile elements and other repeated elements of the genome, was studied to a lesser extent. Activation and transposition of mobile elements accompanied by mutations and chromosome rearrangements are especially dangerous in germline cells. It has been proposed that a specialized class of short RNAs, repeat associated siRNAs (rasiRNAs), can be involved in repression of the expression of mobile elements in Drosophila germ cells. Here we describe the findings on subcellular ribonucleoprotein structures characteristic of germ cells: perinuclear and polar granules containing proteins of the RNA interference and microRNA maturation system. Also, we present our own results revealing the role of genes of the RNA interference system in mobile element silencing in Drosophila.