Drosophila rDNA Genes Shape the Stable Contacts with the Tlk Gene at the Expression Area of Small RNAs and Affect on Looped Domains inside the Gene

In experiments on mouse and human cells it was demonstrated that rDNA plays an important role in epigenetic regulation of many genes. To identify and study rDNA-contacting genes in Drosophila we used the 4С (circular chromosome conformation capture) approach. We detected very stable contacts of rDNA genes within a 5-kb region inside the Tlk gene residing in X chromosome. This 5-kb region corresponds to small RNAs. After heat shock treatment both the amount of contacts, and the expression level of the gene were increased. Tlk and Rala are genes that share the same short bidirectional promoter but exhibit different expression levels. Around the region of rDNA contacts inside the Tlk gene, looped domains were formed. We conclude that rDNA contact-dependent epigenetic regulation is guided by small RNAs and that the contacts are involved in rearrangements of the looped domains.