The effects of Non3 mutations on chromatin organization in Drosophila melanogaster.

The nucleolus is a large membraneless subnuclear structure, the main function of which is ribosome biogenesis. However, there is growing evidence that the function of the nucleolus extends beyond this process. While the nucleolus is the most transcriptionally active site in the nucleus, it is also the compartment for the location and regulation of repressive genomic domains and, like the nuclear lamina, is the hub for the organization of inactive heterochromatin. Studies in human and Drosophila cells have shown that a decrease in some nucleolar proteins leads to changes in nucleolar morphology, heterochromatin organization and declustering of centromeres. This work is devoted to the study of the effects of Novel nucleolar protein 3 (Non3) gene mutations in D. melanogaster on the organization of chromatin in the nucleus. Previously, it was shown that partial deletion of the Non3 gene leads to embryonic lethality, and a decrease in NON3 causes an extension of ontogenesis and formation of a Minute-like phenotype in adult flies. In the present work, we have shown that mutations in the Non3 gene suppress the position effect variegation (PEV) and increase the frequency of meiotic recombination. We have analyzed the classical heterochromatin markers in Non3 mutants and shown that the amount of the HP1 protein as well as the modification of the histone H3K9me2 do not change significantly in larval brains and salivary glands compared to the control in Western blot analysis. Immunostaining with antibodies to HP1 and H3K9me2 did not reveal a significant reduction or change in the localization patterns of these proteins in the pericentromeric regions of salivary gland polytene chromosomes either. We analyzed the localization of the HP1 protein in Non3 mutants using DNA adenine methyltransferase identification (DamID) analysis and did not find substantial differences in protein distribution compared to the control. In hemocytes of Non3 mutants, we observed changes in the morphology of the nucleolus and in the size of the region detected by anti-centromere antibodies, but this was not accompanied by declustering of centromeres and their untethering from the nucleolar periphery. Thus, the NON3 protein is important for the formation/function of the nucleolus and is required for the correct chromatin packaging, but the exact mechanism of NON3 involvement in these processes requires further investigations.