Studies of bacterial defence systems have led to the discovery of restriction-modification and CRISPR-Cas systems, whose proteins, such as restriction endonucleases and Cas nucleases, have become indispensable in DNA manipulation experiments and revolutionized genetic engineering and genome editing, respectively. Currently, about 100 different bacterial defence systems have been identified, which differ in their genetic organization, the composition of various functional domains, and the presumed mechanisms of action. Therefore, detailed studies of their functional, structural and action mechanisms are relevant in the fundamental aspect and may also determine the discovery of new areas of their application. Functional and structural studies of split prokaryotic Argonaute proteins will be carried out during the studies. If necessary other newly discovered prokaryotic defense systems will also be studied. Both in vitro and in vivo studies of selected defence systems will be performed during the studies. Besides biochemical characterisation, structural studies of selected proteins will be carried out using X-ray diffraction and cryo-electron microscopy (cryo-EM) methods. Based on the obtained results, the mechanisms of the defence systems will be proposed.