SURVEY AND SUMMARY Natural covalent complexes of nucleic acids and proteins: some comments on practice and theory on the path from well-known complexes to new ones

More than 20 years have passed since the discovery of natural covalent complexes of nucleic acids and proteins (1–5). Covalent complexes of nucleic acids and proteins are both stable and highly specific because they are nucleoproteins linked by a unique covalent bond. Two important properties are associated with these complexes: (i) a novel structure, describing a new class of natural nucleoproteins; (ii) specific functions, that have led to an understanding of previously undiscovered mechanisms of DNA and RNA replication (i.e. the protein priming mechanism) and that are ultimately needed to change the topology of DNA, for DNA integration, conjugation, DNA structure resolution and other as yet unknown functions. Due to availability and the ability to obtain homogeneous preparations, the majority of data on structure and function have been obtained with covalent nucleic acid and protein complexes of viral origin.