HEPATITIS-B VIRUS GENOME IS ORGANIZED INTO NUCLEOSOMES IN THE NUCLEUSOF THE INFECTED CELL

Hepatitis B virus (HBV) nucleoprotein complexes were isolated from nuc lei of the human hepatoblastoma eel line HepG2.2.15. Under conditions of physiological ionic strength, the complexes sedimented at a rate co rresponding to about 82 S. They contained viral DNA, histone, and nonh istone proteins. For DNA a circular, covalently closed structure was s hown both by CsCl gradient centrifugation and electron microscopy. Spr ead preparations revealed the typical ''beads-on-a-string'' appearance of nucleosomally organized DNA. The average number of nucleosomes was 18, resulting in a biochemical repeat unit of HBV chromatin of approx imately 180 base pairs of DNA. This value was confirmed by experiments analyzing the structure of the HBV chromatin by the use of micrococca l nuclease. Electron microscopy demonstrated that exposure to high ion ic strength conditions resulted in removal of nucleosomes from the com plexes, but also revealed proteinaceous structures remaining bound to viral DNA molecules. The nature of these residual proteins is discusse d. Since native nucleoprotein complexes could be precipitated with HBV -core antibodies, core protein appeared to be one of the nonhistone pr oteins.