Fine structure analysis of Pichinde virus nucleocapsids.

The structure and organization of the ribonucleoprotein (RNP) complex of an arenavirus, Pichinde virus, was investigated. The basic configuration of the RNP was found to be a linear array of globular subunits or nucleosomes, 4 to 5 nm in diameter, that represent individual molecules of the major N polypeptide. This filament appears to fold progressively through a number of intermediate helical structures, 12 to 15 nm in diameter, that reveal an increasing number of nucleosomes associated with each turn of the helix. They range from a fragile configuration of two or three nucleosomes per turn to a more stable fibre in which the nucleosomes cannot be resolved. The strands were shown to form closed circles and it appeared that the twisting of these circular forms resulted in the formation of 20 nm-thick fibres which were seen in isolated viral core structures. The association of these RNP structures with other viral components is discussed.