Y-forms as possible intermediates in the replication of measles virus nucleocapsids

Abstract

THE genome of measles virus, a paramyxovirus, is a non-segmented single stranded RNA molecule which is enclosed within a helix of protein subunits to form a linear nucleocapsid¹. Little is known about its mode of replication. It is known, however, for other paramyxoviruses that during the growth cycle transcription of the RNA occurs from an intact nucleocapsid by a virion-associated enzyme^2–4. The resulting messenger RNA molecules are less than the full genome length and are unencapsidated⁵. By contrast, during viral RNA replication no free genome length RNA can be detected^5,6 and both plus and minus-stranded RNA are found only in the encapsidated form⁷. From this evidence, Kingsbury has suggested⁷ that nucleocapsids of paramyxoviruses replicate without separation of the protein and nucleic acid portions of the template nucleocapsid and has implied that plus and minus-stranded RNAs enclosed within the capsid structure act as templates for each other. If this is so, association of the two types of nucleocapsid in a replicative form should be detectable in infected cells. We have conducted a search by electron microscopy for such structures in measles virus infected cells and the results are described here.