Development of measles virus in vitro

The sequence of development of measles virus in cells cultivated in vitro was studied by electron microscopic techniques. The observations were correlated with virus growth measured by infectivity assay. Viral nucleoprotein tubules were first detected in the cytoplasm of infected cells, and shortly thereafter similar tubules were detected in the nucleus. In any one cell tubules were usually present either in the nucleus or in the cytoplasm, and rarely in both. No evidence suggesting that tubules migrated from one site to the other was obtained. At later stages in the infectious cycle, cytoplasmic tubules were extruded by a budding process at the cell surface, resulting in the formation of enveloped particles, interpreted as measles virions. Cytoplasmic tubule formation was associated with active budding and high yields of virus, whereas when intranuclear formation was predominant, little or no budding was detected and viral yields were low.The distribution of lengths of isolated nucleocapsids in negatively stained preparations showed a peak at 1.4-1.6 μ. When allowance was made for stretching of the nucleocapsid helix, this figure was reduced to 1.1 μ. The number of helical turns per nucleocapsid peaked sharply at a value of 204. On the assumption that the conformation of the measles nucleocapsid is similar to that of tobacco mosaic virus, the length and molecular weight of measles virus RNA were calculated and found to be 6.4μ and 6.2 × 10⁶ daltons, respectively.