Restriction of fusion protein mRNA as a mechanism of measles virus persistence.

Vero cells persistently infected with the hamster neurotropic strain (HNT-PI) of measles virus are deficient in the release of extracellular virus and syncytia formation, suggesting that mutations occur within the viral envelope proteins. Nucleotide sequence comparisons indicated that the coding regions of the matrix (M) and fusion (F) genes of HNT-PI were relatively conserved compared with those of their lytic progenitor virus Philadelphia 26 (Ph26), whereas the hemagglutinin (H) gene differed by 4.2% at the amino acid level. Northern blot analyses demonstrated the predominance of bicistronic M/F transcripts in HNT-PI at a 5:1 ratio over F monocistronic mRNA. Accordingly, no F protein could be detected in the HNT-PI cell line, although both the M and H proteins were produced in amounts comparable to those of Ph26. When the Semliki Forest virus replicon was used, coexpression of the HNT-PI F and Ph26 H genes resulted in the formation of multinucleated syncytia in transfected Vero cell cultures. Since the HNT-PI F protein was fusogenic, the restriction of its monocistronic mRNA is postulated to be a contributing factor in the reduction of cell fusion and ultimately in the maintenance of the persistent infection.