Nucleotide sequence of the 24S subgenomic messenger RNA of a vaccine strain (HPV77) of rubella virus: comparison with a wild-type strain (M33).

A full-length cDNA clone for the 24S subgenomic mRNA of the vaccine strain (HPV77) of rubella virus has been isolated from a cDNA library made from the RNAs of infected cells. Starting from the first Met start codon, the 24S mRNA codes for a precursor protein of 1063 amino acids (aa). This precursor encodes a capsid protein of 300 aa, and two envelope proteins, E1 (481 aa) and E2 (282 aa). Both the E1 and E2 proteins are preceded by a stretch of 21 hydrophobic aa, characteristic of a signal peptide, and each has three putative glycosylation sites in the polypeptide chains. Comparison between the structural proteins of the vaccine and the wild-type (wt; M33) strains of rubella virus, revealed that the E2 protein of the vaccine strain differs, in its apparent Mr, by approx. 3 kDa, from the wt strain. The difference could be due to decreased glycosylation of the vaccine strain E2 protein, as revealed by [3H]mannose incorporation studies. Five single-aa changes in the structural proteins occurred during the attenuation process, one each in the capsid and the E1 protein and three in the E2 protein. The change of Thr-412----Ile in the E2 protein results in the loss of a putative glycosylation site at Asn-410, which offers a plausible explanation for decreased glycosylation of the E2 protein from the vaccine strain of rubella virus.