Characterization of Bunyamwera virus defective interfering particles

In an attempt to isolate conditional lethal amber nonsense mutants of Bunyamwera virus, five variants were found which produced small plaques on BHK and mouse L cells. Characterization of these variants by Northern blotting showed that they synthesized defective (subgenomic) RNAs derived from the L RNA segment. No subgenomic M or S segment RNAs were detected. The defective L RNAs were shown to be packaged into virus particles, and four of five preparations caused interference with the multiplication of standard virus. When defective-containing preparations were mixed with standard virus and grown in doubly infected cells a reduction in titre of standard virus of up to 400-fold was observed. Hence these preparations most probably contained defective interfering (DI) particles. Novel DI-specific polypeptides were synthesized in DI virus-infected cells. These novel proteins could be precipitated by antisera raised against either the N or C terminus, or both, of the L protein. Nucleotide sequence analysis of cloned cDNA to prominent DI RNAs in three different defective virus preparations revealed that the DI RNA in each case had suffered a single internal deletion of the L segment while retaining the 5'- and 3'-terminal sequences. The extent of the deletion ranged between 72% and 77% of the L RNA segment. Our results suggest that these DI particles may have arisen during the attempted isolation of Bunyamwera virus amber mutants on mouse L cells, since defective/subgenomic RNAs derived from the L and M segments were readily generated in mouse L cells but not in BHK cells, following infection with wild-type virus.