No major differences in size were observed when both the double-stranded RNA and the polypeptides of the Australian bluetongue virus (BTV) isolate CSIRO 19 (BTV-20) were compared with those of other BTV serotypes such as BTV-10 and BTV-4. Minor capsid polypeptide P6 of both
BTV-20 and BTV-4, which electrophoreses as a single band on continuous phosphate buffered gels, is separated into 2 distinct bands on discontinuous glycine-buffered gels. This was not the case with BTV-10.
Cross-immune precipitation of BTV-20 with BTV-10, BTV-17, BTV-4 and BTV-3 indicated strong immunological cross-reaction of the group-specific antigen P7 of the different serotypes.
There was also some cross-immune precipitation of the serotype-specific polypeptide P2 of BTV-20 and BTV-4. This result is in agreement with the observed cross neutralization of these 2 viruses. The main distinction between BTV-20 and the other BTV serotypes was observed in crosshybridization experiments. The homology between the nucleic acid of BTV-20 and other BTV serotypes was less than 30%, whereas homology normally found between BTV serotypes is at least 70%.
The hybridization products of the different BTV serotypes were analysed by electrophoresis and fluorography. Two main hybrid segments were observed in all heterologous hybridizations with BTV-20 as compared with 7 hybrid segments in hybridizations between BTV-4 and BTV-10. In order to determine from which genome segment of BTV -20 these 2 hybrid segments were derived, the hybridizations were carried out with individually purified double-stranded RNA segments. These results indicate that the 2 segments of BTV-20 that show the largest homology to corresponding segments of a heterologous BTV serotype are No. 7 and 10.
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