In vitro transcription and translation of bluetongue virus mRNA

Fractionation of in vitro transcribed bluetongue virus (BTV) mRNA by agarose gel electrophoresis resulted in the separation of eight of the 10 species. The relative molar ratio of the mRNAs confirmed that mRNA 5 was transcribed more frequently than would be predicted from the size of the S5 genome segment, while mRNA 10 was transcribed less frequently. In vitro translation of unfractionated BTV mRNAs resulted in the synthesis of the seven known structural proteins (P1 to P7) and two known non-structural proteins (NS1 and NS2). Two additional non-structural proteins (NS3 and NS3A) with Mr of 28K and 25K respectively were identified. The protein coding assignments for the medium- and small-sized double-stranded RNA genome segments of BTV serotype 10 were found to correspond to those reported for BTV-1 and BTV-17. The peptide maps of NS1, NS2, NS3 and NS3A synthesized in vitro corresponded to those of their counterparts synthesized in infected cells. Protein NS3A appeared to be a truncated form of NS3, since its peptide map completely overlapped that of NS3. Proteins NS3 and NS3A were present in very small amounts in the soluble fraction of the cytoplasm of infected cells, and were synthesized in variable amounts in vitro, whereas the other nine viral proteins were synthesized in constant molar ratios. A difference in the relative molar ratios in which some of the BTV proteins were synthesized in vitro and in vivo was observed. In vivo, protein NS1 was translated in the largest amount but in vitro, NS2 was the most efficiently translated protein. Conversely, protein P6 was translated much more efficiently in vitro than in vivo.