Molecular characterization of attenuated vaccine strains of transmissible gastroenteritis virus

Previous studies in our laboratory demonstrated that 2 attenuated strains of transmissible gastroenteritis virus (TGEV) contain deletions affecting messenger (m) RNAs 2, 3, or 4. In this report, we have compared mRNAs of four modified-live virus vaccines for TGEV with the virulent Miller PP3 isolate to determine whether any transcriptional patterns are shared among attenuated strains. Using northern blot analysis, all vaccine viruses expressed mRNAs indistinguishable in size from those of Miller PP3. However, using S1 nuclease protection experiments, alterations in the regions of the genome from which mRNAs 2 and 3 are transcribed were detected in 2 of the vaccine strains. When genomic cDNA fragments derived from the coding region for mRNA 2 were sequenced, a 6-nucleotide deletion, also found in the attenuated strain Purdue-115, was discovered. The product of mRNA 2, a spike glycoprotein, was visualized by western blotting for each vaccine strain, and no profound differences in mobility were detected relative to Miller PP3. Alterations in the region of the genome from which mRNA 3 is transcribed appear to be identical or very similar to sequence alterations already described in this region for Purdue-115, one of which is likely to alter the polypeptide product of mRNA 3. Insertions or deletions in mRNAs 2 or 3 may contribute to attenuation but are not a prerequisite for this phenotype. The S1 nuclease protection analysis is a sensitive tool for differentiating particular strains of TGEV.