Genetic analysis of porcine respiratory coronavirus, an attenuated variant of transmissible gastroenteritis virus.

The genome and transcriptional pattern of a newly identified respiratory variant of transmissible gastroenteritis virus were analyzed and compared with those of classical enterotropic transmissible gastroenteritis virus. The transcriptional patterns of the two viruses indicated that differences occurred in RNAs 1 and 2(S) and that RNA 3 was absent in the porcine respiratory coronavirus (PRCV) variant. The smaller RNA 2(S) of PRCV was due to a 681-nucleotide (nt) deletion after base 62 of the PRCV peplomer or spike (S) gene. The PRCV S gene still retained information for the 16-amino-acid signal peptide and the first 6 amino acid residues at the N terminus of the mature S protein, but the adjacent 227 residues were deleted. Two additional deletions (3 and 5 nt) were detected in the PRCV genome downstream of the S gene. The 3-nt deletion occurred in a noncoding region; however, the 5-nt deletion shortened the potential open reading frame A polypeptide from 72 to 53 amino acid residues. Significantly, a C-to-T substitution was detected in the last base position of the transcription recognition sequence upstream of open reading frame A, which rendered RNA 3 nondetectable in PRCV-infected cell cultures.