Sequence changes of infectious bronchitis virus isolates in the 3' 7.3 kb of the genome after attenuating passage in embryonated eggs.

Attenuated infectious bronchitis virus (IBV) vaccines are available but the relationship between sequence and virulence is not clear. In this study, the sequences of the 3' 7.3 kb of the genome, amplified using reverse transcription-polymerase chain reaction before and after attenuation, were compared to study the relationship between virulence and the sequences of three IBV strains. After attenuation, two to six amino acid substitutions were found in the spike 1 subunit, and two or three amino acid substitutions were found in the spike 2 subunit. None or one amino acid substitution was found in the small membrane protein, and one or three amino acid substitutions were found in the membrane protein. However, no amino acid substitution was found in the nucleocapsid (N) protein, indicating that the N protein might not be related to this attenuation. The 3' untranslated region after the N gene of one strain was partially deleted after attenuation, and might be correlated with virulence. This study is the first demonstration for IBV comparing sequence changes in the 3' 7.3 kb genome after attenuation. The aforementioned information on amino acid changes might be useful in future virulence studies.