Genetic Variation of mRNA1 ORF1 of Avian Infectious Bronchitis Virus LX4 Isolated in China
The fragment of mRNA1 ORF1 of infectious bronchitis virus(IBV)LX4 strain was amplified by RT-PCR method.This fragment was cloned,sequenced and analyzed with the published counterpart of IBV strain.Results showed that there were two open reading frames in the 5′-"unique"region of mRNA1 of LX4,the ORF1a was composed of 11,916 bp encoding for a polypeptide of 3,971 amino acids, the ORF1b contained 7,950bp encoding for 2,649 amino acids.Sequencing data demonstrated that the ORF1a and ORF1b amino acid sequences had 85% and 95% similarity, respectively, in comparison with the published sequences of Beaudette strain.The nucleotide sequence of"slippery sequence"and the basic structure of"pseudoknot"in LX4 ORF1 were similar to that of the Beaudette strain,but there was a point mutation in position 7 of loop2(G→A)of"pseudoknot"in LX4.Compared with Beaudette strain,there was a variable region which was located in position from (2 656) to 3 098 of LX4 and sixty nucleotides(equal to 20 amino acids)were inserted in this region.In addition,nine nucleotides were deleted(equal to 3 amino acids)in the position from 5 168 to 5 181 of ORF1b in LX4.The comparison of proteinase region between LX4 and Beaudette indicated that there was a mutation(S→A)in the N-terminal cleavage site of 3C-like proteinase(3CL~(pro))of LX4,but this mutation didn't affect the self-cleavage of 3CL~(pro) because of the tolerant mutagenesis of this site.The deduced cleavage products of p1a/pp1ab between LX4 and Beaudette had the same cleavage sites,so the mechanisms of maturity and action of the proteins encoded by the 5′-"unique"region of mRNA1 ORF1 between the two strains were perhaps identical during virus duplication.