Infectious bronchitis virus (IBV), a member of Coronaviridae family, causes an acute, highly contagious respiratory disease in poultry that leads to reduce quantity and quality of egg production in chickens. Genome of the virus encodes four structural proteins. Interspersed among the structural protein genes, IBV has two nonstructural protein genes, genes 3 and 5 which have been explained as important factors in pathogenesis of the virus [1]. In this study Open Reading Frames (ORFs) 3a and 3b of gene3 of different IBV reference strains were obtained from Gene Bank and molecular properties of the strains were analyzed using Bioedite and MEGA4 softwares bioinformatically. Based on 3a analysis, nucleotide similarity among different strains of IBV was 99.7-100%. Strain Zj971 from China had 100% identity with B17 from India. The highest difference was observed between H52 and CH3TZ from China. Based on 3a phylogenetic tree, strains from different countries were classified in various clusters regardless of their origins. According to the results of 3b, relatedness of different strains of IBV was 99.62-100% that the highest similarity was between three pairs including: B17 and ZJ971, Mass41 and H52, H120 and Zj971. Phylogenetic tree analysis of 3b gene classified some strains of different origins together and some Chinese strains were located in different groups. High sequence similarity of different reference strains of IBV based on 3a and 3b genes regardless of the geographical region of virus strains, suggests an important potential role of these conserved regions for the virus functions.
