Bluetongue virus evolution: sequence analyses of the genomic S1 segments and major core protein VP7

The S1 segments, encoding the group-specific antigen, VP7, from the five United States prototype BTV serotypes were cloned as full-length entities. The nucleotide and deduced amino acid sequences of segment S1 of BTV-2 were determined and compared with BTV-10, -11, -13, and -17, completing the sequencing of this cognate gene segment from all five US BTV serotypes. Each segment is 1156 bp long and contains an open reading frame encoding the 349-amino acid VP7 protein. Most (greater than 94%) of the amino acids of VP7 among the serotypes are conserved, including the location (position 255) of a single lysine residue. Secondary structure analyses of VP7 predict a putative eight-stranded beta-barrel between amino acid positions 150 and 250, a structure similar to that observed in ssRNA viruses. The S1 genes are flanked by conserved 5' and 3' noncoding regions. Stem-loop structures are predicted at the 3' end of each gene (nucleotide positions 1058-1097). The S1 segments of BTV-2, -10, -11, and -17 have greater than 93% of the nucleotides conserved, while less than 80% of their bases are identical with BTV-13. Analyses of nucleotide mismatches in each codon position of the VP7 open reading frame, transition frequencies, and evolutionary distances show that of the five, BTV-13 is the most distantly related and that BTV-10 and -17 are the most closely related serotypes. Evolutionary distance calculations of segment L2 from BTV-10, -11, and -17 concur with these observations. Comparison of this relationship with hybridization data of segment M3, which codes for VP5, suggests that BTV-17 has evolved by a combination of genetic drift and genomic reassortment. The data also indicate that the five US BTV serotypes are derived from two distinct gene pools. Evolution distances were used to estimate an evolution rate of 2.2 x 10(-3) nucleotide substitution/site/year for BTV segment S1. This rate is similar to the genes of retroviruses and implies an absence of RNA polymerase proofreading activity for dsRNA viruses.