Comparison of the reovirus serotype 1, 2, and 3 S3 genome segments encoding the nonstructural protein sigma NS.
The sequences of the S3 genome segments of reovirus serotype 1 and 2 prototype strains are presented and these sequences are compared with the sequence of the serotype 3 S3 genome segment. The S3 genome segment encodes the nonstructural protein sigma NS which possesses affinity for ssRNA and appears to function in reovirus morphogenesis. The three S3 genome segments are closely related: all are 1198 nucleotides long and possess a single long open reading frame 366 codons long. They exhibit a serotype 1:3 relatedness pattern: there are only 13% mismatches between the S3 genome segments of serotypes 1 and 3, but 27 and 26% mismatches, respectively, between those of serotype 1 and 2 and serotype 3 and 2. The amino acid mismatches for the three sigma NS proteins are much lower (2.7, 13.9, and 13.7%, respectively), because the majority of nucleotide mismatches are in third base codon positions. The three sigma NS proteins possess a conserved secondary structure that is rich in alpha-helix content; in fact, the predicted alpha-helix content of these nonstructural proteins (about 50%) is much higher than that of the three other sigma size class proteins (about 20%), all of which are structural proteins. We also sequenced the S3 genome segment of a ts mutant of serotype 3 generated by treatment with nitrous acid and found a single nucleotide change that specifies an amino acid change that introduces a five-residue-long beta-sheet prone configuration into a long (80 amino acids) highly conserved alpha-helix in the C-terminal half of sigma NS. This change could account for this mutant's ts character. Finally, the three sigma NS proteins have diverged in only about 10% of positions, whereas the three sigma 1 proteins have diverged in about 70%. The rapid evolutionary divergence of the latter may be a result of several factors, including (i) the fact that sigma 1, but not sigma NS, is under immunologic selective pressure; (ii) the fact that the functions of sigma 1 (antigenicity and cell attachment) probably reside in two rather small domains that are not restricted spatially with respect to each other; and (iii) the fact that the functions of sigma NS, namely RNA binding and protein binding (during morphogenesis), require a highly specific overall protein configuration that may permit little variation.
Department of Microbiology and Immunology, Duke University Medical Center, Durham, North Carolina 27710.
J R
JR
Wiener
W K
WK
Joklik
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D000954:Antigens, Surface / Q000276:immunology; D001483:Base Sequence; D005075:Biological Evolution; D005814:Genes, Viral; D008969:Molecular Sequence Data; D011487:Protein Conformation; D012087:Reoviridae / Q000235:genetics*; D014764:Viral Proteins / Q000235:genetics*
D000596:Amino Acids; D000954:Antigens, Surface; D014764:Viral Proteins