FIG. 1.

Schematic of the SARS-CoV genome and the sequence of Nsp15. The approximate locations of polyproteins 1a (pp1a) and 1ab (pp1ab) in the SARS-CoV genome are indicated. Proteins made from subgenomic messages encoded by the 3′ portion of the SARS-CoV genome are shown. For clarity, the individual coding sequences are not shown. The Nsp15 sequence is from the Urbani isolate and is represented by the standard one-letter code. Comparable sequences from MHV and IBV were aligned with the CLUSTAL program (28). Residues that have identical side chains (asterisks), functionally similar side chains (colons), and somewhat similar side chains (periods) are indicated under the sequences. Gaps introduced to maximize alignment are indicated by dashes. The central portion of the MHV protein is more divergent than the comparable sequence from the SARS-CoV and IBV proteins. To help demonstrate the possible relevance of the alignments, the PHD program (21) was used to predict the positions of secondary structures predicted within each polypeptide. The residues putatively involved in the formation of β-strands or α-helices are underlined or shaded, respectively. Sequences under the thick black lines represent the region with similarities to the putative XendoU active site, as described by Snijder et al. (25).