MSA of the arterivirus NSP12 N7-MTase-like domain with N7-MTases across Nidovirales. Panel (A) presents a linear topological comparison of the arterivirus NSP12 N7-MTase-like domain with N7-MTases across Nidovirales. The core α-helices and β-sheet forming the Rossmann fold are shown in blue and red, respectively, with the three-stranded insert in yellow and the C-terminal insert in teal. Secondary structural elements positioned outside of the αβα sandwich is colored gray. (B) Representative MSAs of N7-MTase domains from all five distinct Nidovirales families, with secondary structural elements colored as described. In the coronavirus N7-MTase, a short, inconsistently predicted βS3, which occasionally causes the structure to resemble and portray a seven-strand sheet, is highlighted with a yellow border. A consensus sequence is indicated at the bottom of each alignment. SAM-binding motifs, substrate-binding motifs, and Zn-chelating residues are highlighted in red, black, and magenta, respectively. Excluding the Zn-chelating residues, the SAM and substrate-binding (guanine-binding) residues exhibit erosion and lack conservation in arteriviruses. Arterivirus-specific conserved residues are highlighted in green.