Genome organisation of the mesoniviruses. In ORF1a and ORF1b the putative ribosomal frame-shift (RFS) element and the locations of conserved functional domains, including the 3C-like serine protease (3CLP), RNA-dependent RNA polymerase (RdRp), zinc-binding (Z), helicase (Hel), exoribonuclease (ExoN), N7-methyltransferase (NMT) and 2’-O-methyltransferase (OMT) domains, are shown. Sequence insertions in ORF1a are shown as black blocks. In ORF2a encoding the S glycoprotein the sites of proteolytic cleavage to generate S1, S2 and the uncharacterised N-terminal fragment are shown as black inverted triangles. Putative ORF4 that is relatively conserved in the 3’terminal region of the genome is shown as black rectangles and other small ORFs >180 nt that occur variously in the 3’-terminal domains and elsewhere in the genome are shown as grey rectangles.