Abstract
Theiler's murine encephalomyelitis viruses are usually included in the enterovirus genus of the family Picornaviridae, although there is little physicochemical evidence to support this classification. In this report, the size of the RNA of highly virulent and less virulent representatives of the Theiler's group of viruses has been determined by sucrose gradient centrifugation and electrophoresis in agarose to be the same as that of other enteroviruses. The absence of a poly(C) residue provides evidence that these viruses are not cardioviruses or aphthoviruses. The base composition of the two members are similar to each other but differ from those of other enteroviruses. However the one- and two-dimensional maps of the ribonuclease T1 hydrolysates of the two virus RNAs show considerable differences despite their close serological similarity. Virus-specified RNA synthesis in cells infected with the more virulent strain of the virus was almost 10 times greater than that induced by the less virulent strain, in accord with the yields of virus particles.