Characterization of an acid-resistant mutant of foot-and-mouth disease virus

A foot-and-mouth disease virus mutant which is stable at pH 6.4 has been isolated from a virus of serotype A. In contrast to the parent (P) virus, which gave a mixture of large and small plaques in BHK21 cells and in a bovine kidney cell line, the acid-resistant (AR) virus gave small plaques which did not increase markedly in size after 24 hr. The infectivity titer of the acid-resistant virus was about 100-fold lower in suckling mice than in BHK21 cells, whether the inoculation was made intraperitoneally or intracerebrally, whereas the parent.virus gave similar titers in both systems. Furthermore, in mice the AR virus reached its end point two to three times more slowly. The diameter of the AR virus was almost 20% less than that of the P virus and it had a more distinct topography, but the two viruses cosedimented in sucrose gradients. However, the buoyant density in CsCI of the AR virus was slightly lower (1.42 compared with 1.43 g/cc) in coruns. The RNAs and capsid proteins of the two viruses gave similar profiles in sucrose gradients and by SIDS-PAGE, respectively. However, isoelectric focusing of the capsid proteins revealed considerable differences between the two viruses. Whereas the P virus gave four protein bands, corresponding to VP1–VP4, the AR virus gave one band for VP4, two for VP3, two for VP2, and four for VP1. Sequence analysis of the genes coding for the capsid protein regions of the two viruses showed four changes (one silent), resulting in an Ala-3 → Ser substitution in VP1 and Glu-131 → Lys and Asp-133 → Ser substitutions in VP2.