GENETICALLY-ENGINEERED FOOT-AND-MOUTH-DISEASE VIRUSES WITH POLY(C) TRACTS OF 2 NUCLEOTIDES ARE VIRULENT IN MICE

To determine the role of the poly(C) tract found at the 5' end of the genome of foot-and-mouth disease virus, synthetic RNAs (in vitro trans cripts) with poly(C) tracts of different lengths have been produced an d evaluated. RNAs with poly(C) tracts of 35, 25, 16, 6, or 2 residues displayed similar specific infectivities in baby hamster kidney (BHK) cells. Viruses recovered from cells transfected with in vitro transcri pts containing 6 to 35 Cs had properties similar to those of the wild- type vims in cell culture, and poly(C) tracts present in the synthetic RNA-derived viruses ranged from 75 to 140 bases in length. Viruses re covered from transcripts containing only two Cs showed very different properties. Specifically, viruses grew to much lower levels in cell cu lture and maintained a poly(C) tract of only two residues. The pool of viruses harvested from cells transfected with the synthetic C2 RNA al so contained a small amount of a virus with a 42-base deletion in the region of the poly(C) tract, which appeared to have arisen by recombin ation. Taken together, these data suggest that recombination provides the mechanism of poly(C) elongation and that viruses with poly(C) trac ts over 75 bases in length have a selective advantage in cell culture. Interestingly, all of the in vitro transcript-derived viruses [includ ing viruses with poly(C) tracts of only two residues] were equally vir ulent in mice, indicating that poly(C) tract length has no effect on v irulence in this animal model.