Infectious foot-and-mouth disease virus derived from a cloned full-length cDNA.

A full-length cDNA plasmid of foot-and-mouth disease virus has been constructed. RNA synthesized in vitro by means of a bacteriophage SP6 promoter inserted in front of the cDNA led to the production of infectious particles upon transfection of BHK-21 cells. These particles were also found to be highly infectious for primary bovine kidney cells as well as for baby mice. The difficulty in cloning the foot-and-mouth disease virus cytidyl tract in Escherichia coli was circumvented by joining two separate cloned parts, representing the S and L fragments of the genome, and, in a second step, inserting a dC-dG homopolymer. Homopolymeric sequences of up to 25 cytidyl residues did not lead to the production of virus. Replicons containing poly(C) tracts long enough to permit virus replication were first established in yeast cells. One of these constructs could also be maintained in E. coli and was used to produce infectious RNA in vitro. The length of the poly(C) sequence in this cDNA plasmid was 32 nucleotides. However, the poly(C) tracts of two recombinant viruses found in transfected BHK-21 cells were 60 and 80 nucleotides long, respectively. Possible mechanisms leading to the enlargement of the poly(C) tract during virus replication are discussed.