Isolation and characterization of TK-deficient mutants of African swine fever virus

African swine fever virus induces the synthesis of thymidine kinase (TK) in BHK TK-negative cells as an immediate early protein. The TK gene is not essential for growth of ASFV in cell culture and a stable viral strain deficient in TK has been isolated (E70NTKp). The genetic lesion of this ASFV TK− strain was identified by TK gene nucleotide sequencing, showing a nucleotide deletion leading to a −1 frameshift and a nonsense codon residue downstream of the deletion. The availability of this viable ASFV variant deficient in TK activity allows the insertion of foreign genes in the ASFV genome for genetic and biochemical studies.