African swine fever virus encodes a DNA ligase

Sequencing of the EcoRI N' fragment of African swine fever virus (ASFV) DNA revealed an open reading frame encoding a protein similar to ATP-dependent DNA ligases. When the gene encoding this protein was expressed in Escherichia coli, a protein of the expected molecular mass was labeled in bacterial extracts upon incubation with [alpha-32P]ATP. The recombinant protein comigrated in SDS-PAGE with the putative viral DNA ligase detected in extracts of infected cells. We demonstrate that the recombinant protein is a DNA ligase by dissociation of the protein-[32P]AMP adduct with pyrophosphate and nicked DNA. The putatively adenylylated lysine in ASFV is surrounded by two arginine residues, instead of by two hydrophobic amino acids as in the other ATP-dependent DNA ligases. This might explain the high concentration of pyrophosphate necessary to revert the DNA ligase--AMP adduct in ASFV, 10- to 100-fold higher than that required for other DNA ligases. A comparison of the amino acid sequences reported for ATP-dependent DNA ligases disclosed three new amino acid motifs around the adenylylation site of these enzymes. ASFV DNA ligase has little similarity to the other enzymes at the ends of the molecule, but conserves the amino acid motifs of the central region.