DOMAIN-STRUCTURE OF VACCINIA DNA-LIGASE

The 552 amino acid vaccinia virus DNA ligase consists of three structu ral domains defined by partial proteolysis: (i) an amino-terminal 175 amino acid segment that is susceptible to digestion with chymotrypsin and trypsin; (ii) a protease-resistant central domain that contains th e active site of nucleotidyl transfer (Lys-231); (iii) a protease-resi stant carboxyl domain. The two protease-resistant domains are separate d by a protease-sensitive interdomain bridge from positions 296 to 307 . Adenylyltransferase and DNA ligation activities are preserved when t he N-terminal 200 amino acids are deleted. However, the truncated form of vaccinia ligase has a reduced catalytic rate in strand joining and a lower affinity for DNA than does the full-sized enzyme. The 350 ami no acid catalytic core of the vaccinia ligase is similar in size and p rotease-sensitivity to the full-length bacteriophage T7 DNA ligase.