CONSERVATION OF STRUCTURE AND MECHANISM BETWEEN EUKARYOTIC TOPOISOMERASE-I AND SITE-SPECIFIC RECOMBINASES

Vaccinia DNA topoisomerase breaks and rejoins DNA strands through a DN A-(3'-phosphotyrosyl)-enzyme intermediate. A C-terminal catalytic doma in, Topo(81-314), suffices for transesterification chemistry. The doma in contains a constellation of five amino acids, conserved in all euka ryotic type IB topoisomerases, that catalyzes attack of the tyrosine n ucleophile on the scissile phosphate. The structure of the catalytic d omain, consisting of ten alpha helices and a three-strand beta sheet, resembles the catalytic domains of site-specific recombinases that act via a topoisomerase IB-like mechanism. The topoisomerase catalytic pe ntad is conserved in the tertiary structures of the recombinases despi te scant sequence similarity overall. This implies that the catalytic domains of type IB topoisomerases and recombinases derive from a commo n ancestral strand transferase.