Indispensable, functionally complementing N and C-terminal domains constitute site-specific topoisomerase I.

Mycobacterium smegmatis topoisomerase I differs from the typical type IA topoisomerase in many properties. The enzyme recognizes both single and double-stranded DNA with high affinity and makes sequence-specific contacts during DNA relaxation reaction. The enzyme has a conserved N-terminal domain and a highly varied C-terminal domain, which lacks the characteristic zinc binding motifs found in most of the type I eubacterial enzymes. The roles of the individual domains of the enzyme in the topoisomerase I catalyzed reactions were examined by comparing the properties of full-length topoisomerase I with those of truncated polypeptides lacking the conserved N-terminal or the divergent C-terminal region. The N-terminal larger fragment retained the site-specific binding, DNA cleavage and religation properties, hallmark characteristics of the full-length M.smegmatis topoisomerase I. In contrast, the non-conserved C-terminal fragment lacking the typical DNA binding motif, exhibited non-specific DNA binding behaviour. The two polypeptide fragments, on their own do not catalyze DNA relaxation reaction. The relaxation activity is restored when both the fragments are mixed in vitro reconstituting the enzyme function. These results along with the DNA interaction pattern of the proteins implicate an essential role for the C-terminal region in single-strand DNA passage between the two transesterification reactions catalyzed by the N-terminal domain.