Molluscum contagiosum virus (MCV), the only member of the Molluscipoxvirus 
genus, causes benign papules in healthy people but disfiguring lesions in 
immunocompromised patients. The sequence of MCV has been completed, revealing 
that MCV encodes a probable type I topoisomerase enzyme. All poxviruses 
sequenced to date also encode type I topoisomerases, and in the case of vaccinia 
virus the topoisomerase has been shown to be essential for replication. Thus, 
inhibitors of the MCV topoisomerase might be useful as antiviral agents. We have 
cloned the gene for MCV topoisomerase, overexpressed and purified the protein, 
and begun to characterize its activities in vitro. Like other eukaryotic type I 
topoisomerases, MCV topoisomerase can relax both positive and negative 
supercoils. An analysis of the cleavage of plasmid and oligonucleotide 
substrates indicates that cleavage by MCV topoisomerase is favored just 3' of 
the sequence 5' (T/C)CCTT 3', resulting in formation of a covalent bond to the 
3' T residue, as with other poxvirus topoisomerases. We identified solution 
conditions favorable for activity and measured the rate of formation and decay 
of the covalent intermediate. MCV topoisomerase is sensitive to inhibition by 
coumermycin A1 (50% inhibitory concentration, 32 microM) but insensitive to five 
other previously reported topoisomerase inhibitors. This work provides the point 
of departure for studies of the mechanism of function of MCV topoisomerase and 
the development of medically useful inhibitors.