Nicking and joining activity of banana bunchy top virus replication protein in vitro

The major open reading frame of banana bunchy top virus (BBTV) DNA-1 encodes a putative replication initiation protein (Rep). In vitro, a fusion protein of BBTV Rep linked to a maltose-binding protein exhibited both site-specific nicking and joining activities. These activities were dependent on the presence of Mg2+ or Mn2+, but did not require ATP. The fusion protein specifically cleaved ssDNA between bases +7 and +8 of a conserved nonanucleotide loop sequence which is present in the virion-strand of the stem-loop common region of each BBTV component. During this reaction, the fusion protein became covalently attached to the 5' end of the 3' cleavage product. After the nicking reactions, the fusion protein was also capable of catalysing the joining of two nicked ssDNA fragments in a site-specific manner. Based on these activities, BBTV Rep would appear to be very similar to the Rep proteins of the geminiviruses.