Common sequence motifs in DNA relaxases and nick regions from a variety of DNA transfer systems

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Abstract
Site and strand specific nicking at a transfer origin (oriT) is the central event in initiation of transfer DNA replication during bacterial conjugation. Recently, sequence identity in a DNA region adjacent to the nick sites of IncP plasmids RP4 and R75 1 and part of the border sequences of agrobacterial Ti and Ri plasmids has been demonstrated (Figure 1 and ref. 1). A search for this ‘nick region’ in a variety of transfer origins of other plasmids demonstrates its conservation in R64 (IncIl) and pTF-FC2, a mobilizable plasmid from Thiobacillus ferrooxidans (Figure 1, ref. 2). This finding is paralleled by ococcurrence of a common sequence motif in proteins (DNA-relaxases) encoded by the respective plasmids that interact with their cognate nick regions (Figure 2). Using the corresponding consensus sequence, a search in protein data bases (NBRF-Protein Release 27.0; Swiss-Prot Release 16.0) revealed that the ‘relaxase motif’ is also occurring in potential DNA-relaxing proteins (R1x) of mobilizable plasmids from S.aureus (Figure 2).