Recognition of oriT at the termination of conjugal transfer by MobA, the R1162 DNA strand transferase

R1162 is efficiently mobilized during conjugation by IncP-1 plasmids such as RK2 and R751. Transfer is terminated when the transferred strand, linearized at the 38 base-pair origin of transfer (oriT), is recircularized by the plasmid encoded protein MobA. For strand rejoining, MobA covalently linked to the 5' end of the strand rejoins the ends by a reversible transesterification reaction. The minimal oriT fragment of R1162 contains a highly conserved 12 base region (core) including the cleavage site and a ten base imperfect inverted repeat (IR) that is not highly conserved. From those oligonucleotides with a partially degenerate oriT core base sequence, the subpopulations that are bound by MobA, cleaved and rejoined by this protein and support termination of transfer were identified. Both the IR and the adjacent core bases TAA, are needed for tight binding to MobA, whereas the location of the dinucleotide YG determines the site of strand cleavage. At the IR MobA stabilizes duplex DNA during gel electrophoresis and binds weakly to oligonucleotides lacking the outer arm of the inverted repeat, supporting a model where secondary structure at IR provides a duplex region needed for binding during termination (Zhang and Meyer 1995). Significantly altered IR sequences did allow strong binding to MobA yet completely different IR sequences did not, indicating the IR serves a structural role for binding with low base specificity. A 184 residue aminoterminal MobA fragment capable of binding and cleaving oriT was identified by using phage display and partial enzymatic digestion of the protein. No smaller fragments that could bind or cleave oriT were identified. An active nucleoprotein intermediate consisting of MobA covalently linked to the 5' end of the cleaved oriT was used to show that a single molecule of MobA is sufficient to carry out all the DNA processing steps during transfer.