FIG. 3.

The theta-type replicon of the Pseudomonas plasmid pPS10. The iteron-containing origin (oriV) and motifs found in the replication initiator protein (RepA) are depicted. The minimal origin (oriV) of pPS10 plasmid is a good example for a “canonical” iteron-containing origin. It is composed of four contiguous and identical 22-bp iterons arranged as direct repeats (half arrows), flanked by a 9-bp dnaA box (hatched) and AT- and GC-rich sequences (215). The pPS10 replicon also contains the repA gene, encoding the RepA initiator protein (shadowed ovals). RepA is under a monomer-to-dimer equilibrium, which has consequences for protein activity: RepA dimers bind to an inverted repeat (with a sequence partially homologous to that of the iterons) that overlaps with the repA promoter (P), acting as self-repressor of repA expression, whereas RepA monomers bind to the iterons to form the initiation complex (94). Protein motifs found in RepA are depicted under the protein gene. The LZ motif, responsible for protein dimerization, is represented as a helical wheel projection, in which the hydrophobic spine of Leu residues and the chemical nature of the other displayed residues is indicated (103). For the HTH motif, involved in binding to DNA, the two proposed α-helices are underlined and a stretch of basic residues at the C end of the DNA recognition helix is indicated (+), whereas an arrow points to the Gly residue thought to start the turn (93).