Figure 1. Alternative plasmid structures: (a) supercoiled circular plasmid; (b) linear ‘racket frame’ structure and (c) an ‘endless linear’ plasmid (5¢- and 3¢-DNA termini are joined by a single-strand loop).

Figure 2. Segregation of incompatible plasmids.

Figure 3. Control of replication by antisense RNA in plasmid ColE1. ColE1 illustrates the way that antisense RNA can provide a simple negative feedback loop that controls replication in step with cell growth. RNA I is produced in large quantities but is unstable so its concentration is proportional to plasmid copy number. RNA II is made at low level but is more stable and folds in a way that allows association with the replication origin before RNAaseH processing. RNA II transiently forms a structure that associates with hairpin loops of RNA I and is inactivated, so when copy number is high and RNA I concentration is high, very little RNA I is allowed to proceed to form primer for replication. Rom promotes the association of RNA I and II and so further reduces copy number.

Figure 4. Replication control of the P1 prophage by plasmid handcuffing (a) the R-replicon of phage P1; iterons flanking repA are indicated by arrows (b) following replication, plasmid pairing is mediated by iteron-bound RepA and eventually disrupted by partition and cell division.