Rolling Circle DNA Replication (Molecular Biology)

In DNA replication, the DNA polymerase cannot initiate the synthesis of a new DNA strand and must rely on a priming device. In general, an RNA primer is synthesized at or near a replication origin to start synthesis of the leading strand. However, a DNA primer terminus can be generated by a nuclease-generated nick at a specific place in some circular duplex DNA, and replication will then proceed unidirectionally, as shown in Figure 1. This mode of replication is called rolling circle replication and is found for replication of the replicative form (RF) form of bacteriophage single-stranded genomes of Gram-negative bacteria and of the multicopy plasmids of Gram-positive bacteria (see Single-Stranded DNA Replication). Rolling circle replication is also observed in the late stage of the replication of the lambda phage genome and in the process of the conjugative transfer of bacterial plasmids.

Figure 1. Scheme for rolling circle replication.

Scheme for rolling circle replication.

DNA synthesis initiates using the free 3′ -OH end at the nick as a primer, and a replication fork proceeds around the template. In the process, the newly synthesized strand displaces the old strand from the template. In the case of replication of the RF form of single-stranded phage genomes and of plasmids of Gram-positive bacteria, the displaced old strand is cleaved off after one round of replication and is converted into the circular, double-stranded form. In contrast, in phage lambda replication, the replication fork proceeds a number of revolutions around the template without cleavage of the displaced strand, and the displaced strand becomes double-stranded as it is peeled off. The linear concatemer thus created is cleaved into one unit length and packaged into the phage particles. In the conjugation process of plasmids, the displaced strand is transferred into the new cell.

The initial nick of rolling circle replication is introduced by an endonuclease specific for each system. The similarity of the amino acid sequences of the initiator endonucleases and of the proteins (known as relaxases) involved in the initiation and termination of conjugative DNA strand transfer clearly indicate that they are evolved from a common ancestor (1, 2).

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