Coordinating Synthesis of the Lagging and Leading Strands

KEY CONCEPTS
- Different enzyme units are required to synthesize the leading and lagging strands.
- In E. coli, both of these units contain the same catalytic subunit (DnaE).
- In other organisms, different catalytic subunits might be required for each strand.
Each new DNA strand, leading and lagging, is synthesized by an individual catalytic unit. FIGURE 1. shows that the behavior of these two units is different because the new DNA strands are growing in opposite directions. One enzyme unit is moving in the same direction as the unwinding point of the replication fork and synthesizing the leading strand continuously. The other unit is moving “backward” relative to the DNA, along the exposed single strand. Only short segments of template are exposed at any one time. When synthesis of one Okazaki fragment is completed, synthesis of the next Okazaki fragment is required to start at a new location approximately in the vicinity of the growing point for the leading strand. This requires that DNA polymerase III on the lagging strand disengage from the template, move to a new location, and be reconnected to the template at a primer to start a new Okazaki fragment.

FIGURE 1. A replication complex contains separate catalytic units for synthesizing the leading and lagging strands.
The term enzyme unit avoids the issue of whether the DNA polymerase that synthesizes the leading strand is the same type of enzyme as the DNA polymerase that synthesizes the lagging strand. In the case we know best, E. coli, there is only a single DNA polymerase catalytic subunit used in replication, the DnaE polypeptide. Some bacteria and eukaryotes have multiple replication DNA polymerases .

The active replicase is an asymmetrical dimer with one unit on the lagging strand and one on the leading strand . Each half of the dimer contains DnaE as the catalytic subunit. DnaE is supported by other proteins (which differ between the leading and lagging strands).
The use of a single type of catalytic subunit, however, might be atypical. In the bacterium Bacillus subtilis, there are two different catalytic subunits. PolC is the homolog to E. coli’s DnaE_BS and is responsible for synthesizing the leading strand. A related protein, DnaE is the catalytic subunit that synthesizes the lagging strand.
Eukaryotic DNA polymerases have the same general structure, with different enzyme units synthesizing the leading and lagging strands .
A major problem of the semidiscontinuous mode of replication follows from the use of different enzyme units to synthesize each new DNA strand: How is synthesis of the lagging strand coordinated with synthesis of the leading strand? As the replisome moves along DNA, unwinding the parental strands, one enzyme unit elongates the leading strand. Periodically, the primosome activity initiates an Okazaki fragment on the lagging strand, and the other enzyme unit must then move in the reverse direction to synthesize DNA. The next sections describe how leading and lagging strand replication is coordinated by interactions between the leading and lagging strand enzyme units.