T4 phage replisome

The bacteriophage T4 DNA replisome has been a useful model system for studying cellular DNA replication. Several decades of studies revealed that despite the variation in number and nature of individual proteins in the T4 replisome as compared to other model systems (Escherichia coli and yeast Saccharomyces cerevisiae), the fundamental components that constitute a functional replication fork in bacteriophage T4 faithfully represent other more complex replication systems (Fig. 16.1; Table 16.1). Therefore what has been learned from the T4 replisome can be extended to the replisomes of other organisms, including higher eukaryotes. One advantage of the T4 replisome as a model system is its manipulable complexity. The eight proteins that constitute the T4 replisome are the DNA polymerase (gp43), the clamp loader and clamp proteins (gp44/62 and gp45), the single-stranded DNA-binding protein (gp32), the primase and the helicase (gp61 and gp41), and the helicase-loading protein (gp59). These proteins form sub-complexes including DNA polymerase holoenzyme (gp43, gp45, and gp44/62), primosome (gp61, gp41, and gp59), and single-stranded DNA-binding protein (gp32). In this chapter we will discuss the properties of individual sub-complexes as well as the structural and functional aspects of their components. We will address how these complexes are assembled from individual proteins and how their functions are coordinated to ensure the efficient duplication of the T4 phage genome.