E. coli DNA Polymerase III Holoenzyme

DNA polymerase III holoenzyme is a complex of several proteins. The polC gene encodes a single polypeptide chain of Mr of about 130,000. The protein has an intrinsic polymerase activity, but it is quite low. In cells, however, the PolC protein functions as part of a multiprotein aggregate called the DNA polymerase III holoenzyme. Figure 24.19 shows that the holoenzyme contains 10 different polypeptide chains, each identified with a Greek letter. The functions of these units are summarized as follows:

- PolC gene product. Has polymerase activity. Part of the core polymerase.

- Contains a 3'-5' exonuclease activity comparable to the 3'-5' exonuclease domain of polymerase I. Part of the core polymerase.

- Unknown function. Part of the core polymerase.

- This dimeric protein dimerizes the holoenzyme, holding leading and lagging strand polymerases together so both DNA strands are elongated at the replication fork.

- Mediates the switch from making RNA primers (with primase) to making DNA (with PolC).

- Functions as a sliding clamp to hold the holoenzyme complex to DNA, making the enzyme very processive.

, ', , , - This group of proteins is called the complex (also called the clamp loader) (Figure 24.21). It is composed of one copy of all the proteins except , of which there are 2 or 3 copies. The clamp loader wraps the clamp onto the DNA. Figure 24.21 shows some of the steps in this process.