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 M r 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.

See also: E. coli DNA Polymerases, Eukaryotic DNA Polymerases, Figure 24.8, Table 24.2

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