Which of the following nucleotides is involved in the proofreading activity of DNA Pol III?

The nucleotide involved in the proofreading activity of DNA Polymerase III is epsilon. DNA Polymerase III has a complex structure that includes several subunits, and one of these subunits is the epsilon subunit, which possesses 3' to 5' exonuclease activity. This activity is crucial for the proofreading function because it allows the enzyme to remove incorrectly incorporated nucleotides during DNA replication.

When DNA Polymerase III adds nucleotides to the growing DNA strand, there is the potential for errors, such as the incorporation of a mismatched base. The epsilon subunit can identify these mismatches due to their incorrect bonding and, through its exonuclease activity, excise the erroneous nucleotide from the newly synthesized strand. This correction mechanism enhances the fidelity of DNA replication, which is vital for maintaining genetic integrity during cell division.

The other subunits mentioned do not have proofreading functionality; for instance, the alpha subunit is responsible for the polymerase activity, while beta and tau subunits play roles in clamp loading and stability of the polymerase complex but do not participate in proofreading.