Question

The β (beta) subunit of E. coli DNA polymerase III possesses which of the following enzymatic activities?

1. 3’- 5’ polymerase

2. 3’- 5’ exonuclease

3. 5’ - 3’ exonuclease

4. 5’ - 3’ polymerase

5. the beta subunit is not an enzyme

Answer 1

The correct option is the 5th one ( beta subunit has no enzymatic activity)

Beta subunit codes for a clamp. This clamps locks the enzymatic parts on DNA strand and increases the processivity (No of nucleotides added at each binding with DNA or template) ,

DNA polymerase III is the main enzyme for DNA synthesis (replication) in prokaryotes.This enzyme has very high processivity. Since, it is the main enzyme involved in DNA replication, it has dual activity

DNA synthesizing activity in 5'-3' direction

DNA exonuclease activity in 3'-5' direction (proof reading)

DNA pol III is a complex holoenzyme. It has following subunits with different functions-

2 subunits of core enzyme- each consisting alpha, epsilon and theta

a sliding clamp- 2 beta subunits

and a clamp loading complex- with subunits like delta, gamma, psi subunits.

option 1 is incorrect- as 3'-5' polymerase is not found in any DNA polymerase. DNA synthesis always takes place in 5' to 3' direction. Hence, this option is wrong

Option 2 i incorrect as epsilon subunit of DNA polymerase III has 3'- 5' exonuclease (proof reading) activity, not the beta subunit. Hence, this option is wrong

Option 3 is incorrect as- No subunit of DNA polymerase III has 5'-3' exonuclease activity. This activity is shown by DNA polymerase I. Hence, this option is wrong

Option 4 is incorrect as alpha subunit of DNA polymerase possess 5'-3' polymerase activity. Beta subunit shows no polymerase activity . Hence, this option is wrong