DNA POLYMERASE III Presented by Group 3 INTRODUCTION DNA polymerase 3 (Pol III) is the primary enzyme complex involved in prokaryotic DNA replication. It was discovered by Thomas Kornberg and Malcolm Gefter in 1970 Generally, DNA polymerases function optimally within a specific pH range. For many DNA polymerases, including DNA Polymerase III, the optimal pH is around 7.4 to 8.0. However, the exact pH optimum can vary depending on the specific organism and conditions. It consists of 4005 base pairs coding for 1335 amino acids (from the start to the stop codon), giving a molecular weight of 151,273. STRUCTURE DNA polymerase III is a complex enzyme containing 10 different polypeptides (α, γ, Ɛ etc.). All those polypeptides must be present for proper replicative function. " The 5' to 3' polymerase activity and the 5'to 3' exonuclease activity both present on the α polypeptides of DNA polymerase III. The 3'to 5' proofreading activity of polymerase Ill present on the γ-polypeptide. MECHANISM OF ACTION DNA polymerase III holoenzyme (Pol III HE) is a complex molecular machine that catalyzes the synthesis of DNA during prokaryotic replication. Its mechanism of action involves a series of coordinated steps that ensure the high fidelity and efficiency of DNA synthesis. INITIATION DNA unwinding: At the replication fork, the DNA helicase unwinds the double helix, creating a single-stranded template for DNA polymerase III to act upon. Primer synthesis: Primase, an RNA polymerase, synthesizes short RNA primers, which provide a 3'-OH end for DNA polymerase III to start DNA synthesis. ELONGATION Substrate binding: DNA polymerase III binds to the DNA template, the RNA primer, and the incoming deoxyribonucleotide triphosphate (dNTP) that corresponds to the next base to be added. Base pairing: The enzyme aligns the incoming dNTP with its complementary base on the template strand, ensuring accurate base pairing. Phosphodiester bond formation: DNA polymerase III catalyzes the formation of a covalent phosphodiester bond between the 3'-OH end of the growing DNA strand and the αphosphate group of the incoming dNTP. Proofreading: The enzyme possesses proofreading activity, allowing it to check the newly incorporated base for mismatches. If an incorrect base is detected, the enzyme removes it through its 3'-5' exonuclease activity before adding the correct base. Processivity: The β clamp, a ring-shaped protein, encircles the DNA template and tethers DNA polymerase III to it, enhancing the enzyme's processivity, enabling it to synthesize long stretches of DNA without dissociating from the template. TERMINATION Reaching a termination signal: DNA polymerase III continues DNA synthesis until it encounters a termination signal, a sequence of nucleotides that signals the end of a replication unit. Removal of RNA primers: RNase H digests the RNA primers, leaving gaps in the newly synthesized DNA strand. Gap filling: DNA polymerase I, an enzyme with both polymerase and exonuclease activities, fills in the gaps left by the RNA primers and removes any remaining mismatched bases. Ligation: DNA ligase seals the nicks between adjacent DNA fragments, completing the synthesis of a continuous DNA strand. CLINICAL CORRELATION Hypomyelinating leukodystrophy (HLD) is a group of rare genetic disorders that affect the development of myelin, the fatty substance that insulates nerve cells. People with HLD have a variety of neurological symptoms, including muscle weakness, spasticity, seizures, and intellectual disability. Mutations in the POLR3A, POLR3B, and POLR1C genes are the most common causes of HLD. Scleroderma is an autoimmune disease that causes the skin and connective tissues to harden and thicken. People with scleroderma may also have problems with their blood vessels, lungs, and kidneys. Mutations in the POLR3A gene have been associated with scleroderma. Cancer Mutations in the POLR3A gene have also been associated with cancer. One study found that mutations in POLR3A were present in 6 of 8 patients with a type of cancer called mesothelioma. Immunodeficiency: Mutations in DNA polymerase III can also lead to immunodeficiency disorders, which are characterized by a weakened immune system. This can make individuals more susceptible to infections. GROUP MEMBERS 1. EXCEL DOPAMU - BU21NSC1101 2. ANEJUKWO DIVINE - BU21MED1012 3. BOYE FISAYOMI - BU21MED1002 4. AMOSU ERIAGBARAOLUWA - BU21MED1007 5. FASOLA OREOLUWAPO - BU21MED1013 6. AROGUNDADE GRACE - BU22MED2014 7. OLAWALE FAVOUR - BU21NSC1085
