Prokaryotic DNA Repair: SOS Response

Summary

The SOS response is a complex DNA repair mechanism activated when DNA is damaged. It begins with the accumulation of single-stranded DNA when DNA polymerase encounters damage. This activates RecA, which interacts with LexA to de-repress SOS genes. Early proteins, such as UvrD and UvrA, attempt simpler nucleotide excision repair. If this fails, the cell activates late proteins like SulA, which inhibits cell division, leading to mutagenic repair. Although effective, the process is costly for the cell, balancing repair with cellular resources in response to DNA damage.

Takeaways

• 😀 DNA damage triggers repair mechanisms like base excision repair and nucleotide excision repair.
• 😀 The SOS response is a more complex process involving gene translation and cellular interaction.
• 😀 LexA is a repressor protein that binds to the SOS box, controlling the expression of repair genes.
• 😀 DNA damage, such as UV irradiation, alkylating agents, and structural changes, activates the SOS response.
• 😀 DNA polymerase encounters damage, leading to an accumulation of single-stranded DNA, which activates RecA.
• 😀 RecA protein forms filaments around single-stranded DNA, consuming ATP in the process.
• 😀 Active RecA cleaves LexA, leading to the derepression of SOS repair genes and the activation of early proteins.
• 😀 Early proteins (e.g., UVR, UVRD) attempt simpler repair processes like nucleotide excision repair.
• 😀 If early repair mechanisms fail, the SOS response escalates to late repair mechanisms, consuming more cellular energy.
• 😀 Late proteins (e.g., SolA, UDC) cause filamentation by halting cell division, allowing more extensive repair.
• 😀 The SOS response is a costly process for the cell, as it involves mutagenic repair to fix severe DNA damage.

Q & A

What are the primary DNA repair mechanisms mentioned in the script?

The primary DNA repair mechanisms mentioned are base excision repair (BER) and nucleotide excision repair (NER), both of which use enzymes like nucleases and helicases to fix DNA damage.

What is the SOS response, and why is it considered more complex than other repair mechanisms?

The SOS response is a complex DNA repair system activated when the DNA is severely damaged, often by agents like UV irradiation. Unlike simpler mechanisms, it involves translation of genes and widespread interaction within the cell.

What role does RecA play in the SOS response?

RecA is activated by the accumulation of single-stranded DNA. It forms filaments and uses ATP to facilitate the cleavage of LexA, which derepresses genes involved in the SOS response.

How does LexA contribute to regulating the SOS response?

LexA is a repressor that binds to the SOS box, inhibiting the expression of downstream genes. When RecA activates, it cleaves LexA, allowing the expression of SOS genes involved in DNA repair.

What triggers the activation of the SOS response?

The SOS response is triggered by DNA damage that stalls DNA polymerase, creating an accumulation of single-stranded DNA, which activates RecA and begins the repair process.

What happens when the early repair mechanisms (e.g., nucleotide excision repair) are insufficient?

If early repair mechanisms fail to resolve the damage, the single-stranded DNA accumulation continues, leading to further activation of RecA and the activation of late SOS genes.

What is the role of SulA in the SOS response?

SulA is a late protein in the SOS response that inhibits cell division by binding to FtsZ, causing filamentation, which is the elongation of the cell without division.

What is filamentation, and how does it relate to the SOS response?

Filamentation is a process where bacterial cells elongate without dividing. This occurs as a result of the inhibition of cell division by SulA during the SOS response.

Why is the SOS response considered costly for the cell?

The SOS response becomes costly because it involves mutagenic repair mechanisms, halts normal cellular processes like division, and requires significant energy to fix DNA damage that couldn't be repaired by simpler methods.

What are the key proteins involved in the early SOS response, and what do they do?

The key early proteins include uvrD and uvrA. They are involved in simpler nucleotide excision repair processes, addressing DNA damage before more complex and energy-intensive repairs are required.