LexA repressor

Bacterial transcriptional repressor

LexA repressor

The LexA repressor is a protein that plays a crucial role in the SOS response of bacteria. It is a transcriptional repressor that regulates the expression of genes involved in DNA repair, cell cycle control, and mutagenesis. The LexA protein is a key component in maintaining genomic stability by controlling the bacterial response to DNA damage.

Structure of LexA repressor

Structure

The LexA repressor is a dimeric protein, meaning it consists of two identical subunits. Each subunit contains a DNA-binding domain and a catalytic domain. The DNA-binding domain is responsible for recognizing and binding to specific sequences in the promoter regions of target genes, while the catalytic domain is involved in the self-cleavage reaction that inactivates the repressor under certain conditions.

The structure of LexA has been elucidated through X-ray crystallography, revealing how it interacts with DNA and undergoes conformational changes upon activation. The DNA-binding domain typically contains a helix-turn-helix motif, which is a common structural motif in DNA-binding proteins.

Function

The primary function of the LexA repressor is to inhibit the transcription of genes involved in the SOS response. Under normal conditions, LexA binds to the operator regions of these genes, preventing their expression. However, when DNA damage occurs, the RecA protein becomes activated and facilitates the self-cleavage of LexA. This cleavage inactivates the repressor, allowing the expression of SOS genes.

The SOS response includes the induction of DNA repair enzymes, cell cycle arrest, and, in some cases, error-prone repair mechanisms that can lead to increased mutation rates. This response is crucial for bacterial survival under conditions of DNA damage, such as exposure to ultraviolet light or chemical mutagens.

Mechanism of Action

The activation of the SOS response begins with the detection of single-stranded DNA, which is a signal of DNA damage. The RecA protein binds to single-stranded DNA and forms a nucleoprotein filament. This activated RecA filament interacts with LexA, promoting its autocatalytic cleavage. The cleavage occurs at a specific site within the LexA protein, separating the DNA-binding domain from the rest of the protein and rendering it inactive.

Once LexA is inactivated, the repression of SOS genes is lifted, and the bacterial cell can initiate the repair of damaged DNA. After the damage is repaired, the levels of single-stranded DNA decrease, leading to the deactivation of RecA and the re-synthesis of intact LexA repressor, which restores the repression of SOS genes.

Evolutionary Significance

The LexA repressor and the SOS response are highly conserved among bacterial species, indicating their importance in bacterial evolution. The ability to rapidly respond to DNA damage and repair it is a critical survival mechanism, especially in environments with fluctuating conditions that can cause genetic damage.

The SOS response also plays a role in the evolution of antibiotic resistance, as the error-prone repair mechanisms can introduce mutations that confer resistance to antibiotics. This highlights the dual role of the SOS response in both protecting the genome and facilitating genetic diversity.

Related pages

• SOS response
• RecA
• DNA repair
• Transcriptional regulation