Explain why it makes sense for the lexA regulatory gene of the SOS regulon to be expressed constitutively.

Verified step by step guidance

1. Understand the role of the lexA gene: The lexA gene codes for the LexA protein, which is a repressor that regulates the expression of the SOS regulon genes involved in DNA repair and cell cycle control in response to DNA damage.
2. Recognize the meaning of 'constitutively expressed': This term means that the gene is expressed continuously, regardless of external conditions.
3. Consider the function of the SOS regulon: The SOS regulon is activated in response to DNA damage. For a rapid response to DNA damage, the repressor of this system (LexA) needs to be readily available to be cleaved and inactivated, allowing the expression of DNA repair genes.
4. Link constant availability to effective response: By being expressed constitutively, LexA ensures that it is always present in the cell. This allows for a quick response to DNA damage by immediate cleavage of LexA, derepression of the SOS genes, and initiation of the DNA repair process.
5. Conclude the necessity of constitutive expression: Constitutive expression of lexA is crucial for maintaining a state of readiness, enabling the cell to swiftly respond to genotoxic stress without the delay that would be involved in first needing to synthesize the LexA protein.