80. A bacterial response regulator turns on gene A in its phosphorylated form. The
    amount of “A” shows a sharp and sleep rise at a threshold concentration of the signal
    sensed by the cognate sensor. This is most likely due to
    (1) increased phosphatase activity of the sensor at the threshold concentration
    (2) decreased phosphorylation of the response regulator by the sensor
    (3) cooperativity in binding of the response regulator to the target gene
    (4) A negative feedback in gene A expression

Sharp Gene Expression Thresholds in Bacterial Response Regulators: Role of Cooperativity

Understanding the Problem

A bacterial response regulator activates gene A when phosphorylated. The expression of gene A shows a sharp and steep increase at a threshold concentration of the sensed signal by its sensor. This type of switch-like response strongly suggests an ultrasensitive regulatory mechanism.

Evaluating the Options

1. Increased phosphatase activity of the sensor at threshold concentration: This would decrease phosphorylated response regulator levels and thus reduce gene expression, not cause a sharp increase.

2. Decreased phosphorylation of the response regulator by the sensor: Similar to above, this would lower activation, causing less gene expression, not a sharp rise.

3. Cooperativity in binding of the response regulator to the target gene: When multiple response regulator molecules bind cooperatively to the gene’s regulatory region, the binding affinity increases sharply once a threshold concentration is reached, producing a sigmoidal and ultrasensitive gene activation. This explains the steep rise in expression.

4. A negative feedback in gene A expression: Negative feedback tends to dampen or stabilize responses, often preventing abrupt changes, so it’s unlikely to cause a sharp expression threshold.

Supporting Evidence from Literature

Studies in bacterial gene regulation frequently highlight cooperativity as a mechanism that drives ultrasensitive responses and threshold effects (Levine et al., 2008). Small RNAs and transcription factors often work cooperatively to generate sharp gene expression boundaries, preventing noise from triggering unwanted responses.​

This cooperative binding enhances the effective response once the phosphorylated regulator crosses a critical concentration, aligning well with the observed steep induction of gene A.

Conclusion

The most plausible explanation for the sharp and steep rise in gene A expression at a threshold signal concentration is:

(3) Cooperativity in binding of the response regulator to the target gene.