​Which one of the following options represents the combination of all correct statements?

SLN1 receptor, a part of a two-component system, is required for osmoregulation in yeast. The yeast mutant (sln1) lacking the SLN1 receptor protein dies. A researcher tries to rescue the mutant by expressing an Arabidopsis gene for the cytokinin receptor, CRE1, which like SLN1 is also a histidine kinase that acts through a two-component system.

The following statements are made regarding the outcome of the experiment.
A. SLN1 and CRE1 proteins respond to the same external signals and therefore, CRE1 rescues the yeast mutant.
B. As CRE1 cannot interact with the downstream signaling pathway in the yeast, it will not rescue the yeast mutant.
C. CRE1 will rescue the yeast mutant, only if cytokinin is present.
D. The effector domains of CRE1 and SLN1 proteins are sufficiently similar and therefore, CRE1 can induce the downstream signaling pathway and rescue the yeast mutant.

Which one of the following options represents the combination of all correct statements?

Correct option is B

• A. "SLN1 and CRE1 proteins respond to the same external signals and therefore, CRE1 rescues the yeast mutant."
  This statement is incorrect. While SLN1 and CRE1 are both part of two-component systems, they do not necessarily respond to exactly the same external signals in yeast. The interaction and similarity in signals are not as direct as implied here.

• B. "As CRE1 cannot interact with the downstream signaling pathway in the yeast, it will not rescue the yeast mutant."
  This is false. CRE1 can interact with the downstream signaling pathway, but it requires cytokinin to do so. This statement contradicts the actual process of how CRE1 can rescue the mutant, provided cytokinin is present.

• C. "CRE1 will rescue the yeast mutant, only if cytokinin is present."
  This statement is correct. For CRE1 to activate the downstream signaling pathway and rescue the yeast mutant, the presence of cytokinin is crucial. Cytokinin acts as a signal for activating the receptor CRE1.

• D. "The effector domains of CRE1 and SLN1 proteins are sufficiently similar and therefore, CRE1 can induce the downstream signaling pathway and rescue the yeast mutant."
  This is correct. CRE1 and SLN1 share similarities in their effector domains, allowing CRE1 to interact with the signaling pathway. However, for this to be effective, cytokinin must be present to activate the pathway.

Thus, the correct combination of statements is C and D, making option (2) the correct answer.

Information Booster:

1. Cytokinin is required for the activation of CRE1’s downstream signaling pathway in yeast.
2. SLN1 and CRE1 belong to similar two-component signaling systems but do not necessarily respond to exactly the same external signals.
3. The effector domains of CRE1 and SLN1 proteins are similar, which allows CRE1 to trigger the downstream signaling pathway.
4. The presence of cytokinin is a critical factor in enabling CRE1 to rescue the yeast mutant.
5. The study of two-component systems in yeast and plants highlights how external signals regulate complex cellular processes.
6. This experiment provides insight into the molecular rescue mechanisms that can compensate for the lack of a particular receptor protein in mutant organisms.