Question

The ethylene response in plants involves a dimeric transmembrane receptor. When the receptor is not bound to ethylene, the receptor binds to and activates a protein kinase, which activates an intracellular signaling pathway that leads to the degradation of a transcriptional regulator important for transcribing the ethylene response genes (see Fig. Q16-50). You discover a phosphatase that is important for ethylene signaling, and you name it PtpE. Plants lacking PtpE never turn on ethylene-response genes, even in the presence of ethylene. You find that PtpE dephosphorylates serine 121 on the transcriptional regulator. Furthermore, plants lacking PtpE degrade the transcriptional regulator in the presence of ethylene.

Which of the following statements below is inconsistent with your data?
(a) When the transcriptional regulator is phosphorylated, it activates transcription of the ethylene-response genes.
(b) When the transcriptional regulator is not phosphorylated, it binds to DNA.
(c) Activation of the protein kinase that binds to the ethylene receptor leads to inactivation of PtpE.
(d) Binding of ethylene to its receptor leads to the activation of PtpE.

Answer 1

The inconsistent statement with the provided data is (a), as it suggests that phosphorylation activates transcription, while the data indicate that dephosphorylation by PtpE is required for ethylene-response gene transcription.

Step-by-step explanation:

The statement inconsistent with the provided data is (a) "When the transcriptional regulator is phosphorylated, it activates transcription of the ethylene-response genes." Based on the scenario, the presence of phosphatase PtpE is important for ethylene signaling, and its role is to dephosphorylate a specific serine residue (serine 121) on the transcriptional regulator. In PtpE-lacking plants, the transcriptional regulator is degraded even in the presence of ethylene, suggesting that dephosphorylation, not phosphorylation, of the transcriptional regulator, is necessary for the transcription of ethylene-response genes. Moreover, since ethylene signaling requires PtpE and the degradation of the transcriptional regulator occurs when PtpE is absent, it is implied that dephosphorylated (not phosphorylated) form of the transcriptional regulator is needed for gene expression.