Chapter 15, Problem 13CONQ

Transcription factors such as the glucocorticoid receptor and the CREB protein form homodimers and activate transcription. Other transcription factors form heterodimers. For example, a transcription factor known as myogenic bHLH forms a heterodimer with a protein called the E protein. This heterodimer activates the transcription of genes that promote muscle cell differentiation. However, when myogenic bHLH forms a heterodimer with a protein called the Id protein, transcriptional activation does not occur. (Note: Id stands for “Inhibitor of differentiation.”) Which of the following possibilities best explains this observation? Only one possibility is correct.

	Myogenic bHLH	E Protein	Id Protein
Possibility 1
DNA-binding domain:	Yes	No	No
Leucine zipper:	Yes	No	Yes
Possibility 2
DNA-binding domain:	Yes	Yes	No
Leucine zipper:	Yes	Yes	Yes
Possibility 3
DNA-binding domain:	Yes	No	Yes
Leucine zipper:	Yes	No	No