Transcription factors control gene expression by ____.

A) initiating or repressing gene transcription only
B) regulating the rate of gene transcription only
C) promoting RNA degradation only
D) initiating or repressing gene transcription and regulating the rate of gene transcription
E) initiating or repressing gene transcription and promoting RNA degradation

Transcription factors can either initiate or repress gene transcription and also regulate the rate of gene transcription. They can do so by binding to specific DNA sequences in the promoter or enhancer regions of the gene, which can activate or inhibit RNA polymerase from binding to the DNA template to initiate transcription. Therefore, option D is the best choice. Option E is incorrect because transcription factors do not promote RNA degradation.