Final answer:
The next step in gene expression regulation is that the transcription factor binds to DNA and recruits RNA polymerase to initiate transcription.
Step-by-step explanation:
The next step in gene expression regulation once a signal triggers the expression of a transcription regulator is that the transcription factor binds to DNA and recruits RNA polymerase.
This binding of the transcription factor to DNA and recruitment of RNA polymerase leads to the initiation of transcription, where RNA polymerase begins synthesizing mRNA from the DNA template.
Therefore, option 1) Transcription factor binds to DNA and recruits RNA polymerase is the correct answer.