Final answer:
The core promoter sequence was used as a competitor in the GST pull-down experiment to obtain the sharp decline in pulled-down radioactivity.
Step-by-step explanation:
In the GST pull-down experiment, a competitor known as the core promoter sequence was used to obtain the sharp decline in pulled-down radioactivity. The core promoter sequence is an important element in the promoter region that binds to the transcription machinery and initiates transcription of a gene. By using the core promoter sequence as a competitor, it effectively competed with the isolated GST-alpha CTD subunit for binding, resulting in a decrease in radioactivity in the pulled-down fraction.