Final answer:
The addition of a single missing protein, which is a transcription factor, can significantly alter gene expression by acting as an activator or repressor in the complex regulatory network that controls transcription.
Step-by-step explanation:
Transcription factors play a critical role in the regulation of gene expression. They bind to specific DNA sequences, including both promoters and enhancers, influencing the transcription of adjacent genes. The effect of transcription regulators is usually combinatorial, meaning that multiple factors act together to regulate gene expression. Thus, the addition of a single, missing protein, or transcription factor, can significantly alter gene expression. For instance, it can switch on a gene that was previously off if the protein is an activator, or it can repress a gene that was formerly being expressed if the protein acts as a repressor. The precise outcome depends on the nature of the transcription factor and the regulatory network in which it operates.