Final answer:
The appearance of regulatory proteins and histone proteins in metazoans expanded the capacity of DNA looping to be utilized as a mechanism of transcriptional regulation.
Step-by-step explanation:
The appearance of regulatory proteins and elements in metazoans expanded the capacity of DNA looping to be utilized as a mechanism of transcriptional regulation. These regulatory proteins, such as transcription factors, bind to specific DNA sequences, including enhancers or silencers, and enhance or suppress transcription of genes. Additionally, modifications of histone proteins also play a role in epigenetic regulation and gene expression.