Final answer:
Transcriptional repression is mediated by repressor proteins binding to DNA, attenuation mechanisms that involve stem-loop structures in mRNA, and inducers that modify repressor or activator functions.
Step-by-step explanation:
The three main mechanisms of transcriptional repression ensure the regulation of gene expression in both prokaryotic and eukaryotic cells. In prokaryotes, the mechanisms involve repressor proteins, which can bind to operator regions to block transcription. Another way is through attenuation, which relies on the formation of stem-loop structures that help in deciding whether transcription will continue. Lastly, inducers can interact with repressors or activators to either inhibit or promote transcription based on cellular needs.