Final answer:
Transcription factors could explain differences in the levels of the mRNAs for different operons.
Step-by-step explanation:
The mechanism that could explain differences in the levels of the mRNAs for different operons is a) Transcription factors. Transcription factors are proteins that bind to specific regulatory sequences on the DNA and either enhance or inhibit gene transcription. By binding to the promoter region of an operon, transcription factors can increase or decrease the rate of transcription, ultimately affecting the levels of mRNAs produced.
For example, if a transcription factor enhances the binding of RNA polymerase to the promoter, it can increase transcription and subsequently increase the levels of mRNAs produced. On the other hand, if a transcription factor inhibits the binding of RNA polymerase to the promoter, it can decrease transcription and reduce the levels of mRNAs.
Therefore, the presence or absence of specific transcription factors can play a significant role in regulating the levels of mRNAs for different operons.