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Consider the <E. Coli K-12 MG1655 operon thrLABC>.

That single <operon> can produce two different <mRNA> <transcription units>:
* thrL only, the transcription unit is also called thrL: https://biocyc.org/ECOLI/NEW-IMAGE?object=TU0-42486
* thrL + thrA + thrB + thrC all together, the transcription unit is called thrLABC: https://biocyc.org/ECOLI/NEW-IMAGE?type=OPERON&object=TU00178

The reason for this appears to be that there is a <rho-independent termination> region after thrL. But then under certain conditions, that must get innactivated, and then the thrLABC is produced instead.


Discussion on "Operon vs transcription unit" by Ciro Santilli 32