The pairing of U-rich tracts and the formation of unpaired regions in these internal terminators contributed to their folding energies, affecting the stability of their upstream SBP transcripts. Internal terminators had a lower U content in their 3′ U-rich tracts and longer GC-rich stems, which distinguishes them from canonical terminators and potentially endows them with special termination efficiencies. This was determined by their termination efficiencies.

We found that there were usually stem-loop structures downstream of SBP genes, which could prematurely terminate the transcription of ABC importers and were putative internal intrinsic terminators, resulting in high transcript levels of upstream SBP genes and low transcript levels of downstream cognate translocator genes.

Here, we elucidated a mechanism contributing to differential gene expression in operons encoding ABC importers by employing cellulolytic Clostridia species, specifically Ruminiclostridium cellulolyticum. The extracellular substrate-binding proteins (SBPs) of ATP-binding cassette (ABC) importers tend to be expressed in excess relative to their cognate translocators, but how the stoichiometry of ABC transporters is controlled remains unclear.

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