Deciphering the coupling state-dependent transcription termination in the Escherichia coli galactose operon

The distance between the ribosome and the RNA polymerase active center, often referred to as the mRNA loop length, is a critical determinant of transcription-translation coupling. While structural biology studies have indicated the existence of multiple expressomes with varying mRNA loop lengths, their in vivo roles and functional significance remain largely unexplored. This study delves into the mechanisms governing transcription termination within the Escherichia coli galactose operon, revealing a crucial role in the transcription and translation coupling state. The operon employs both Rho-independent and Rho-dependent terminators. Our findings demonstrate that long-loop coupled transcription-translation complexes preferentially terminate at the upstream Rho-independent terminator. In contrast, short-loop coupled complexes bypass the Rho-independent terminator and terminate at the downstream Rho-dependent terminator. The efficiency of the Rho-independent terminator is enhanced by an extended U-track, suggesting a novel mechanism for overcoming ribosome inhibition. These results challenge the traditional view of transcription termination as a random process, highlighting a predetermined mechanism contingent on the coupling state. This study emphasizes the intricate interactions between transcription and translation in prokaryotes. Understanding how these processes affect the RNA polymerases selection of transcriptional terminators is critical for developing strategies to regulate gene expression.