Wang, C.; Molodtsov, V.; Kaelber, J.; Blaha, G.; Ebright, R. H.

Structures recently have been reported of molecular assemblies that mediate transcription-translation coupling in Escherichia coli. In these molecular assemblies, termed \"coupled transcription-translation complexes\" or \"TTC-B\", RNA polymerase (RNAP) interacts directly with the ribosome, the transcription elongation factor NusG or its paralog RfaH forms a bridge between RNAP and ribosome, and the transcription elongation factor NusA optionally forms a second bridge between RNAP and ribosome. Here, we have determined structures of coupled transcription-translation complexes having mRNA spacers between RNAP and ribosome longer than the maximum-length mRNA spacer compatible with formation of TTC-B. The results define a new class of coupled transcription-translation complex, termed \"TTC-LC,\" where \"LC\" denotes \"long-range coupling.\" TTC-LC differs from TTC-B by a ~60o rotation and ~70 Angstrom translation of RNAP relative to ribosome, resulting in loss of direct interactions between RNAP and ribosome and creation of a ~70 Angstrom gap between RNAP and ribosome. TTC-LC accommodates long mRNA spacers by looping out mRNA from the gap between RNAP and ribosome. We propose that TTC-LC is an intermediate in assembling and disassembling TTC-B, mediating pre-TTC-B transcription-translation coupling before a ribosome catches up to RNAP, and mediating post-TTC-B transcription-translation coupling after a ribosome stops moving and RNAP continues moving.