Distinct Spatial Organisation of Rho and RNA Polymerase in Salmonella Cells.

Rho is a conserved, ATP-dependent RNA translocase that terminates transcription at hundreds of sites across bacterial genomes. Although the molecular mechanism of Rho-dependent termination is well characterized, its spatial interplay with RNA polymerase (RNAP) within bacterial cells remains elusive. To address this question, we constructed intragenic, in-frame fusions inserting mCherry or sfGFP 48 amino acid residues downstream of the N-terminus of Rho in Salmonella. Strikingly, mCherry-but not sfGFP-renders the first 48 residues of Rho dispensable. RhoΔ48::mCherry is viable in single copy and exhibits wild-type termination activity in vitro, whereas the full-length Rho::sfGFP fusion, although viable, slows growth and shows strongly reduced activity. Structured illumination microscopy (SIM) revealed that, despite these functional differences, both constructs exhibit similar localisation patterns relative to fluorescently tagged RNAP in single cells. During exponential growth, both Rho and RNAP form discrete clusters, but with markedly distinct spatial organisations: RNAP clusters associate with the nucleoid, whereas Rho is distributed throughout the cell body. This spatial partitioning persists in stationary phase, where RNAP becomes diffusely associated with a compacted nucleoid while Rho accumulates at the cell periphery. The widespread distribution of Rho at cytoplasmic locations is unexpected and suggests participation in cellular functions beyond its canonical role in transcription termination.