FIGURE 1.

Growth characteristics of the Rho* mutants. A, screening strategies for isolating Rho mutants that can function in the absence of NusG. The Rho-dependent transcription termination requires the interaction of Rho and NusG-CTD (9), absence of which leads to termination read-through and cell death. The NTD of NusG has more affinity for the RNAP than the full-length NusG (13). So, its overexpression replaces the latter from the elongation complex leading to conditional toxicity of the cells. Schematics show the representation of the toxicity of the overexpressed NusG-NTD in the presence of WT Rho. It is expected that the Rho mutants, which can overcome this NusG-NTD mediated toxicity, do not require interaction with NusG implying that they can terminate in the absence of NusG. In the screening protocol, NTD of NusG was cloned under a pBAD promoter and was overexpressed with 0.2% arabinose (see the picture of the growth plate). B, transductants obtained following the deletion of chromosomal nusG by P1 transduction of a MG1655rac⁻rho⁻ strain. In these strains, WT and Rho* mutants (R221C, I168V, P235H) are expressed from pCL1920 plasmids as indicated. Effect of Rho P103L is shown for comparison. Plates were incubated for ∼20 h at 37 °C. C, homology model of E. coli NusG depicting the Rho binding and termination defective mutations. D, transductants obtained after removing the chromosomal nusG by P1 transduction of MG1655rac⁻rho⁻ strains that express different combinations of Rho and NusG derivatives expressed from pCL1920 and pHYD3011 plasmids, respectively, as indicated. These NusG mutants are defective for Rho binding (9).