Effects of NusA protein on transcription termination in the tryptophan operon of Escherichia coli.

Termination of transcription at the end of the tryptophan (trp) operon of E. coli at the trp t site is very efficient in vivo, but is only 25% efficient in vitro. To try to resolve this discrepancy, we have altered numerous parameters and report here on the modifications that bring the in vitro results into closer agreement with the in vivo ones. Lowering the concentration of UTP (but not ATP, CTP or GTP) in the transcription mix can greatly improve termination at trp t. With three other terminators structurally similar to trp t, there is no detectable effect of reducing the concentration of any of the four triphosphates. This response at trp t to low UTP is therefore both nucleotide-specific and terminator-specific, suggesting that apparently minor structural differences may still have profound effects upon termination. Increased specificity and sensitivity may also be provided by the NusA protein, which causes RNA polymerase to pause at trp t and at the 1:2 stem of the trp attenuator. NusA protein also enhances termination at trp t, an effect similar to the low UTP response. Further, termination can be slightly improved by including rho factor, resulting in an overall efficiency of almost 100% at trp t.