NusA directly interacts with antitermination factor Q from phage λ

Antitermination (AT) is a ubiquitous principle in the regulation of bacterial transcription to suppress termination signals. In phage lambda antiterminator protein Q controls the expression of the phage's late genes with loading of lambda Q onto the transcription elongation complex halted at a sigma -dependent pause requiring a specific DNA element. The molecular basis of lambda Q-dependent AT and its dependence on N-utilization substance (Nus) A is so far only poorly understood. Here we used solution-state nuclear magnetic resonance spectroscopy to show that the solution structure of lambda Q is in agreement with the crystal structure of an N-terminally truncated variant and that the 60 residues at the N-terminus are unstructured. We also provide evidence that multidomain protein NusA interacts directly with lambda Q via its N-terminal domain (NTD) and the acidic repeat (AR) 2 domain, with the lambda Q:NusA-AR2 interaction being able to release NusA autoinhibition. The binding sites for NusA-NTD and NusA-AR2 on lambda Q overlap and the interactions are mutually exclusive with similar affinities, suggesting distinct roles during lambda Q-dependent AT, e.g. the lambda Q:NusA-NTD interaction might position NusA-NTD in a way to suppress termination, making NusA-NTD repositioning a general scheme in AT mechanisms.