Identification of a structural element that is essential for two functions of transcription factor NusG

The transcription factor NusG from Escherichia coli modulates the rate of transcript elongation by RNA polymerase and the efficiency of Rho-dependent transcript termination. It consists of two globular domains with an extra loop extending out of the amino-terminal domain in the position that is occupied by a third globular domain in some NusG homologues. We have tested the role of this appended mini-domain by assaying the elongation and termination enhancement activities of variants. The results show that variants with changes in their sequence do not cause a loss of functions, whereas variants with the deletions of the residues in that domain are much less active for both functions. This finding suggests that the mini-domain serves as a structural element for an interaction rather than as a site for residue-specific contacts.