NusG/Spt5: are there common functions of this ubiquitous transcription elongation factor?

NusG/Spt5 is a transcription elongation factor that assists in DNA-templated RNA synthesis by cellular RNA polymerases (RNAP). The modular domain composition of NusG/Spt5 and the way it binds to RNAP are conserved in all three domains of life. NusG/Spt5 closes RNAP around the DNA binding channel, thereby increasing transcription processivity. Recruitment of additional factors to elongating RNAP may be another conserved function of this ubiquitous protein. Eukaryotic Spt5 couples RNA processing and chromatin modification to transcription elongation, whereas bacterial NusG participates in a wide variety of processes, including RNAP pausing and Rho-dependent termination. Elongating RNAP forms a transcriptional bubble in which 12 bp of the two DNA strands are locally separated. Within this transcription bubble the growing 3'-end of nascent RNA forms an 8-9 bp long hybrid with the template DNA strand. Because of their location in the transcriptional bubble, NusG and its paralog RfaH recognize specific sequences in the nontemplate DNA strand and regulate transcription elongation in response to these signals.