Clamping the clamp of RNA polymerase

Bacterial NusG and its archaeal and eukaryal orthologues Spt5 are the only general transcription factors conserved across the three domains of life. The best studied among them, NusG, is found to be associated with the majority of the transcribed genes in the genome (with its paralogue RfaH picking up the slack) (Belogurov et al, 2009), and is implicated in regulating the lateral mobility of RNA polymerase (RNAP) (Bar-Nahum et al, 2005; Herbert et al, 2010), transcription termination and anti-termination (Nudler and Gottesman, 2002), coordinating transcription and translation (Burmann et al, 2010; Proshkin et al, 2010), and silencing horizontally transferred genes (Cardinale et al, 2008). Understanding of the mechanism of these factors requires detailed structural information about the complexes they are a part of, most importantly that of transcription elongation. Several high-resolution structures of proteins from this family are available, but until now the attempts to co-crystallize NusG or Spt5 with its primary target, RNAP, have failed. Two recent works from the labs of Murakami (Klein et al, 2011) and Cramer (in this issue of The EMBO Journal) succeeded in circumventing the problems that plagued the conventional strategies.