A NusE:NusG Complex Links Transcription and Translation

Abstract

Bacterial NusG is a highly conserved transcription factor that is required for most Rho activity in vivo. We show by nuclear magnetic resonance spectroscopy that Escherichia coli NusG carboxyl-terminal domain forms a complex alternatively with Rho or with transcription factor NusE, a protein identical to 30S ribosomal protein S10. Because NusG amino-terminal domain contacts RNA polymerase and the NusG carboxy-terminal domain interaction site of NusE is accessible in the ribosomal 30S subunit, NusG may act as a link between transcription and translation. Uncoupling of transcription and translation at the ends of bacterial operons enables transcription termination by Rho factor, and competition between ribosomal NusE and Rho for NusG helps to explain why Rho cannot terminate translated transcripts.

Modulation of Rho-Dependent Transcription Termination in Escherichia coli by the H-NS Family of Proteins

“…The maintenance of transcription-translation coupling in a prokaryotic cell would require dynamic inter-regulation between the binding and progression of a pioneer ribosome on the nascent transcript on the one hand and the rate of transcription elongation on the other (9,45); however, the detailed mechanisms by which such regulation is achieved are not known. Furthermore, in bacteria such as Escherichia coli, nascent transcripts that are not simultaneously translated are subject to a mechanism of factor-dependent transcription termination (also referred to as transcriptional polarity) (reviewed in references 1, 6, 15, 40, 44, 47, and 52), so that the occurrence of translation-uncoupled transcription is minimized within the cells (11,43).…”

“…NusG has two domains that bind Rho and RNA polymerase, respectively (13,36). The Rho-binding domain in NusG has also recently been shown to interact with the S10 protein subunit of the ribosome, which has implications for the mechanism by which translation-uncoupled nascent transcripts may be subject to termination of their synthesis (9). Although null mutations in rho or nusG confer inviability in wild-type E. coli (11), missense mutations in the two genes are known that confer a transcription termination-defective (that is, polarity relief) phenotype (1,12,13,27,36).…”

“…Finally, the polarity defects of rho and nusG mutants were exacerbated by Hha and YdgT deficiency. A model is proposed that invokes a novel role for the polymeric architectural scaffold formed on DNA by H-NS and StpA independent of the gene-silencing functions of these nucleoid proteins, in modulating Rho-dependent transcription termination such that interruption of the scaffold (as obtained by expression either of the H-NS oligomerization variants or of YdgT) is associated with improved termination efficiency in the rho and nusG mutants.Translation is a cotranscriptional process in both eubacteria and archaebacteria (1,9,25,45,50), and it has been proposed that such coupling is a defining characteristic of prokaryotic life (34, 64). The maintenance of transcription-translation coupling in a prokaryotic cell would require dynamic inter-regulation between the binding and progression of a pioneer ribosome on the nascent transcript on the one hand and the rate of transcription elongation on the other (9, 45); however, the detailed mechanisms by which such regulation is achieved are not known.…”

“…Translation is a cotranscriptional process in both eubacteria and archaebacteria (1,9,25,45,50), and it has been proposed that such coupling is a defining characteristic of prokaryotic life (34, 64). The maintenance of transcription-translation coupling in a prokaryotic cell would require dynamic inter-regulation between the binding and progression of a pioneer ribosome on the nascent transcript on the one hand and the rate of transcription elongation on the other (9, 45); however, the detailed mechanisms by which such regulation is achieved are not known.…”

Evolutionary optimization of speed and accuracy of decoding on the ribosome

“…Translation of a particular codon depends on both the nature and abundance of the respective tRNAs, particularly on the non-random use of synonymous codons and the availability of the respective isoacceptor tRNAs [8]. The overall rate of translation is limited by the codon-specific rates of cognate ternary complex delivery to the A site and is further attenuated by other factors, such as collisions between individual ribosomes in polysomes [3], controlled ribosome stalling (for a recent review, see [9]), or the cooperation between translating ribosomes and the RNA polymerase machinery [4,10].Estimations of error frequencies of translation range between 10 25 and 10 23 , depending on the type of measurement, concentrations and nature of tRNAs that perform misreading, and the mRNA context [11][12][13]. Different approaches were taken to measure these values.…”