Nus transcription elongation factors and RNase III modulate small ribosome subunit biogenesis in Escherichia coli.

Escherichia coli NusA and NusB proteins bind specific sites, such as those in the leader and spacer sequences that flank the 16 S region of the ribosomal RNA transcript, forming a complex with RNA polymerase that suppresses Rho-dependent transcription termination. Although antitermination has long been the accepted role for Nus factors in rRNA synthesis, we propose that another major role for the Nus-modified transcription complex in rrn operons is as an RNA chaperone insuring co-ordination of 16 S rRNA folding and RNase III processing that results in production of proper 30 S ribosome subunits. This contrarian proposal is based on our studies of nusA and nusB cold-sensitive mutations that have altered translation and at low temperature accumulate 30 S subunit precursors. Both phenotypes are suppressed by deletion of RNase III. We argue that these results are consistent with the idea that the nus mutations cause altered rRNA folding that leads to abnormal 30 S subunits and slow translation. According to this idea, functional Nus proteins stabilize an RNA loop between their binding sites in the 5′ RNA leader and on the transcribing RNA polymerase, providing a topological constraint on the RNA that aids normal rRNA folding and processing. [ABSTRACT FROM AUTHOR]