Toxin-antitoxin (TA) systems are widespread in prokaryotes. Among these, the mazEF TA system encodes an endoribonucleolytic toxin, MazF, that inhibits growth by sequence-specific cleavage of single-stranded RNA. Defining the physiological targets of a MazF toxin first requires the identification of its cleavage specificity, yet the current toolkit is antiquated and limited. We describe a rapid genome-scale approach, MORE (mapping by overexpression of an RNase in Escherichia coli) RNA-seq, for defining the cleavage specificity of endoribonucleolytic toxins. Application of MORE RNA-seq to MazF-mt3 from Mycobacterium tuberculosis reveals two critical ribosomal targets-the essential, evolutionarily conserved helix/loop 70 of 23S rRNA and the anti-Shine-Dalgarno (aSD) sequence of 16S rRNA. Our findings support an emerging model where both ribosomal and messenger RNAs are principal targets of MazF toxins and suggest that, as in E. coli, removal of the aSD sequence by a MazF toxin modifies ribosomes to selectively translate leaderless mRNAs in M. tuberculosis.
The bacteriophage λ Q protein is a transcription antitermination factor that controls expression of the phage late genes as a stable component of the transcription elongation complex. To join the elongation complex, λQ binds a specific DNA sequence element and interacts with RNA polymerase that is paused during early elongation. λQ binds to the paused early-elongation complex through interactions between λQ and two regions of RNA polymerase: region 4 of the σ(70) subunit and the flap region of the β subunit. We present the 2.1 Å resolution crystal structure of a portion of λQ containing determinants for interaction with DNA, interaction with region 4 of σ(70), and interaction with the β flap. The structure provides a framework for interpreting prior genetic and biochemical analysis and sets the stage for future structural studies to elucidate the mechanism by which λQ alters the functional properties of the transcription elongation complex.
Transcription elongation is interrupted by sequences that inhibit nucleotide addition and cause RNA polymerase (RNAP) to pause. Here, by use of native elongating transcript sequencing (NET-seq) and a variant of NET-seq that enables analysis of mutant RNAP derivatives in merodiploid cells (mNET-seq), we analyze transcriptional pausing genome-wide in vivo in Escherichia coli. We identify a consensus pause-inducing sequence element, G₋₁₀Y₋₁G(+1) (where -1 corresponds to the position of the RNA 3' end). We demonstrate that sequence-specific interactions between RNAP core enzyme and a core recognition element (CRE) that stabilize transcription initiation complexes also occur in transcription elongation complexes and facilitate pause read-through by stabilizing RNAP in a posttranslocated register. Our findings identify key sequence determinants of transcriptional pausing and establish that RNAP-CRE interactions modulate pausing.