Antitermination mechanisms in rRNA operons of Escherichia coli

Abstract

In this review I assess the available information about antitermination mechanisms in rRNA operons (rrn operons) of Escherichia coli. The reader is referred to reviews on rrn operon structure (18) and lambda antitermination (6) for references supporting statements on these topics.Why rrn operons have antitermination mechanisms. The idea that rrn operons might have antitermination mechanisms evolved from studies of transcriptional polarity in operons that code for translated mRNAs. Transcriptional polarity is known to be caused by premature transcription termination resulting from the uncoupling of transcription and translation when translation is interrupted by premature nonsense codons or by other mechanisms (1). The essence of coupling is that ribosomes translating an mRNA concurrent with mRNA synthesis act, in a way not yet clear, to prevent transcription termination at termination sites within proteincoding regions. Termination at sites in protein-coding regions has been found to be largely or solely Rho dependent in all cases examined. Presumably, Rho-dependent termination signals are frequent in sequences that have not specifically evolved as termination signals.The phenomenon of polarity demonstrates that coupling is required for efficient end-to-end transcription of mRNA. However, 6,000-base-pair rrn operons are transcribed endto-end without detectable transcription termination despite the fact that coupling during transcription of rrn operons is unlikely because rrn transcripts are not translated. These facts led to early experiments to demonstrate that antitermination mechanisms in rrn operons compensate for the inability of coupling to reduce premature termination (17). rrn operon structure. The promoter-leader regions of the seven E. coli rrn operons each consists of two highly conserved tandem promoters (Pi and P2) separated by 109 to 119 base pairs, followed by a highly conserved, transcribed but untranslated leader region that is 171 to 173 base pairs in length from the start of P2 transcripts to the start of the 16S rRNA gene. Between the 16S and 23S genes is a region of 350 to 450 base pairs (the spacer region) containing precursor-specific sequences and one or two tRNA genes. The 23S rRNA genes are followed by 5S rRNA genes and, in some cases, one or two tRNA genes. As discussed below, both the leader and spacer regions are suspected of being involved with antitermination.Evidence for rrn antiternmination. Early evidence for rrn antitermination was obtained when insertions of Tn9, TnlO,and IS] were observed to cause far less transcriptional polarity in rrn operons than in protein-coding operons (4,17,26). These experiments indicated that transcription termination was reduced in rrn operons. It was suggested that modifications of RNA polymerase to reduce termination could be responsible for the low polarity of these insertions, but it was also pointed out that the frequent transcription of rrn operons, structural properties of rrn transcripts, or association of rRNA with ribosomal proteins could...

Elongation factor NusG interacts with termination factor rho to regulate termination and antitermination of transcription.

“…The boxA components of the X n u t sites are closely related to the boxA sequences of the rrn operons of E. coli (Olsen et al 1982;Morgan 1986;Berg et al 1989). NusB and NusG are present in transcription complexes synthesizing rRNA in vitro, and the presence of NusB and NusG in these elongation complexes depends on the rrn operon boxA sequence (Li et al 1992).…”

The nut site of bacteriophage lambda is made of RNA and is bound by transcription antitermination factors on the surface of RNA polymerase.

“…Two adenine residues in the loop of boxB that were protected by 50% have been shown previously to be important for antitermination (Doelling and Franklin 1990). The three residues ACA, located 3' to the boxA sequence in the extended boxA homology region (Morgan 1986), were protected by 60%. The DMS reactivity of five nucleotides was enhanced by N and the host factors.…”

“…The nutL and nutR sites of bacteriophage h have two genetically defined components: boxA and boxB. The k boxA sequence is traditionally defined as CGCTCTT (Olson et al 1982;Peltz et al 1985); however the 9 or 10 nucleotides immediately downstream of it (the extended boxA homology region) are evolutionarily conserved in lambdoid bacteriophages (Morgan 1986) and are known to be important for transcription antitermination by N (Zuber et al 1987). boxB is located 9 or 10 bp downstream from the core boxA sequence.…”

Cloning and Expression in Escherichia coli of Proteus vulgaris Genes for 16S Ribosomal RNA

“…vulgaris rRNA clone pPM2 (Fig. 3) contains putative promoters, a discriminator region, a box A region responsible for antitermination function (Morgan, 1986) and the 5' region of the stem containing the recognition site of RNAase 111. The dimensions of these functional parts were adopted from published E. coli sequences.…”