Transcription Attenuation Mechanism Research Articles

Transcription of ribosomal genes during a nutritional shift-up of Escherichia coli

We measured the differential transcription rates of individual ribosomal operons after a nutritional shift-up. All operons showed a transient increase in transcription. However, the response of the S10 ribosomal protein operon was much stronger than that of any other operon. We propose that only the S10 operon is autogenously regulated by a transcription attenuation mechanism.

Mutations that convert the four leucine codons of the Salmonella typhimurium leu leader to four threonine codons

In Salmonella typhimurium, expression of the leucine operon is regulated by a transcription attenuation mechanism. According to a current model of attenuation, elevated expression of this operon requires that a ribosome stall at one of four adjacent codons for leucine on a leader RNA. We used oligonucleotide-directed mutagenesis to convert the four leucine codons of the S. typhimurium leu leader to four threonine codons. Analysis of the resulting mutant operon showed that almost all regulation by leucine had been abolished. The mutant operon was, instead, partially derepressed by a limitation for charged threonine tRNA. These results provide direct evidence for the function for the four leucine codons postulated by the attenuator model. An unexpected observation made during these studies was that the wild-type leu operon was partially derepressed by starvation for threonine.

Regulatory regions of two transport operons under nitrogen control: nucleotide sequences.

We have determined the nucleotide sequences of the regulatory regions from two amino acid transport operons from Salmonella typhimurium: dhuA, which regulates the histidine transport operon, and argTr, which regulates argT, the gene encoding the lysine-arginine-ornithine-binding protein, LAO. The promoter for the histidine transport operon has been identified from the sequence change in the promoter-up mutation dhuA1. Neither regulatory region has any of the features typical of the regulatory regions of the amino acid biosynthetic operons, indicating that regulation of at least these transport genes does not involve a transcription attenuation mechanism. We have identified three interesting features, present in both of these sequences, which may be of importance in the regulation of these and other operons: a "stem-loop-foot" structure, a region of specific homology, and a mirror symmetry. The region of mirror symmetry may be a protein recognition site important is regulating expression of these and other operons in response to nitrogen availability. Mirror symmetry as a structure for DNA-protein interaction sites has not been proposed previously.

Control of leu operon expression in Escherichia coli by a transcription attenuation mechanism

The DNA sequence of 306 base-pairs preceding and within the first structural gene of the leu operon of Escherichia coli, leuA, was determined. A prominent feature of this sequence is a signal for transcription termination. In vitro, transcription does terminate at this site, yielding a leader RNA of about 165 nucleotides. This leader RNA is potentially translatable into a peptide containing 28 amino acids, four of which are adjacent Leu residues. Several regions of base complementarity exist within the leader RNA. When the nucleotide sequences of the leu leaders of E. coli and Salmonella typhimurium are compared these segments are conserved while other segments vary appreciably. The highly conserved regions are those thought to play a functional role in attentuation control of leu operon expression.

Alternative secondary structures of leader RNAs and the regulation of the trp, phe, his, thr, and leu operons.

The trp, phe, his, thr, and leu operons of enteric bacteria are regulated by a transcriptional attenuation mechanism. Under conditions of amino acid sufficiency, transcription terminates at an attenuator site after a leader of about 150 nucleotides has been synthesized. Under conditions of limitation of a controlling amino acid, transcription continues past the attenuator into adjacent structural genes. As demonstrated by others, each of the five leader RNAs contains two regions of potential secondary structure which are partially overlapping. One of these regions occurs at the 3' terminus of the leader and is named the "terminator." The other region, which potentially can preclude the formation of the terminator, is named the "preemptor." Conditions that allow the preemptor to form result in derepression. We report here that the five published leader RNA sequences contain an additional potential region of secondary structure, which we call the "protector." The protector partially overlaps the preemptor in such a way that pairing of the former precludes pairing of the latter. For derepression to occur, a ribosome that is translating the leader must block the protector without blocking the preemptor, a condition that is met when the ribosome is arrested at the 3' end of a set of control codons. Including the protector in the model for attenuation explains why derepression of the operon does not result from the arrest of a ribosome at a codon preceding the control set. It also explains why termination is the outcome when transcription occurs in the absence of ribosomes. Finally, termination is the predicted outcome when unfettered translation of the leader RNA occurs, resulting in release of the ribosome at the translational stop signal.