The RNA antiterminator causes transcription pausing in the leader region of the tryptophan operon

Regions of the Bacillus subtilis ilv-leu operon involved in regulation by leucine

The ilv-leu operon of Bacillus subtilis is regulated in part by transcription attenuation. The cis-acting elements required for regulation by leucine lie within a 683-bp fragment of DNA from the region upstream of ilvB, the first gene of the operon. This fragment contains the ilv-leu promoter and 482 bp of the ilv-leu leader region. Spontaneous mutations that lead to increased expression of the operon were shown to lie in an imperfect inverted repeat encoding the terminator stem within the leader region. Mutations within the inverted repeat of the terminator destroyed most of the leucine-mediated repression. The remaining leucine-mediated repression probably resulted from a decrease in transcription initiation. A systematic analysis of other deletions within the ilv-leu leader region identified a 40-bp region required for the derepression that occurred during leucine limitation. This region lies within a potential RNA stem-and-loop structure that is probably required for leucine-dependent control. Deletion analysis also suggested that alternate secondary structures proximal to the terminator are involved in allowing transcription to proceed beyond the terminator. Additional experiments suggested that attenuation of the ilv-leu operon is not dependent on coupling translation to transcription of the leader region. Our data support a model proposed by Grundy and Henkin (F. J. Grundy and T. M. Henkin, Cell 74:475-482, 1993) in which uncharged tRNA acts as a positive regulatory factor to increase gene expression during amino acid limitation.

IS10 mRNA stability and steady state levels in Escherichia coli: indirect effects of translation and role of rne function

Translation of the IS10 transposase gene is known to be very infrequent. We have identified mutations whose genetic properties suggest that they act directly to increase or decrease the intrinsic level of translation initiation. Also, we have analysed in detail the effects of these mutations on IS10 mRNA using one particular IS10 derivative. In this case, increases or decreases in translation are accompanied by increases or decreases in both the steady state level and the half-life of transposase mRNA; effects on steady state levels are much more dramatic than effects on message half-life. At wild-type levels of translation initiation, the rate-limiting step in physical decay of full length IS10 message for a particular IS10 derivative is shown to be rne-dependent endonucleolytic cleavage; 3' exonucleases appear to play a secondary role, degrading primary cleavage products. Analysis of interplay between translation mutations and rne function, together with the above observations, suggests that translation stabilizes messages in a general way against rne-dependent endonucleolytic cleavage, and that significant protection may be conferred by one or a few ribosomes. However, dramatic effects of translation on steady state message levels are still observed in an rne mutant and involve the 3' end of the transcript; we propose that these additional effects reflect translation-mediated stimulation of transcript release.

Genetic analysis of the attenuator of the Rhizobium meliloti trpE(G) gene

It was previously reported that transcription of the Rhizobium meliloti trpE(G) gene starts at the adenine residue of the AUG codon of the leader peptide coding sequence (trpL), suggesting that translation of the trpL sequence starts without the Shine-Dalgarno sequence. We constructed mutations replacing the AUG codon of the trpL sequence with AAG or ACG. These mutations reduced the expression of a trpL'-'lacZ fusion gene to 0.1 and 0.2% of the wild-type level, respectively, indicating that the AUG codon is the translation initiation codon for the trpL coding sequence. In addition, these mutations, as well as a mutation converting the eighth codon (UCG) of the trpL sequence to UGA, abolished regulation by attenuation when introduced upstream of the tandem tryptophan codons in a trpE'-'lacZ fusion. Mutations affecting the stability of the probable antiterminator and terminator secondary structures in trpL mRNA were also constructed. Studies using these mutations indicate that the attenuator of R. meliloti functions in a way analogous to that of the Escherichia coli trp attenuator.