Premature termination of in vivo transcription of a gene encoding a branched-chain amino acid transport protein in Escherichia coli

Intracistronic transcriptional polarity enhances translational repression: a new role for Rho

Transcriptional polarity in Escherichia coli occurs when cryptic Rho-dependent transcription terminators become activated as a consequence of reduced translation. Whether this is due to an increased spacing between the RNA polymerase and the leading ribosome or to prior functional inactivation of a subpopulation of the mRNAs has been a matter of discussion. Transcriptional polarity results in decreased synthesis of inefficiently translated mRNAs and therefore in decreased expression of downstream genes in the same operon (intercistronic polarity). By analogy, expression of the gene in which the conditional termination occurs is also expected to decrease, but this has so far not been demonstrated experimentally. To study the relevance of this intracistronic polarity for expression regulation in vivo, the polarity-prone IacZ reporter gene was fused to a range of mutated ribosome binding sites, repressed to different degrees by local RNA structure. Quantitative analysis of protein and mRNA synthesis shows that polarity occurs on functionally active mRNA molecules and that it indeed affects expression of the cistron carrying the terminator, thus enhancing the effect of translational repression. These findings point to a novel regulatory function of transcriptional polarity, reminiscent of transcriptional attenuation but opposite in effect.

Biosynthesis of Glutamate, Aspartate, Asparagine, L-Alanine, and D-Alanine

Glutamate, aspartate, asparagine, L-alanine, and D-alanine are derived from intermediates of central metabolism, mostly the citric acid cycle, in one or two steps. While the pathways are short, the importance and complexity of the functions of these amino acids befit their proximity to central metabolism. Inorganic nitrogen (ammonia) is assimilated into glutamate, which is the major intracellular nitrogen donor. Glutamate is a precursor for arginine, glutamine, proline, and the polyamines. Glutamate degradation is also important for survival in acidic environments, and changes in glutamate concentration accompany changes in osmolarity. Aspartate is a precursor for asparagine, isoleucine, methionine, lysine, threonine, pyrimidines, NAD, and pantothenate; a nitrogen donor for arginine and purine synthesis; and an important metabolic effector controlling the interconversion of C3 and C4 intermediates and the activity of the DcuS-DcuR two-component system. Finally, L- and D-alanine are components of the peptide of peptidoglycan, and L-alanine is an effector of the leucine responsive regulatory protein and an inhibitor of glutamine synthetase (GS). This review summarizes the genes and enzymes of glutamate, aspartate, asparagine, L-alanine, and D-alanine synthesis and the regulators and environmental factors that control the expression of these genes. Glutamate dehydrogenase (GDH) deficient strains of E. coli, K. aerogenes, and S. enterica serovar Typhimurium grow normally in glucose containing (energy-rich) minimal medium but are at a competitive disadvantage in energy limited medium. Glutamate, aspartate, asparagine, L-alanine, and D-alanine have multiple transport systems.

Multiple loci of Pseudomonas syringae pv. syringae are involved in pathogenicity on bean: restoration of one lesion-deficient mutant requires two tRNA genes

A mutational analysis of lesion-forming ability was undertaken in Pseudomonas syringae pv. syringae B728a, causal agent of bacterial brown spot disease of bean. Following a screen of 6,401 Tn5-containing derivatives of B728a on bean pods, 26 strains that did not form disease lesions were identified. Nine of the mutant strains were defective in the ability to elicit the hypersensitive reaction (HR) and were shown to contain Tn5 insertions within the P. syringae pv. syringae hrp region. Ten HR+ mutants were defective in the production of the toxin syringomycin, and a region of the chromosome implicated in the biosynthesis of syringomycin was deleted in a subset of these mutants. The remaining seven lesion-defective mutants retained the ability to produce protease and syringomycin. Marker exchange mutagenesis confirmed that the Tn5 insertion was causal to the mutant phenotype in several lesion-defective, HR+ strains. KW239, a lesion- and syringomycin-deficient mutant, was characterized at the molecular level. Sequence analysis of the chromosomal region flanking the Tn5 within KW239 revealed strong similarities to a number of known Escherichia coli gene products and DNA sequences: the nusA operon, including the complete initiator tRNA(Met) gene, metY; a tRNA(Leu) gene; the tpiA gene product; and the MrsA protein. Removal of sequences containing the two potential tRNA genes prevented restoration of mutant KW239 in trans. The Tn5 insertions within the lesion-deficient strains examined, including KW239, were not closely linked to each other or to the lemA or gacA genes previously identified as involved in lesion formation by P. syringae pv. syringae.