Modulation of Escherichia coli tryptophan (trp) attenuation by the UGA readthrough process

Codon context

The analysis of coding sequences reveals nonrandomness in the context of both sense and stop codons. Part of this is related to nucleotide doublet preference, seen also in non-coding sequences and thought to arise from the dependence of mutational events on surrounding sequence. Another nonrandom context element, relating the wobble nucleotides of successive codons, is observed even when doublet preference, codon usage and bias in amino acid doublets are all allowed for. Several phenomena related to protein synthesis have been shown in vivo to be affected by the nucleotide sequence around codons. Thus, nonsense and missense suppression, elongation rate, precision of tRNA selection and polypeptide chain termination are all affected by codon context. At present, it remains unclear how these phenomena may influence the evolution of nonrandomness in the context of codons in natural sequences.

Stop is not the end: physiological implications of translational readthrough

The translational readthrough mechanism permits the occasional misreading of termination codons by normal charged tRNAs causing extended translation beyond the stop signal. In both prokaryotes and eukaryotes translational readthrough is involved in the regulation of gene expression, as for example in the synthesis of the enzyme reverse transcriptase of the Murine Leukemia Virus (MuLV) (Yoshinaka et al., 1985). Here we particularly deal with the sensitivity of the translational readthrough process to two parameters which are affected by changes in physiological conditions: (1) fluctuations in the concentration of readthrough tRNAs and (b) The affinity of the tRNAs to termination codons. We also discuss the possible role of translational readthrough during major changes in cell physiology.

Cloning, nucleotide sequence and expression of the pyrBI operon of Salmonella typhimurium LT2

The pyrB-pyrI region of the Salmonella typhimurium LT2 chromosome has been cloned and sequenced. The two genes were found to constitute an operon, with pyrI being the distal gene and separated from pyrB by a 15-bp intercistronic region. Sequence analysis revealed the presence of two potential promoters; transcription initiated from the promoter proximal to pyrB would produce a transcript which could direct the synthesis of a 33-amino-acid leader peptide. The leader sequence possesses the requisite features of a rho-independent transcriptional terminator (attenuator) which is positioned 22 bp upstream from the pyrB structural gene. A regulatory mutation imparting a 30-fold elevated expression of pyrBI was identified as a two-base-pair deletion in the track of A X T base pairs located on the 3' side of the region of dyad symmetry of the attenuator. The leader sequence also has an additional region of dyad symmetry (putative transcriptional pause site) located 33 nucleotides upstream from the start of the proposed attenuator. The intervening sequence between the putative pause site and the indicated attenuator is characterized by encoding a high content of uracil residues in the transcript. Construction and analysis of transcriptional and translational fusions provided evidence that the leader region has the necessary features to mediate polypeptide synthesis in vivo, the removal of the region corresponding to the pause site and attenuator results in constitutive expression and the more distant potential promoter does not appear to facilitate significant transcriptional activity. Strong homology exists with the pyrBI operon from Escherichia coli K-12 but notable differences are observed.

Characterization of a cloned Bacillus subtilis gene that inhibits sporulation in multiple copies

We have isolated a 1.0-kilobase fragment of the Bacillus subtilis chromosome which, when present in high-copy-number plasmids, caused a sporulation-proficient strain to become phenotypically sporulation deficient. This is referred to as the sporulation inhibition (Sin) phenotype. This DNA fragment, in multicopy, also inhibited the production of extracellular protease activity, which normally appears at the beginning of stationary growth. The origin of the fragment was mapped between the dnaE and spo0A genes on the B. subtilis chromosome, and its complete DNA sequence has been determined. By analysis of various deletions and a spontaneous mutant the Sin function was localized to an open reading frame (ORF) predicted from the DNA sequence. Inactivation of this ORF in the chromosome did not affect the ability of cells to sporulate. However, the late-growth-associated production of proteases and alpha-amylase was elevated in these cells. The predicted amino acid sequence of the protein encoded by this ORF had a DNA-binding domain, typically present in several regulatory proteins. We propose that the sin ORF encodes a regulatory protein that is involved in the transition from vegetative growth to sporulation.