Amber suppression relaxes stringent control by elongating stringent factor

Global transcriptional effects of a suppressor tRNA and the inactivation of the regulator frmR

Expression of an amber suppressor tRNA should result in read-through of the 326 open reading frames (ORFs) that terminate with amber stop codons in the Escherichia coli genome, including six pseudogenes. Abnormal extension of an ORF might alter the activities of the protein and have effects on cellular physiology, while suppression of a pseudogene could lead to a gain of function. We used oligonucleotide microarrays to determine if any effects were apparent at the level of transcription in glucose minimal medium. Surprisingly, only eight genes had significantly different expression in the presence of the suppressor. Among these were the genes yaiN, adhC, and yaiM, forming a single putative operon whose likely function is the degradation of formaldehyde. Expression of wild-type yaiN was shown to result in repression of the operon, while a suppression-mimicking allele lacking the amber stop codon and extended 7 amino acids did not. The operon was shown to be induced by formaldehyde, and the genes have been renamed frmR, frmA, and frmB, respectively.

Intramolecular regulation of the opposing (p)ppGpp catalytic activities of Rel(Seq), the Rel/Spo enzyme from Streptococcus equisimilis

Catalytic and regulatory domains of the Rel/Spo homolog of Streptococcus equisimilis affecting (p)ppGpp synthesis and degradation activities have been defined, and opposing activities of the purified protein and its fragments have been compared. Two major domains of the 739-residue Rel(Seq) protein are defined by limited proteolytic digestion. In vitro assays of the purified N-terminal half-protein reveal synthesis of (p)ppGpp by an ATP-GTP 3'-pyrophosphotransferase as well as an ability to degrade (p)ppGpp by a Mn(2+)-dependent 3'-pyrophosphohydrolase. Removal of the C-terminal half-protein has reciprocal regulatory effects on the activities of the N-terminal half-protein. Compared to the full-length protein, deletion activates (p)ppGpp synthesis specific activity about 12-fold and simultaneously inhibits (p)ppGpp degradation specific activity about 150-fold to shift the balance of the two activities in favor of synthesis. Cellular (p)ppGpp accumulation behavior is consistent with these changes. The bifunctional N-terminal half-protein can be further dissected into overlapping monofunctional subdomains, since purified peptides display either degradation activity (residues 1 to 224) or synthetic activity (residues 79 to 385) in vitro. These assignments can also apply to RelA and SpoT. The ability of Rel(Seq) to mediate (p)ppGpp accumulation during amino acid starvation in S. equisimilis is absent when the protein is expressed ectopically in Escherichia coli. Fusing the N-terminal half of Rel(Seq) with the C-terminal domain of RelA creates a chimeric protein that restores the stringent response in E. coli by inhibiting unregulated degradation and restoring regulated synthetic activity. Reciprocal intramolecular regulation of the dual activities may be a general intrinsic feature of Rel/Spo homolog proteins.

Mutation in the structural gene for release factor 1 (RF-1) of Salmonella typhimurium inhibits cell division

A temperature-sensitive mutant of Salmonella typhimurium LT2 was isolated. At the nonpermissive temperature cell division stopped and multinucleated filaments were formed. DNA, RNA, or protein synthesis was not affected until after about two generations. Different physiological conditions, such as anaerobiosis and different growth media, suppress the division deficiency at high temperatures. Certain mutations causing a reduced polypeptide chain elongation rate also suppress the division deficiency. The mutation is recessive and shown to be in the structural gene for release factor I (prfA). DNA sequencing of both the wild-type (prfA+) and mutant (prfA101) allele revealed a GC-to-AT transition in codon 168. Like other known prfA mutants, prfA101 can suppress amber mutations. The division defect in the prfA101 mutant strain could not be suppressed by overexpression of the ftsQAZ operon. Moreover, at the nonpermissive temperature the mutant shows a normal heat shock and SOS response and has a normal ppGpp level. We conclude that the prfA101-mediated defect in cell division is not directed through any of these metabolic pathways, which are all known to affect cell division. We speculate that the altered release factor I induces aberrant synthesis of an unidentified protein(s) involved in the elaborate process of septation.

Actions of the anticodon arm in translation on the phenotypes of RNA mutants

In previous publications, we have shown that it is practical to study the translational activity of tRNAs by replacement and alteration of the anticodon arm sequence of the genus on a plasmid clone. Experiments in which the anticodon arm sequence is transplanted between tRNA genes suggest that the translational activity is determined by these sequences. We have therefore made every variant of the anticodon loop and the three base-pairs of the stem proximal to the loop, in order to resolve the relation between the structure of Su7Am tRNATrp, and its function. All derivatives conserved the normal secondary structure of the molecule, which was known to be essential for translational activity. The probability of translation of the amber codon by these suppressors is measured in this work. This translational activity in vivo is rationalized in terms of data on the copy numbers of the plasmid clones, the nucleotide modifications of the tRNAs, the steady-state level of the mature tRNA, and the aminoacylation of these molecules. Nucleotide modification levels vary among these tRNAs, giving information about the specificities of modification systems that make O-methylribose, pseudouridine, and modified A in the anticodon arm. However, for this series of tRNAs, none of these modifications has a strong effect on translational efficiency of the tRNAs. A few of the substitutions reduce aminoacylation of the tRNAs with glutamine, as determined by comparison of suppression in normal strains and related strains, which have 25-fold elevated levels of the glutaminyl-tRNA synthetase (GlnRS). The substitutions that have the largest effect on GlnRS action are, unexpectedly, purines for conserved pyrimidines on the 5' side of the anticodon loop. Data on the concentrations of tRNA in vivo suggest that the anticodon loop and helix contribute similarly to the determination of the steady-state level of the tRNAs. This level varies sevenfold, though all tRNAs are processed from a homologous precursor made from the same transcription unit. Effects on levels appear to be mediated by changes in anticodon arm structure. A robust equation that relates aminoacyl-tRNA levels to suppressor efficiency is developed in order to resolve effects on tRNA levels and on ribosomal steps: E = A/(K + A), where E is efficiency, A is aminoacyl-tRNA concentration, and K is the effective concentration, or cellular tRNA content required for an individual tRNA to have an efficiency of 0.50. The tRNAs vary in their intrinsic ability to function on the ribosome (represented by K), after other influences have been normalized.(ABSTRACT TRUNCATED AT 400 WORDS)

Genetic screen for cloned release factor genes

Nonsense suppression reflects competition between a nonsense suppressor tRNA and a translational release factor. This provides a simple way to screen for release factor genes cloned into a multicopy plasmid. We have confirmed that the expected competition occurs with the gene for release factor 2, cloned by C.T. Caskey et al. (C.T. Caskey, W.C. Forrester, W. Tate, and C.D. Ward, J. Bacteriol. 158:365-368, 1984), and used it to clone the gene for release factor 1.