Suppression of dnaE nonsense mutations by pcbA1.

DNA polymerase III has been recognized as the required replication enzyme in Escherichia coli. The synthesis subunit of DNA polymerase III holoenzyme (alpha subunit) is encoded by the dnaE gene. We have reported that E. coli cells can survive and grow in the absence of a functional dnaE gene product if DNA polymerase I and the pcbA1 mutation are present. Existing mutations in the dnaE gene have been conditionally defective thermolabile mutations. We report here construction of nonsense mutations in the dnaE gene by use of a temperature-sensitive suppressor mutation to permit survival at the permissive temperature (32 degrees C). Introduction of the pcbA1 mutation eliminated the temperature-sensitive phenotype. We confirmed by immunoblotting the lack of detectable alpha subunit at 43 degrees C.     

Mutations in the rpoH (htpR) gene of Escherichia coli K-12 phenotypically suppress a temperature-sensitive mutant defective in the sigma 70 subunit of RNA polymerase.

Escherichia coli K-12 strain 285c contains a mutation in rpoD, the gene encoding the sigma subunit of RNA polymerase. The 70-kilodalton sigma polypeptide encoded by this allele is unstable, and this instability leads to temperature-sensitive growth. We describe the isolation and characterization of four temperature-resistant pseudorevertants of 285c that can grow at high temperature. Each of these revertants increased the stability of the sigma 70 mutant protein. The map position of the suppressor mutations was close to that of the rpoH (htpR) gene. A multicopy plasmid containing the intact rpoH gene restored the temperature-sensitive phenotype. Marker rescue experiments established the positions of three of the alleles within the rpoH gene. One mutation has been sequenced and causes a leucine-to-tryptophan change 7 amino acids from the carboxyl terminus of the rpoH gene product.     

Gene rpmF for ribosomal protein L32 and gene rimJ for a ribosomal protein acetylating enzyme are located near pyrC (23.4 min) in Escherichia coli

Summary An Escherichia coli mutant harbouring altered ribosomal protein L32 has been isolated and genetically characterized. The mutation leading to this alteration (rpmF) and the temperature-sensitive mutation (ts-1517) present in the same strain were found to map near pyrC (23.4 min), being cotransducible not only with pyrC but also with fabD, flaT and purB in P1 phage mediated transductions. Furthermore, we found that the gene rimJ, which encodes an enzyme that acetylates the N-terminal alanine of protein S5 and the temperature-sensitive mutation, ts-386, present in the rimJ mutant strain (Cumberlidge and Isono 1979) also mapped in this region. Thus, the order of genes is deduced to be: ts-386-pyrC-ts-1517-rimJ-flaT-fabD-rpmF-purB.     

Isolation and characterization of a new temperature-sensitive polynucleotide phosphorylase mutation in Escherichia coli K-12

Polynucleotide phosphorylase (PNPase) has been studied in detail since its discovery in 1955 [1]. In an attempt to determine what role, if any, it has in mRNA decay in Escherichia coli, we have isolated and characterized a temperature-sensitive mutation, pnp-200, in the pnp gene. In vitro phosphorolysis, polymerization and exchange activities of the partially purified Pnp-200 enzyme are all reduced to 30-40% of wild-type activity at 50 degrees C compared to 32 degrees C. The pnp-200 mutation alone does not affect cell growth or mRNA stability. A triple mutant strain containing pnp-200 in combination with other temperature-sensitive mutations in genes known to affect mRNA metabolism (rnb-500 and ams-1) is conditionally lethal and shows increased mRNA stability after shift to the non-permissive temperature.     

Localization of the structural gene for ribosomal protein L19 (rplS) in Escherichia coli

Summary A temperature-sensitive mutant derived from an E. coli K12 strain, PA3092, was found to have an alteration in the ribosomal protein L19 (Isono et al., 1977). This mutant is a double mutant with a temperature-sensitivity mutation and a mutation leading to the structural alteration of L19 protein. Crosses with various Hfr strains and transductions with P1kc have revealed that the latter mutation maps at 56.4 min, between pheA and alaS. From the fact that two other mutations causing different types of alterations in L19 protein also map at this locus, the gene affected by these mutations was concluded to be the structural gene for the ribosomal protein L19 (rplS).     

Synthesis of tryptophanase in Escherichia coli: Isolation and characterization of a structural-gene mutant and two regulatory mutants

Summary A mutant of E. coli has been isolated that is temperature-sensitive in respect of tryptophanase. When incubated at 60°C, cell-free extracts of the mutant suffer inactivation of enzyme activity much more rapidly than similar extracts of the wild type. After lysogeny with a specialized transducing phage carrying the wild-type tryptophanase gene, the mutant is able to synthesize tryptophanase that is wild-type in its response to treatment at 60°C. It is concluded that the mutation lies in the structural gene for the enzyme.Two further mutants have been isolated that synthesize tryptophanase constitutively. One mutation renders synthesis of the enzyme indifferent to the presence of inducer; the other mutation allows synthesis of the enzyme in the absence of inducer at about 35% of the fully induced wild-type rate. Neither mutation alleviates catabolite repression. Genetic mapping shows that the constitutive mutations lie very close to the structural-gene mutation, on the side of the structural gene distant from bglR.     

Genetic characterization of a filament-forming, lipoprotein-deficient mutant of Escherichia coli.

The fam-715 allele of Escherichia coli ST715, previously described as a temperature-sensitive filament former with reduced levels of lipoprotein at the nonpermissive temperature (S. V. Torti and J. T. Park, Nature [London] 263: 323--326, 1976), was mapped at 74 min. This mutation appears to be amber. It is recessive and can be complemented by F' plasmids carrying the wild-type allele or by an F' plasmid carrying an amber suppressor. Isotopic labeling experiments as well as map position differentiate the fam-715 allele from lipoprotein structural gene mutations.     

Mutational analysis of the mitochondrial Rieske iron-sulfur protein of Saccharomyces cerevisiae. I. Construction of a RIP1 deletion strain and isolation of temperature-sensitive mutants

A protocol has been devised to permit mutational analysis of the Rieske iron-sulfur protein of the mitochondrial cytochrome bc1 complex of Saccharomyces cerevisiae. The gene for this iron-sulfur protein (RIP1) has recently been cloned and sequenced (Beckmann, J. D., Ljungdahl, P. O., Lopez, J. L., and Trumpower, B. L. (1987) J. Biol. Chem. 262, 8901-8909). We have constructed a stable yeast deletion strain, JPJ1, in which the chromosomal copy of RIP1 was displaced by the yeast LEU2 gene by homologous recombination. A linear DNA fragment containing the LEU2 gene was inserted at the breakpoints of an 800-base pair deletion of the iron-sulfur protein gene and used to transform a leu- yeast strain. Leu+ transformants were obtained which were unable to grow on nonfermentable carbon sources. Southern analysis of the transformant, JPJ1, confirmed that the chromosomal copy of the RIP1 gene was deleted and replaced by the LEU2 gene. The genotype of JPJ1 was confirmed by genetic crosses. JPJ1 cannot grow on nonfermentable carbon sources but can be complemented to respiratory competence and transformed by yeast vectors containing the wild type RIP1 gene. The ability to complement strain JPJ1 with episomally encoded iron-sulfur protein provided the basis of a selection protocol by which mutagenized plasmids containing the RIP1 gene were assayed for mutations affecting respiratory growth. Five mutants of RIP1 were identified by their ability to complement JPJ1 to temperature-sensitive respiratory growth. DNA sequence analysis demonstrated that temperature-sensitive respiratory growth resulted from single point mutations within the protein coding region of RIP1. These mutations altered a single amino acid residue in each case. Mutations were dispersed throughout the terminal two-thirds of the protein. Each mutation was recessive and did not affect fermentative growth on dextrose. However, each mutation exerted unique temperature-sensitive growth characteristics on media containing the nonfermentable carbon source glycerol.     

Mutant isolation of mouse DNA topoisomerase II alpha in yeast.

For characterizing in vivo functions of a mammalian protein, it is informative to obtain conditional mutations and apply them to the mouse genetic system. However, the isolation of conditional mutations has been quite difficult in cultured cells. We report here that functional expression of a heterologous mammalian gene in the yeast Saccharomyces cerevisiae provides a system for isolating mutated genes. We found that the cloned mouse TOP2 alpha cDNA, which encodes mouse DNA topoisomerase II (topo II) alpha, could rescue the lethal phenotype caused by yeast top2 null mutation. In order to generate and select temperature-sensitive mouse topo II alpha, an expression plasmid was mutagenized in vitro and was transformed, using the plasmid shuffling method, into the yeast strain, in which the endogenous TOP2 gene had been disrupted. We observed that one of such clone of yeast cells harboring a mutagenized mouse TOP2 alpha showed temperature-sensitive growth. Enzymatic assays and sequencing analysis revealed that this phenotype was caused by the thermosensitive nature of the mutant mouse protein, which has isoleucine at amino acid 961 instead of threonine. Therefore we have isolated the first conditional mutation in the mouse TOP2 alpha.     

Genetic analysis of Escherichia coli K-12 chromosomal mutants defective in expression of F-plasmid functions: identification of genes cpxA and cpxB.

Two temperature-sensitive, chromosomal mutants of Escherichia coli were selected for their inability to express deoxyribonucleic acid donor activity and other activities associated with the conjugative plasmid F. These mutants were also auxotrophic for isoleucine and valine at 41 degrees C. Each mutant strain contained two altered genes: cpxA, located at 88 min on the E. coli K-12 genetic map, and cpxB, located at 41 min. Mutations in both genes were required for maximal expression of mutant phenotypes. The parent strain of mutants KN401 and KN312 already contained the cpxB mutation that is present in both mutants (cpxB1). This mutation by itself was cryptic. The cpxA mutations represent different mutant alleles since they are of independent origin. A cpxA mutation by itself significantly affected the expression of plasmid functions and growth at 41 degrees C in the absence of isoleucine and valine, but strains containing both a cpxA and cpxB mutation were more severely affected. Along with the observation that both cpxA mutations were revertable, the temperature sensitivity of cpxA cpxB+ cells suggests that both cpxA alleles contain point mutations that do not completely destroy the activity of the cpxA gene product.     

Isolation of an Escherichia coli K-12 dnaE mutation as a mutator.

Using a papillation method, a large number of Escherichia coli K-12 mutator mutations have been isolated. Only one of these (out of 1,250) mutator mutations has proved to be conditionally lethal at high temperatures. In vivo complementation tests indicated that this mutation, dnaE9, lies in dnaE, the structural gene for DNA polymerase III. The dnaE9 polymerase was not thermolabile in vitro; however, it showed a slow decline in specific activity in vivo at the nonpermissive temperature. Cultures of this mutant exhibited a comparably slow shutoff of DNA synthesis on shift to a nonpermissive temperature. dnaE9 showed temperature-sensitive mutator activity, which is not dependent on recA.     

Mutations in a gene encoding the alpha subunit of a Saccharomyces cerevisiae G protein indicate a role in mating pheromone signaling.

Mutations which allowed conjugation by Saccharomyces cerevisiae cells lacking a mating pheromone receptor gene were selected. One of the genes defined by such mutations was isolated from a yeast genomic library by complementation of a temperature-sensitive mutation and is identical to the gene GPA1 (also known as SCG1), recently shown to be highly homologous to genes encoding the alpha subunits of mammalian G proteins. Physiological analysis of temperature-sensitive gpa1 mutations suggests that the encoded G protein is involved in signaling in response to mating pheromones. Mutational disruption of G-protein activity causes cell-cycle arrest in G1, deposition of mating-specific cell surface agglutinins, and induction of pheromone-specific mRNAs, all of which are responses to pheromone in wild-type cells. In addition, mutants can conjugate without the benefit of mating pheromone or pheromone receptor. A model is presented where the activated G protein has a negative impact on a constitutive signal which normally keeps the pheromone response repressed.     

Escherichia coli K-12 mutants in which viability is dependent on recA function.

A gene required for growth and viability in recA mutants of Escherichia coli K-12 was identified. This gene, rdgB (for Rec-dependent growth), mapped near 64 min on the E. coli genetic map. In a strain carrying a temperature-sensitive recA allele, recA200, and an rdgB mutation, DNA synthesis but not protein synthesis ceased after 80 min of incubation at 42 degrees C, and there was extensive DNA degradation. The rdgB mutation alone had no apparent effect on DNA synthesis or growth; however, mutant strains did show enhanced intrachromosomal recombination and induction of the SOS regulon. The rdgB gene was cloned and its-gene product identified through the construction and analysis of deletion and insertion mutations of rdgB-containing plasmids. The ability of a plasmid to complement an rdgB recA mutant was correlated with its ability to produce a 25-kilodalton polypeptide as detected by the maxicell technique.     

Cloning and expression in Escherichia coli of a yeast mannosyltransferase from the asparagine-linked glycosylation pathway

The yeast Saccharomyces cerevisiae temperature-sensitive lethal mutant alg1-1, has been previously shown to lack the activity necessary for the addition of the first mannose residue in the synthesis of lipid-linked precursor oligosaccharide. The gene ALG1 has been cloned by complementation of the temperature-sensitive mutation alg1-1 with a total genomic DNA library. The original DNA fragment isolated was 11,300 base pairs and has been subcloned to a 1,500-base pair fragment which is still capable of complementing alg1-1. The gene ALG1 has been mapped on chromosome II at a distance of 2.1 map units from LYS2. The ALG1 gene product has been shown to catalyze the transfer of a mannosyl residue from GDP-mannose to the lipid-linked acceptor GlcNAc2, yielding Man beta 1-4GlcNAc2-lipid, in lysates from Escherichia coli transformants. This result proves that ALG1 is the structural gene for the first mannosyltransferase in lipid-linked oligosaccharide assembly.