Mutation of the Ser2 codon of the light-harvesting B870 alpha polypeptide of Rhodobacter capsulatus partially suppresses the pufX phenotype.

The exact function of the pufX gene product of Rhodobacter capsulatus is uncertain, but deletion of the pufX gene renders cells incapable of phototrophic growth on a minimal medium, and photosynthetic electron transfer is impaired in vitro. However, suppressor mutants that are able to grow phototropically are readily isolated. Two such suppressor mutants were characterized as to their phototrophic growth properties, their fluorescence at different incident light intensities, the integrity of their chromatophores, and their abilities to generate a transmembrane potential. We found that the photosynthetic apparatus in the suppressor mutants was less stable than that of the pseudo-wild-type and primary mutant strains and that the suppressor mutants used light energy less efficiently than the pseudo-wild-type strain. Therefore, the suppressor strains are more precisely designated partial suppressor mutants. The locations and sequences of the suppressor mutations were determined, and both were found to change the second codon of the pufA gene. It is hypothesized that the serine residue specified by this codon is important in interactions between the B870 alpha protein and other membrane-bound polypeptides and that suppressor mutations at this position partially compensate for loss of the PufX protein. A model is proposed for the function of the PufX protein.     

Rough mutants of Salmonella typhimurium: immunochemical and structural analysis of lipopolysaccharides from rfaH mutants

Lipopolysaccharides, extracted by phenol/chloroform/petroleum ether, from two rough mutants of Salmonella typhimurium of class rfaH were studied by passive haemagglutination inhibition and by methylation analysis. The structural and immunochemical analyses showed that (i) formation of the galactose I unit of the core is defective, but the defect is not complete, and (ii) of those core chains which do receive the galactose I residue, many are not continued to form complete core, but instead terminate at intermediate points. This suggests that the rfaH gene, though involved in formation of the galactose I unit, is not the structural gene for the galactosyltransferase which adds this unit. The rfaH product may be a positive regulator for several rfa genes specifying glycosyltransferases, or it may be a protein needed for the efficient action of several such transferases.     

New suppressor mutation sur0B of spo0B and spo0F mutations in Bacillus subtilis

Two extragenic suppressor mutations, sur0B20 and sur0F1, which restore the sporulation of spo0B or spo0F mutants of Bacillus subtilis to the wild-type level, were obtained. These suppressor mutations were located in the spo0A gene. Their location is close to that of the sof-1 mutation, which suppresses spo0B, spo0E and spo0F mutations. However, spo0 strains bearing the sur0B20 mutation differed in several phenotypic characteristics from spo0 mutants bearing the sof-1 suppressor. Nucleotide sequence analysis revealed that the sur0B20 and sur0F1 mutations resulted in Glu14 to Val and Asn12 to Lys conversion, respectively, in the spo0A gene. This result indicates that sur0B20 is a new suppressor of spo0b and spo0F mutations, whereas sur0F1 is identical to sof-1.     

Genetic selection of intragenic suppressor mutations that reverse the effect of common p53 cancer mutations.

Several lines of evidence suggest that the presence of the wild-type tumor suppressor gene p53 in human cancers correlates well with successful anti-cancer therapy. Restoration of wild-type p53 function to cancer cells that have lost it might therefore improve treatment outcomes. Using a systematic yeast genetic approach, we selected second-site suppressor mutations that can overcome the deleterious effects of common p53 cancer mutations in human cells. We identified several suppressor mutations for the V143A, G245S and R249S cancer mutations. The beneficial effects of these suppressor mutations were demonstrated using mammalian reporter gene and apoptosis assays. Further experiments showed that these suppressor mutations could override additional p53 cancer mutations. The mechanisms of such suppressor mutations can be elucidated by structural studies, ultimately leading to a framework for the discovery of small molecules able to stabilize p53 mutants.     

Transcriptional regulation through RfaH contributes to intestinal colonization by Escherichia coli

The Escherichia coli regulatory protein RfaH contributes to efficient colonization of the mouse gut. Extraintestinal pathogenic (ExPEC) as well as non-pathogenic probiotic E. coli strains rapidly outcompeted their isogenic rfaH mutants following oral mixed infections. LPS-core and O-antigen side-chain as well as capsular polysaccharide synthesis are among the E. coli virulence factors affected by RfaH. In respect of colonization, deep-rough LPS mutants (waaG) but not capsular (kps) mutants were shown to behave similarly to rfaH mutants. Furthermore, alteration in the length of O-antigen side-chains did not modify colonization ability either indicating that it was the regulatory effect of RfaH on LPS-core synthesis, which affected intestinal colonization. Loss of RfaH did not significantly influence adhesion of bacteria to cultured colon epithelial cells. Increased susceptibility of rfaH mutants to bile salts, on the other hand, suggested that impaired in vivo survival could be responsible for the reduced colonization capacity.     

Patterns of Genetic and Phenotypic Suppression of lys2 Mutations in the Yeast SACCHAROMYCES CEREVISIAE

A total of 358 lys2 mutants of Saccharomyces cerevisiae have been characterized for suppressibility by the following suppressors: UAA and UAG suppressors that insert tyrosine, serine or leucine; a putative UGA suppressor; an omnipotent suppressor SUP46; and a frameshift suppressor SUF1–1. In addition, the lys2 mutants were examined for phenotypic suppression by the aminoglycoside antibiotic paromomycin, for osmotic remediability and for temperature sensitivity. The mutants exhibited over 50 different patterns of suppression and most of the nonsense mutants appeared similar to nonsense mutants previously described. A total of 24% were suppressible by one or more of the UAA suppressors, 4% were suppressible by one or more of the UAG suppressors, while only one was suppressible by the UGA suppressor and only one was weakly suppressible by the frameshift suppressor. One mutant responded to both UAA and UAG suppressors, indicating that UAA or UAG mutations at certain rare sites can be exceptions to the specific action of UAA and UAG suppressors. Some of the mutants appeared to require certain types of amino acid replacements at the mutant sites in order to produce a functional gene product, while others appeared to require suppressors that were expressed at high levels. Many of the mutants suppressible by SUP46 and paromomycin were not suppressible by any of the UAA, UAG or UGA suppressors, indicating that omnipotent suppression and phenotypic suppression need not be restricted to nonsense mutations. All of the mutants suppressible by SUP46 were also suppressible by paromomycin, suggesting a common mode of action of omnipotent suppression and phenotypic misreading.     

Thermo-labile stability of sigmaH (Spo0H) in temperature-sensitive spo0H mutants of Bacillus subtilis can be suppressed by mutations in RNA polymerase beta subunit

We isolated novel temperature-sensitive mutants of spo0H, spo0H1 and spo0H5, having E61K and G30E amino-acid substitutions within the sigmaH protein, respectively, and located in the highly conserved region, "2", among prokaryotic sigma factors that participates in binding to core enzyme of RNA polymerase. These mutants showed a sporulation-deficient phenotype at 43 degrees C. Moreover, we successfully isolated suppressor mutants that were spontaneously generated from the spo0H mutants. Our genetic analysis of these suppressor mutations revealed that the suppressor mutations are within the rpoB gene coding for the beta subunit of RNA polymerase. The mutations caused single amino-acid substitutions, E857A and P1055S, in rpoB18 and rpoB532 mutants that were generated from spo0H1 and spo0H5, respectively. Whereas the sigmaH-dependent expression of a spo0A-bgaB fusion was greatly reduced in both spo0H mutants, their expression was partially restored in the suppressor mutants at 43 degrees C. Western blot analysis showed that the level of sigmaH protein in the wild type increased between T0 and T2 and decreased after T3, while the level of sigmaH protein in spo0H mutants was greatly reduced throughout growth, indicating that the mutant sigmaH proteins were rapidly degraded by some unknown proteolytic enzyme(s). The analysis of the half-life of sigmaH protein showed that the short life of sigmaH in spo0H mutants is prolonged in the suppressor mutants. These findings suggest that, at least to some extent, the process of E-sigmaH formation may be involved in stabilization of sigmaH at the onset of sporulation.     

Alternative route for biosynthesis of amino sugars in Escherichia coli K-12 mutants by means of a catabolic isomerase.

By inserting a lambda placMu bacteriophage into gene glmS encoding glucosamine 6-phosphate synthetase (GlmS), the key enzyme of amino sugar biosynthesis, a nonreverting mutant of Escherichia coli K-12 that was strictly dependent on exogenous N-acetyl-D-glucosamine or D-glucosamine was generated. Analysis of suppressor mutations rendering the mutant independent of amino sugar supply revealed that the catabolic enzyme D-glucosamine-6-phosphate isomerase (deaminase), encoded by gene nagB of the nag operon, was able to fulfill anabolic functions in amino sugar biosynthesis. The suppressor mutants invariably expressed the isomerase constitutively as a result of mutations in nagR, the locus for the repressor of the nag regulon. Suppression was also possible by transformation of glmS mutants with high-copy-number plasmids expressing the gene nagB. Efficient suppression of the glmS lesion, however, required mutations in a second locus, termed glmX, which has been localized to 26.8 min on the standard E. coli K-12 map. Its possible function in nitrogen or cell wall metabolism is discussed.     

A novel family of yeast chaperons involved in the distribution of V-ATPase and other membrane proteins.

Null mutations in genes encoding V-ATPase subunits in Saccharomyces cerevisiae result in a phenotype that is unable to grow at high pH and is sensitive to high and low metal-ion concentrations. Treatment of these null mutants with ethylmethanesulfonate causes mutations that suppress the V-ATPase null phenotype, and the mutant cells are able to grow at pH 7.5. The suppressor mutants were denoted as svf (suppressor of V-ATPase function). The frequency of svf is relatively high, suggesting a large target containing several genes for the ethylmethanesulfonate mutagenesis. The suppressors' frequency is dependent on the individual genes that were inactivated to manifest the V-ATPase null mutation. The svf mutations are recessive, because crossing the svf mutants with their corresponding V-ATPase null mutants resulted in diploid strains that are unable to grow at pH 7.5. A novel gene family in which null mutations cause pleiotropic effects on metal-ion resistance or sensitivity and distribution of membrane proteins in different targets was discovered. The family was defined as VTC (Vacuolar Transporter Chaperon) and it contains four genes in the S. cerevisiae genome. Inactivation of one of them, VTC1, in the background of V-ATPase null mutations resulted in svf phenotype manifested by growth at pH 7.5. Deletion of the VTC1 gene (DeltaVTC1) results in a reduced amount of V-ATPase in the vacuolar membrane. These mutant cells fail to accumulate quinacrine into their vacuoles, but they are able to grow at pH 7.5. The VTC1 null mutant also results in a reduced amount of the plasma membrane H(+)-ATPase (Pma1p) in membrane preparations and possibly mis-targeting. This observation may provide an explanation for the svf phenotype in the double disruptant mutants of DeltaVTC1 and DeltaVMA subunits.     

Identification and characterization of the smbA gene, a suppressor of the mukB null mutant of Escherichia coli.

The mukB gene encodes a protein involved in chromosome partitioning in Escherichia coli. To study the function of this protein, we isolated from the temperature-sensitive mukB null mutant and characterized 56 suppressor mutants which could grow at 42 degrees C. Ten of the mutants also showed cold-sensitive growth at 22 degrees C. Using one of the cold-sensitive mutants as host, the wild type of the suppressor gene was cloned. The cloned suppressor gene complemented all of the 56 suppressor mutations. DNA sequencing revealed the presence of an open reading frame of 723 bp which could encode a protein of 25,953 Da. The gene product was indeed detected. The previously undiscovered gene, named smbA (suppressor of mukB), is located at 4 min on the E. coli chromosome, between the tsf and frr genes. The smbA gene is essential for cell proliferation in the range from 22 to 42 degrees C. Cells which lacked the SmbA protein ceased macromolecular synthesis. The smbA mutants are sensitive to a detergent, sodium dodecyl sulfate, and they show a novel morphological phenotype under nonpermissive conditions, suggesting a defect in specific membrane sites.     

Specific Suppression of Mutations in Genes 46 and 47 by das, a New Class of Mutations in Bacteriophage T4D

Mutants in T4 genes 46 and 47 exhibit early cessation of deoxyribonucleic acid (DNA) synthesis ("DNA arrest") and decreased synthesis of late proteins and phage. In addition, mutants in genes 46 and 47 fail to degrade host DNA to acidsoluble products. It is shown here that this complex phenotype can be partially suppressed by mutation of a T4 gene external to genes 46 and 47 which has been named das for "DNA arrest suppressor." The das mutations were discovered as third-site mutations in spontaneous pseudorevertants of [46, 47] mutants; the pseudorevertants make small plaques on Escherichia coli B, whereas [46, 47] mutants make none. The [das, 46, 47] triple mutant exhibits increased DNA, late protein, and viable phage production compared to the double mutant [46, 47]. The [das, 46, 47] mutant also degrades more of the host DNA to acid-soluble products than does the [46, 47] mutant. The suppressor effect of the das mutation appears to be gene-specific: it suppresses both amber and temperature-sensitive mutations in genes 46 and 47 and does not suppress amber mutations in any of the other genes tested. The [das] single mutants make normal-sized plaques on E. coli B and exhibit nearly normal host DNA degradation, DNA synthesis, late protein synthesis, and viable phage production. The das mutations either define a new gene between genes 33 and 34 or are special mutations within gene 33.     

Suppressors of the ras2 Mutation of SACCHAROMYCES CEREVISIAE

Saccharomyces cerevisiae contains two members of the ras gene family. Strains with disruptions of the RAS2 gene fail to grow efficiently on nonfermentable carbon sources. This growth defect can be suppressed by extragenic mutations called sra. We have isolated 79 independent suppressor mutations, 68 of which have been assigned to one of five loci. Eleven additional dominant mutations have not been assigned to a specific locus. Some sra1 and SRA4 and all SRA3 mutations were RAS independent, allowing growth of yeast cells that lack a functional RAS gene. Mutations in sra1, SRA3, SRA4 and sra6 are linked to his6, ino1, met3 and ade6, respectively. Some sra mutants have pleiotropic phenotypes that affect glycogen accumulation, sporulation, viability, respiratory capacity and suppression of two cell-division-cycle mutations, cdc25 and cdc35. The proposed functions of many of the suppressor genes are consistent with the model in which RAS activates adenylate cyclase.     

Studies of opal mutations in bacteriophage T4B

Hydroxylamine-induced amber and opal mutants are localized on the map of gene 47 of bacteriophage T4B. The matched map of amber and opal mutations showed the presence of four paired sites which seemed to have arisen in the triplet coding for tryptophan.In growth studies o opal mutants in gene 47 in a series of Su⁺ strains the number of strains bearing a gene-suppressor for amber or ochre mutations also had a weak suppressor activity for some opal mutants. This suppressor acitivity is supposedly due to a second mutation in gene Su_uga.A comparative study of the phage yield with amber and opal mutations located in the same (paired) triplet in gene 47 has shown that the suppressor activity depends on the location of the mutant site along the gene.Experiments dealing with the induction of reversions by nitrous acid in amber and opal mutants with mutational sites located in the same triplet of gene 47 (mutant pairs) have shown the essential influence of the nucleotide sequence in the triplet on the frequency of induced reverse mutations at the given site.     

OmpF assembly mutants of Escherichia coli K-12: isolation, characterization, and suppressor analysis.

This paper describes a novel genetic method used to isolate mutations that alter proper assembly of OmpF in the outer membrane. The thermolabile nature of assembly intermediates allowed selection of temperature-sensitive mutations within the ompF gene. A variant allele of ompF (ompF-Dex) was used because it provided a convenient selectable phenotype (Dex+). Assembly mutants were isolated in two steps. First, amber mutations were obtained that mapped in ompF-Dex. This resulted in a Dex- phenotype. Starting with these Dex- strains, Dex+ revertants were isolated. Mutants that displayed a temperature-sensitive Dex+ phenotype were further characterized. Three such mutants possessed a single substitution within ompF that reverted the nonsense codon to a sense codon which replaced W214 with either an E or Q and Y231 with a Q residue in the mature OmpF protein. All three mutant OmpF proteins showed an assembly defect. This defect led to a substantial reduction in the amount of stable OmpF trimers with the concomitant increase of a high-molecular-weight form of OmpF which migrated at the top of the gel. Suppressor mutations were sought that corrected the assembly defect of OmpF. These extragenic suppressor mutations were mapped at 45 min on the Escherichia coli chromosome. The suppressor mutations displayed no allele specificity and were recessive to the wild-type allele. In the presence of a suppressor, mutant stable trimers appeared in an almost normal manner. The appearance of stable trimers concurred with a substantial loss of the high-molecular-weight OmpF species. At this stage, it is not clear whether the high-molecular-weight species of OmpF is a normal assembly intermediate or a dead-end assembly product. The results presented in this study raise the intriguing possibility of a chaperone-like activity for the wild-type suppressor gene product.     

Novel non-suppressing mutants of Escherichia coli tRNATyr su+3.

Several addition and deletion mutations were constructed in the region of the gene for Escherichia coli tRNATyr su+3 corresponding to the dihydrouracil loop of the mature tRNA. None of these resulting mutants had detectable suppressor function compared to the parent gene yet some directed the synthesis of mature tRNA. These latter mutants may affect the ability of the tRNA to be aminoacylated or to interact with the translational machinery on the ribosome.     

Isolation and characterization of FliK-independent flagellation mutants from Salmonella typhimurium.

A flagellum of Salmonella typhimurium and Escherichia coli consists of three structural parts, a basal body, a hook, and a filament. Because the fliK mutants produce elongated hooks, called polyhooks, lacking filament portions, the fliK gene product has been believed to be involved in both the determination of hook length and the initiation of the filament assembly. In the present study, we isolated two mutants from S. typhimurium which can form flagella even in the absence of the fliK gene product. Flagellar structures were fractionated from these suppressor mutants and inspected by electron microscopy. The suppressor mutants produced polyhook-filament complexes in the fliK mutant background, while they formed flagellar structures apparently indistinguishable from those of the wild-type strain in the fliK+ background. Genetic and sequence analyses of the suppressor mutations revealed that they are located near the 3'-end of the flhB gene, which has been believed to be involved in the early process of the basal body assembly. On the basis of these results, we discuss the mechanism of suppression of the fliK defects by the flhB mutations and propose a hypothesis on the export switching machinery of the flagellar proteins.     

Nonsense mutations in the amylomaltase gene and other loci of Streptococcus pneumoniae

Summary Maltose-negative mutations in the amylomaltase gene of Streptococcus pneumoniae were examined for the presence of nonsense mutations. Out of 28 single-site mutants tested, 3 were shown to be suppressible by an amber suppressor previously found by Gasc et al. (1979). In the presence of the suppressor these mutants manifested 10–30% of wild type amylomaltase activity. In addition to the amylomaltase governed by malM, and the maltosaccharide phosphorylase governed by malP (which maps to the side of malM distal to the regulatory gene, malR), a new maltose-inducible protein, governed by another gene, malX, was observed in gel electrophoretic patterns. The malX gene maps on the side of malM proximal to the malR gene. The approximate molecular weights of the amylomaltase, phosphorylase and malX polypeptides are 62,000, 87,000 and 50,000, respectively. There appear to be no polar effects of the nonsense mutations in the malM gene on synthesis of the gene products of either malP or malX. In a search for nonsense mutants at other loci, one was found in the end gene, which governs the major endonuclease, a membrane enzyme. None were detected among 5 mismatch-repair defective hex mutants analyzed.