[Genetics and biochemistry of the bacterial ribosome]

Antisuppressor mutations reduce misreading of the genetic code in Aspergillus nidulans

Antisuppressor mutations were isolated in a strain containing the omnipotent suppressor suaC109. The antisuppressors reduce the activity of translational suppressors in vivo and counteract most aspects of the pleiotropic phenotype associated with the suaC and the suaA suppressor mutations. Using an homologous system for cell-free translation, we have measured translational accuracy in two antisuppressor strains with the genotype suaC109 and either the asuB11 or the asuD14 antisuppressor mutation. Ribosomes from antisuppressor mutants have higher levels of translation accuracy than those from the suppressor strain (suaC109, asu+). Mistranslation levels depended solely on the source of the sucrose-cleaned ribosomes. However, the increased accuracy associated with sucrose-cleaned ribosomes from antisuppressor strains can be nullified by salt-washing, suggesting that the component responsible can be washed off.

Every ribosomal suppressor mutation in Aspergillus nidulans has a unique and highly pleiotropic phenotype

18 suppressors of alcR125 have been selected in Aspergillus nidulans. They have been located in genes as follows: 12 in suaA, 1 in suaB and 5 in suaC. Suppressors have been examined to see whether their phenotype is diagnostic for their genotype. Several new traits are described: conidial viability, cycloheximide resistance, fertility, suppression of niaD500, niaD501 and fwA1. These tests, added to those already in use, provide a battery of tests suitable for assigning suppressor mutations to physiological type (tRNA or ribosomal), and in one case to a specific gene since only suaA mutations suppressed fwA1. A very broad range of phenotypes was associated with suppressors such that every mutation had a unique phenotype. This indicates that the ribosomal suppressor mutations are in genes which code directly for ribosomal proteins, rather than genes which code for modifying enzymes.

Antisuppressor mutations in Aspergillus nidulans: cold-resistant revertants of suppressor suaC109

Cold-resistant revertants of the cold-sensitive, ribosomal suppressorsuaC109have been isolated, with a view to obtaining mutations in new ribosomal protein genes. Many revertants had reduced suppressor activity and were classified as antisuppressor mutants. Both intragenic and extragenic reversions were found. In seven strains the extragenic reversion to cold resistance segregated with the antisuppressor phenotype, and these were designatedasumutations. Three of the fiveasugenes, C, B and D were mapped to linkage groups, I, II and V respectively. The antisuppressors are not gene-specific, although they mainly antagonize the activity of ribosomal suppressors. The antisuppressors altered all aspects of the phenotype of suppressorsuaC109including sensitivity to aminoglycoside antibiotics, and are therefore thought to be mutations in ribosomal protein genes.

Ribosomal suppressors and antisuppressors in Podospora anserina: altered susceptibility to paromomycin and relationships between genetic and phenotypic suppression

Informational suppressors and antisuppressors have been previously isolated in Podospora anserina, and their properties suggest that they could be ribosomal mutants involved in the control of translational fidelity. In this paper we present results concerning relationships between these mutants and paromomycin, an aminoglycoside antibiotic known to stimulate translational errors. The mutants were found to manifest an altered growth sensitivity to this drug as compared with the wild-type strain: Most of the suppressors were more sensitive and, in contrast, most of the antisuppressors were more resistant to paromomycin. Moreover, phenotypic suppression of an auxotrophic mutation by paromomycin was observed only if a suppressor and an antisuppressor had been introduced in the strain. These results suggest that ambiguity levels could be altered in the suppressor and antisuppressor strains. In addition, paromomycin was shown to abolish sporulation, which suggests relationships between mistranslation and a step of cellular differentiation.

Search for ribosomal mutants in Podospora anserina: genetic analysis of cold-sensitive mutants

Twenty-four cold-sensitive (prototrophic) mutants were isolated after UV mutagenesis of protoplasts of the fungusPodospora anserina. Genetic analysis of these mutants was performed in order to detect those among them which were most likely to be impaired in translational fidelity. The 24 mutations belonged to 24 different genes. One half of the mutants were pleiotropic and displayed an altered phenotype: growth rate at the permissive temperature, germination of the spores, fertility and/or sporulation. Nine mutants differed from wild-type in their resistance levels to cycloheximide, trichodermin and/or paromomycin. Several mutations were linked to known ribosomal loci. Two mutations behaved like informational antisuppressors: one is allelic to the previously describedAs3gene and the other one defines a new antisuppressor gene,AS6.

Search for ribosomal mutants in Podospora anserina: genetic analysis of mutants resistant to paromomycin

It has recently been shown that paromomycin, an antibiotic of the aminoglycoside family, is also active on eukaryotic cytoplasmic ribosomes. In the fungus Podospora anserina, genetic analysis of ten mutants resistant to high doses of paromomycin shows that this resistance is caused by mutations in two different nuclear genes. These mutants display pleiotropic phenotypes (cold sensitivity, mycelium and spore appearance and coloration, cross-resistance to other antibiotics). Double mutants are either lethal or very altered and unstable. Moreover, the cytochrome spectra of these mutants seem to indicate that cytoplasmic protein synthesis is affected. The mutants also display a slight suppressor effect. We can therefore assume that these mutations affect cytoplasmic ribosomes.

A missense mutation in the gene coding for ribosomal protein S17 (rpsQ) leading to ribosomal assembly defectivity in Escherichia coli

The conditionally lethal mutation, 2861 mis, has been mapped inside the ribosomal protein gene cluster at 72 minutes on the Escherichia coli chromosome and was found to cotransduce at 97% with rpsE (S5). The 2861 mis mutation leads to thermosensitivity and impaired assembly in vivo of 30S ribosomal particles at 42 degrees C. The strain carrying the mutation has an altered S 17 ribosomal protein; the mutational alteration involves a replacement of serine by phenylalanine in protein S 17. Spontaneous reversion to temperature independence can restore the normal assembly in vivo of 30 S ribosomal subunits at 42 degrees C and the normal chromatographical behaviour of the S 17 ribosomal protein in vitro. We conclude therefore that the 2861 mis mutation affects the structural gene for protein S 17 (rpsQ).

The physical localization of the gene for ribosomal protein S20

As a prerequisite to the examination of the structure and properties of the promoter for ribosomal protein S20 of Escherichia coli, I have determined the cleavage sites in lambdadapB2cI857S7 DNA for four restriction endonucleases. Subsequently, purified fragments obtained after digestion of this DNA with BamI or HindIII have served as templates for coupled transcription and translation. This has permitted the localization of the structural gene for S20 within a 1000 base pair segment of lambdadapB2cIB57S7 DNA . Cleavage of this DNA with HindIII partially inactivates the expression of S20 in vitro, implying that one HindIII site lies in or near a region essential for the expression of S20.

Amber mutations in Escherichia coli essential genes: isolation of mutants affected in the ribosomes

A method to obtain amber mutations in ribosomal protein genes is described. tit relies on the P1-mediated localized mutagenesis (Hong and Ames, 1971) and on the fact that the recipient strain contains (a) an efficient but genetically unstable suppressor, (b) a particular thermoinducible lambda prophage which kills suppressor hosts at 42 degrees C. Exposure of these bacteria to the high temperature yields frequent suppressor-free derivatives while none will be found if the strain carries an amber mutation in an essential gene. Eleven mutants have been isolated by this method, of which at least six appear to carry amber mutations. All of them map close to, and to the right of spcA, in a region which codes mostly for ribosomal proteins. Three mutants were studied biochemically; all three show defective ribosomal assembly in vivo upon loss of suppression.