INTERACTION OF STREPTOMYCIN AND A SUPPRESSOR FOR GALACTOSE FERMENTATION IN E. COLI K-12

Biosynthetic Protein Folding and Molecular Chaperons

The problem of linear polypeptide chain folding into a unique tertiary structure is one of the fundamental scientific challenges. The process of folding cannot be fully understood without its biological context, especially for big multidomain and multisubunit proteins. The principal features of biosynthetic folding are co-translational folding of growing nascent polypeptide chains and involvement of molecular chaperones in the process. The review summarizes available data on the early events of nascent chain folding, as well as on later advanced steps, including formation of elements of native structure. The relationship between the non-uniformity of translation rate and folding of the growing polypeptide is discussed. The results of studies on the effect of biosynthetic folding features on the parameters of folding as a physical process, its kinetics and mechanisms, are presented. Current understanding and hypotheses on the relationship of biosynthetic folding with the fundamental physical parameters and current views on polypeptide folding in the context of energy landscapes are discussed.

Cilia - The sensory antennae in the eye

Cilia are hair-like projections found on almost all cells in the human body. Originally believed to function merely in motility, the function of solitary non-motile (primary) cilia was long overlooked. Recent research has demonstrated that primary cilia function as signalling hubs that sense environmental cues and are pivotal for organ development and function, tissue hoemoestasis, and maintenance of human health. Cilia share a common anatomy and their diverse functional features are achieved by evolutionarily conserved functional modules, organized into sub-compartments. Defects in these functional modules are responsible for a rapidly growing list of human diseases collectively termed ciliopathies. Ocular pathogenesis is common in virtually all classes of syndromic ciliopathies, and disruptions in cilia genes have been found to be causative in a growing number of non-syndromic retinal dystrophies. This review will address what is currently known about cilia contribution to visual function. We will focus on the molecular and cellular functions of ciliary proteins and their role in the photoreceptor sensory cilia and their visual phenotypes. We also highlight other ciliated cell types in tissues of the eye (e.g. lens, RPE and Müller glia cells) discussing their possible contribution to disease progression. Progress in basic research on the cilia function in the eye is paving the way for therapeutic options for retinal ciliopathies. In the final section we describe the latest advancements in gene therapy, read-through of non-sense mutations and stem cell therapy, all being adopted to treat cilia dysfunction in the retina.

Suppression of premature termination codons as a therapeutic approach

In this review, we describe our current understanding of translation termination and pharmacological agents that influence the accuracy of this process. A number of drugs have been identified that induce suppression of translation termination at in-frame premature termination codons (PTCs; also known as nonsense mutations) in mammalian cells. We discuss efforts to utilize these drugs to suppress disease-causing PTCs that result in the loss of protein expression and function. In-frame PTCs represent a genotypic subset of mutations that make up ~11% of all known mutations that cause genetic diseases, and millions of patients have diseases attributable to PTCs. Current approaches aimed at reducing the efficiency of translation termination at PTCs (referred to as PTC suppression therapy) have the goal of alleviating the phenotypic consequences of a wide range of genetic diseases. Suppression therapy is currently in clinical trials for treatment of several genetic diseases caused by PTCs, and preliminary results suggest that some patients have shown clinical improvements. While current progress is promising, we discuss various approaches that may further enhance the efficiency of this novel therapeutic approach.

Sensitivity and Resistance to Spectinomycin in Escherichia coli

Inhibition of growth and division of Escherichia coli by spectinomycin is reversible, and the kinetics of its interference with deoxyribonucleic and ribonucleic acid synthesis may be interpreted as secondary effects of inhibition of protein synthesis on the ribosome. Spontaneous mutations to spectinomycin resistance occur in E. coli K-12 at a rate of about 2 x 10(-10). Resistance is transducible with a discrete lag in phenotypic expression, and the kinetics of its development is about the same as that for streptomycin resistance. All spectinomycin-resistant mutants tested contain resistant ribosomes, and all map in a locus (spc) counterclockwise to and 70% cotransducible with the classical str locus. Differences in the residual drug sensitivity of various spectinomycin-resistant mutants, and of their ribosomes, indicate the existence of more than one phenotypic class of resistance.

Kinetic impairment of restrictive streptomycin-resistant ribosomes

Comparisons in vivo and in vitro of wild-type and otherwise isogenic bacteria with five different mutant alleles of the gene (rpsL) specifying ribosomal protein S12, all resistant to high levels of streptomycin, show that the streptomycin-resistant (Smr) phenotype can be subdivided into major groups: restrictive and non-restrictive. The restrictive bacteria have a characteristically lower frequency of nonsense suppression in vivo, and are also slower than the wild type in their rate of protein synthesis. Non-restrictive Smr bacteria on the other hand do not differ significantly from the wild type either in nonsense suppression frequencies or in the rate of translation. A complementary pattern is seen in vitro, where ribosomes from the restrictive Smr bacteria translate poly(U) with a significantly lower missense error frequency than wild-type ribosomes, and also show an increased Michaelis constant (KM) with respect to their substrate, i.e. ternary complexes. Both effects are correlated with the more aggressive proofreading function that is characteristic of these restrictive ribosomes. In contrast, ribosomes isolated from the non-restrictive Smr bacteria do not show any major difference in either proofreading or missense error in vitro when compared to the wild type.

Spectinomycin resistance due to a mutation in an rRNA operon of Escherichia coli

A spectinomycin resistance mutation was isolated in an Escherichia coli rRNA operon (rrnH) located on a multicopy plasmid. Cell-free protein-synthesizing extracts made from cells containing the plasmid were partially resistant to spectinomycin. Although spectinomycin is an aminoglycoside antibiotic, the mutation did not confer resistance to any other aminoglycoside antibiotic tested.

Assignment of two mitochondrially synthesized polypeptides to human mitochondrial DNA and their use in the study of intracellular mitochondrial interaction

Two mitochondrially synthesized marker polypeptides, MV-1 and MV-2, were found in human HeLa and HT1080 cells. These were assigned to the mitochondrial DNA in HeLa-HT1080 cybrids and hybrids by demonstrating their linkage to cytoplasmic genetic markers. These markers include mitochondrial DNA restriction site polymorphisms and resistance to chloramphenicol, an inhibitor of mitochondrial protein synthesis. In the absence of chloramphenicol, the expression of MV-1 and MV-2 in cybrids and hybrids was found to be directly proportional to the ratio of the parental mitochondrial DNAs. In the presence of chloramphenicol, the marker polypeptide linked to the chloramphenicol-sensitive mitochondrial DNA continued to be expressed. This demonstrated that resistant and sensitive mitochondrial DNAs can cooperate within a cell for gene expression and that the CAP-resistant allele was dominant or codominant to sensitive. Such cooperation suggests that mitochondrial DNAs can be exchanged between mitochondria.

Assignment of two mitochondrially synthesized polypeptides to human mitochondrial DNA and their use in the study of intracellular mitochondrial interaction

Two mitochondrially synthesized marker polypeptides, MV-1 and MV-2, were found in human HeLa and HT1080 cells. These were assigned to the mitochondrial DNA in HeLa-HT1080 cybrids and hybrids by demonstrating their linkage to cytoplasmic genetic markers. These markers include mitochondrial DNA restriction site polymorphisms and resistance to chloramphenicol, an inhibitor of mitochondrial protein synthesis. In the absence of chloramphenicol, the expression of MV-1 and MV-2 in cybrids and hybrids was found to be directly proportional to the ratio of the parental mitochondrial DNAs. In the presence of chloramphenicol, the marker polypeptide linked to the chloramphenicol-sensitive mitochondrial DNA continued to be expressed. This demonstrated that resistant and sensitive mitochondrial DNAs can cooperate within a cell for gene expression and that the CAP-resistant allele was dominant or codominant to sensitive. Such cooperation suggests that mitochondrial DNAs can be exchanged between mitochondria.

Cycloheximide resistance of Physarum polycephalum

In the presence of cycloheximide, wild-type plasmodia of Physarum polycephalum exhibit an immediate decrease in deoxyribonucleic acid synthesis, a reduction in the incorporation of [3H]thymidine into thymidine triphosphate, and an increase in the level of thymidine triphosphate, as well as a decrease in protein synthesis. In this study, we have utilized a cycloheximide-resistant (Cycr) amoebic strain selected from a population of cells mutagenized with nitrosoguanidine. Segregation data indicate that the resistance is due to a single mutation. We have used this Cycr mutant to construct Cycr plasmodial strains. Ribosomes isolated from such Cycr plasmodia showed resistance to cycloheximide in vitro, in contrast to ribosomes isolated from wild-type plasmodia. The Cycr plasmodia showed none of the cycloheximide-induced biochemical effects. Plasmodia heterozygous for the resistance marker were sensitive to cycloheximide with regard to growth but showed an intermediate response in the biochemical parameters. Heterokaryons formed by fusion of various proportions of the sensitive and resistant plasmodia showed a resistance with regard to both growth and biochemical parameters which was directly related to the fraction of Cycr plasmodia present in the heterokaryons. The data are consistent with the hypothesis that the effects of cycloheximide on deoxyribonucleic acid synthesis and nucleoside metabolism are secondary to the effect of the drug on protein synthesis in this organism.

Genetic and biochemical analysis of cycloheximide resistance in the fungus Podospora anserina

Genetic analysis of cycloheximide-resistant mutants has shown that at least three genes control the resistance to cycloheximide in Podospora anserina and that the antibiotic resistance is recessive to sensitivity. In vitro and in vivo studies of protein synthesis indicated that for two mutants cycloheximide resistance is associated with the ribosomes. For one of these mutants, the elongation step in protein biosynthesis is insensitive to cycloheximide over a wide range of concentration. In this mutant the resistance to cycloheximide is a property of the 60S subunit.

[3H] dihydrostreptomycin accumulation and binding to ribosomes in Rhizobium mutants with different levels of streptomycin resistance

Rhizobium trifolii B1, a symbiotic nitrogen fixer, is sensitive to streptomycin (10 microgram/ml) and spontaneously produces spheroplast-like forms during cultivation. Streptomycin-resistant mutants selected with high doses of antibiotic (1,000 microgram/ml) showed pleiotropic changes, including loss of spheroplast formation and infectivity to plants, whereas mutants selected with low doses of streptomycin (10 to 100 microgram/ml) retained properties of parent strain B1 (I. Zelazna-Kowalska, Acta Microbiol. Pol., in press). The present studies revealed that strain B1 and its mutant with a high level of streptomycin resistance, B1 strH, accumulated the antibiotic at similar rates. Mutant B1 strL, with a low level of streptomycin resistance (up to 100 microgram/ml), accumulated the antibiotic at a lower rate. Ribosomes isolated from strains B1 and B2 strL bound [3H]dihydrostreptomycin, whereas those from strain B1 strH did not. These observations indicate that, in R. trifolii B1, mutation to a high level of streptomycin resistance affects ribosomal structure, whereas low-level resistance involves a change in membrane permeability.

Altered surface properties of Escherichia coli associated with a specific amino acid change in the S12 ribosomal protein of streptomycin-resistant mutants

Escherichia coli mutants resistant to streptomycin exhibited differences in countercurrent distribution from the parental strains. The degree of difference from the parental strain correlated with the degree of restriction of translation and thus the particular strA allele. The changes in countercurrent distribution in the phase systems used probably resulted predominantly from surface charge alterations. The differences in countercurrent distribution in these and other mutants may be a useful selective technique to obtain different types of mutants for which specific selective techniques may not be available. In addition, it appears that the surface properties of cells, which determine their position in countercurrent distribution, are a function of the translational efficiency and fidelity, and that the surface of cells consists of a mosaic that is an expression of this translational fidelity.

Increased loss of duplicated genes in streptomycin-resistant (strA) mutants of Escherichia coli k-12

The recombination-dependent loss of a duplicated portion of the Escherichia coli chromosome is five- to tenfold greater in strains containing streptomycin resistance (strA) mutations than in the strA+ parental strain. Streptomycin (500 mug/ml) partially reverses the increase. These results suggest an interaction between strA mutations and recombination.

Male-specific bacteriophage MS2 propagation in fluorophenylalanine-resistant Escherichia coli K12

Mutation of Escherichia coli K12 HfrH to resistance to fluorophenylalanine resulted in changes in the plaque morphology of bacteriophage MS2 on this strain and led to an increased efficiency of propagation of the phage in liquid cultures. Evidence was obtained that the mutation resulted in inhibition of early lysis in infected cells and that lysis involved the production of a lysozyme. Genetic studies suggested that the observed pleiotropy of the resistance mutation was due to informational suppression.

Bactericidal action of streptomycin and comparison with spectinomycin in heterozygotes of Escherichia coli

Str(s)/str(r) heterozygotes of Escherichia coli K-12 are shown to be sensitive to the lethal as well as the inhibitory action of streptomycin. The rate of killing was lower in heterozygotes than in sensitive homozygotes, and among heterozygotes it was lower in those with a higher proportion of resistant ribosomes. These strains also differed, in a parallel manner, in the kinetics of inhibition of growth and protein synthesis by streptomycin. Similar results were obtained with spectinomycin and corresponding merodiploid strains. Since spectinomycin is purely bacteriostatic and stabilizes polysomes, it must block resistant ribosomes behind inhibited sensitive ribosomes; hence, these results are consistent with an initial similar polysomal blockade by streptomycin. However, since streptomycin causes gradual polysome breakdown, its dominant lethal action must involve some mechanism other than a permanent polysomal blockade.

Pleiotrophy of p-fluorophenylalanine-resistant and antibiotic hypersensitive mutants of Pseudomonas aeruginosa

Mutants ofPseudomonas aeruginosaisolated as being resistant top-fluorophenylalanine (FPA-r) are pleiotropic, showing patterns of phenotypic alteration of host-controlled modification, growth rate at 37 and 43 °C, ability to grow on minimal medium, response to antibiotics and ribosomal characteristics. Alterations in host-controlled modification are also found in FPA-r mutants ofEscherichia coliB.

The pleiotropy arising in these FPA-r mutants ofP. aeruginosaappears to result from alterations in the specificity of protein synthesis. Phenotypic variations in host-controlled modification of the type found in the FPA-r mutants may provide a method for the detection of suppressor mutations which act by miscoding.

New class of conditional colicin-tolerant mutants

A new class of temperature-dependent tol mutants of Escherichia coli expresses tolerance to colicins only after growth at higher temperatures. Colicin sensitivity or tolerance, acquired by growth at 30 or 41 C, respectively, is not lost upon incubation at the other temperature in the presence of chloramphenicol. The kinetics of conversion from sensitivity to tolerance and vice versa have been analyzed. In one instance, the tol VIII temperature-dependent phenotype was due to the modifying role of a str-r mutation on the suppression of a tol VIII amber mutant by the suppressor mutation suII. The bearing of the present findings on the role of the tol genes is discussed.

T6r+-induced proteins and nucleic acids in Escherichia coli infected in the presence of streptomycin

Streptomycin does not strongly inhibit T-even phage multiplication in the streptomycin-susceptible polyauxotroph, Escherichia coli strain T(-)H(-)U(-). The relatively slight inhibition, observed earlier, on production of late proteins has now been studied further. The phage-induced ribonucleic acid, synthesized in T6 phage infection in the presence of streptomycin, has been characterized by its base composition, size distribution, and behavior in hybridization tests. Comparison of these properties to those of control samples, taken during either early or late periods of infection, have not shown any significant differences. Phage-induced proteins, synthesized at different times during infection, were studied by disc-gel electrophoresis. Staining and autoradiography of the patterns of pulse-labeled proteins, formed in the absence and presence of the antibiotic showed only slight quantitative changes in the appearance of early proteins. More marked quantitative effects were detected later in infection. Nevertheless, changes in the mobilities of the different proteins were not observed in the streptomycin-treated cultures at any time after infection, suggesting the absence of gross misreading sufficiently great to alter the distinctive electrophoretic patterns of the extracts. Cells infected and incubated in the presence of the antibiotic were found to contain intact virus particles, as shown by electron microscopy. Such infected cells contained extensive deoxyribonucleic acid pools and did not develop the rounded nucleoids with enclosed dense bodies characteristic of the lethal action of the antibiotic. On the other hand, infected bacteria previously exposed to lethal concentrations of streptomycin were unable to synthesize the early enzymes, deoxycytidylate (dCMP) hydroxymethylase and dihydrofolate reductase, or to make phage deoxyribonucleic acid and phage. Such previously killed cells contained the rounded and clotted nucleoids and were unable to unravel this pathological structure after phage infection.

Effect of mutation to streptomycin resistance on amber suppressor genes

Three classes of nonidentical streptomycin-resistant mutations were distinguished in Escherichia coli by their effect on the efficiency of suppression by an amber suppressor gene, sup E. The first class of mutation caused a strong restriction in efficiency of suppression of an amber codon in various cistrons of phage lambda and in an alkaline phosphatase structural gene of E. coli. The second class caused weak restriction, and the third class caused no restriction. The restrictive effect of the streptomycin resistance mutation of the first class on the sup E gene was reduced by addition of streptomycin. This mutation had little effect on efficiencies of suppression by amber suppressor genes sup D and sup F. Analyses on the alkaline phosphatase formed in the suppressor strain indicated that mutation to restrictive streptomycin resistance causes a reduction in translation of the amber codon in the alkaline phosphatase structural gene.

Effects of streptomycin deprivation on enzyme synthesis in streptomycin-dependent Escherichia coli

During growth of streptomycin-dependent strains of Escherichia coli in the absence of streptomycin (deprived growth), both constitutive and inducible synthesis of beta-galactosidase were preferentially inhibited. A similar preferential inhibition of constitutive and derepressed synthesis of alkaline phosphatase was observed. Catabolite repression accounted for part, but not all, of the inhibition of the inducible beta-galactosidase synthesis. Serological experiments indicated that that part of the inhibition specificially associated with streptomycin deprivation was not a result of the production of altered beta-galactosidase. It is suggested that during deprived growth the ribosomes of streptomycin-dependent bacteria become impaired in their ability to translate certain messages.

Phenotype masking and streptomycin dependence

In attempting to define the role of ribosomes in the mechanism of streptomycin dependence, a new phenomenon has been discovered. Analysis of this phenomenon-called phenotypic masking-leads to the conclusion that "streptomycin dependent" mutants are actually "drug dependent" because their dependence is equally satisfied by several drugs. These drugs, some of which are totally unrelated chemically, act on the ribosome and induce misreading in vitro and suppression in vivo.

Suppression of fluorophenylalanine resistance by mutation to streptomycin resistance in Pseudomonas aeruginosa

Waltho, Judith A. (University of Melbourne, Victoria, Australia), and B. W. Holloway. Suppression of fluorophenylalanine resistance by mutation to streptomycin resistance in Pseudomonas aeruginosa. J. Bacteriol. 92:35-42. 1966.-Fluorophenylalanine-resistant mutants (fpa-r) of Pseudomonas aeruginosa have been isolated. By cotransduction analysis, the mutations were shown to have at least two chromosomal locations. One locus (fpaA) showed linkage to three other markers, str, try-3bi, and arg-3, and the order of these four linked markers was found to be try-3bi, arg-3, fpaA, str. The linkage relationships of the other fpa loci are not yet known. The phenotypic expression of resistance at the fpaA locus can be suppressed by mutation of the str locus from str-s to str-r, whereas that at an unlinked fpa locus cannot.

Suppression of amber and ochre rII mutants of bacteriophage T4 by streptomycin

Orias, E. (University of California, Santa Barbara), and T. K. Gartner. Suppression of amber and ochre rII mutants of bacteriophage T4 by streptomycin. J. Bacteriol. 91:2210-2215. 1966.-Streptomycin-induced suppression of amber and ochre rII mutants of phage T4 was studied in a streptomycin-sensitive strain of Escherichia coli and four nearly isogenic streptomycin-resistant derivatives of this strain, in the presence and in the absence of an ochre suppressor. Most of the 12 rII mutants tested were suppressed by streptomycin in the streptomycin-sensitive su(-) strain. This streptomycin-induced suppression in the su(-) strain was eliminated by the independent action of at least two of the four nonidentical mutations to streptomycin resistance. In two of the su(+)str-r strains, streptomycin markedly augmented the suppression caused by the ochre suppressor. In those su(-)str-r hosts in which significant streptomycin-induced suppression could be measured, the amber mutants were more suppressible than the ochre mutants.

Effects of mutations to streptomycin resistance on the rate of translation of mutant genetic information

Gartner, T. K. (University of California, Santa Barbara), and E. Orias. Effects of mutations to streptomycin resistance on the rate of translation of mutant genetic information. J. Bacteriol. 91:1021-1028. 1966.-The effects of mutations to streptomycin resistance of independent origin upon the translation of suppressible mutant information were studied in an isogenic series of strains of Escherichia coli. The group of suppressible mutants included 1 mutation in the z gene of the lac operon of E. coli (O(0) (2) allele), 12 mutations distributed among the two rII cistrons of T4, and 13 mutations distributed among at least five cistrons of phage T7. It was concluded that the mutations to streptomycin resistance cause a significant decrease in the rate of translation of the suppressible codons, and that this effect is limited to a few types of codons.

Streptomycinoid Antibiotics: Synergism by Puromycin

Puromycin synergizes the lethal action of streptomycin and related antibiotics. This is interpreted to mean that puromycin action uncovers a sensitive site (or sites) on the 30S ribosome. The streptomycinoid antibiotics can then associate more readily with the ribosome and inhibit further synthesis of valid protein.