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

Inhibition of bacterial ribosome assembly: a suitable drug target?

The assembly of bacterial ribosomes is viewed with increasing interest as a potential target for new antibiotics. The in vivo synthesis and assembly of ribosomes are briefly reviewed here, highlighting the many ways in which assembly can be perturbed. The process is compared with the model in vitro process from which much of our knowledge is derived. The coordinate synthesis of the ribosomal components is essential for their ordered and efficient assembly; antibiotics interfere with this coordination and therefore affect assembly. It has also been claimed that the binding of antibiotics to nascent ribosomes prevents their assembly. These two contrasting models of antibiotic action are compared and evaluated. Finally, the suitability and tractability of assembly as a drug target are assessed.

Parental age dependent changes as a source of genetic variation in Drosophila melanogaster

It has been shown repeatedly that numerous cumulative changes occur in chromosomes of D. melanogaster, as an effect of ageing which, especially in the homozygous state, significantly affect different fitness components of their carriers. It appears that the observed age-affected events are produced by systematic and ontogenetically programmed changes in genetic loads at specific chromosomes, which are transferable to following generations. It has been suggested that such changes could be of mutational origin, and that they could be more frequent at gene loci which are epigenetically active during ontogenesis. It was demonstrated that a large sample of identical chromosomes behave quite differently in the homozygous state when obtained from aged compared to non-aged parents, producing a significant decrease in relative viability, length of preadult development, and longevity of their carriers, as well as in the frequency of recombinations of corresponding chromosomes. A specific treatment by streptomycin resulted in remarkably milder effects of ageing, which is in accordance with the statement of some authors that such a treatment may diminish the frequency of spontaneous recessive mutations in their carriers. Thus the observed age-affected changes could be an important source of developmental and evolutionary variation of living organisms.

Amber suppressors of Erwinia chrysanthemi

Mutations trp1 and thyA1, both of a polyauxotrophic derivative of the Erwinia chrysanthemi strain B374, were characterized as amber mutations with an Escherichia coli suppressor, supA1P2, which inserts a glutamine in response to UAG. Simultaneous reversion of both mutations allowed us to isolate amber suppressor mutants of E. chrysanthemi. These suppressors were tested with a set of amber mutants of bacteriophage Mu which had been previously characterized on E. coli. The two independently isolated suppressors behaved as supD and supE mutants, respectively, of E. coli.

Mutations affecting replication and copy number control in plasmid mini-F both reside in the gene for the 29-kDa protein

We have isolated and characterized cop, copts, and repam mutants of plasmid mini-F after in vitro mutagenesis with hydroxylamine. cop mutants exhibit a copy number of about 10 per cell. The copts mutants are cold-sensitive and have, at 25 degrees C, a copy number of about 30-40 copies per cell, which drops to 4 copies at 42 degrees C. The cop and repam mutations affect the 29-kDa E protein. The Copts phenotype results from the simultaneous occurrence of two mutations, a cop mutation in the E protein and a temperature-dependent mutation (termed ecp) enhancing the Cop phenotype at low temperature. The latter new type of mutation is located within the DNA region 44.1-44.85F. Complementation experiments with plasmid cointegrates show that the wild-type gene is dominant over the cop allele. The nucleotide sequences of the cop and the repam mutations have been determined.

Blue ghosts: a new method for isolating amber mutants defective in essential genes of Escherichia coli

We describe a technique which permits an easy screening for amber mutants defective in essential genes of Escherichia coli. Using this approach, we have isolated three amber mutants defective in the rho gene. An extension of the technique allows the detection of ochre mutants and transposon insertions in essential genes.

Amber-mutants of plasmid mini-F defective in replication

Amber mutants of the mini-F plasmid pML31 were isolated with the mutagen hydroxylamine. Under non-permissive conditions amber mutants segregate and show no incorporation of label into supercoiled plasmid DNA in double-label experiments. Wild-type and one mutant of mini-F were integrated by recombinant DNA techniques into the single EcoRI site of plasmid vector pBR322. Plasmid specific proteins were labeled in minicells and analysed by SDS-PAGE. A 34,000 dalton molecular weight protein was identified to be missing in the amber mutant of plasmid mini-F.

High efficiency temperature-sensitive amber suppressor strains of Escherichia coli K12: isolation of strains with suppressor-enhancing mutations

Two independent high efficiency ts amber suppressor strains have been isolated as derivatives of a well-characterized supD ts suppressor strain (Oeschger and Woods, 1976). The mutations which raise suppressor activity have been shown by Hfr mapping to be distinct from each other and the supD locus. One of the isolates provides up to 100% efficient suppressor activity at low temperatures.

Polarity of amber mutations in ribosomal protein genes of Escherichia coli

Two amber mutations have been mapped inside the spcA-strA region (now called rpsE-rpsL) on the bacterial genome. Derivatives of the transducing phage lambda fus3 carrying each mutation were constructed and assayed in ultraviolet-irradiated bacteria to identify the mutated genes and measure the polarity of the mutations. The data indicated that both mutations, 3162(Am) and 3161(Am), affect genes coding for ribosomal proteins: rplC (L3) and rpsN (S14), respectively. It was shown also that each mutation exerts, inside of its respective operon (S10 and spc units), a relatively strong polar effect on genes distal to the mutated locus.

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).