Different Degrees Of Compensation Research Articles

Donor Levels in n‐InSb and Their Ionization in Electric Field

AbstractThe temperature dependences of the Hall coefficient RH, the conductivity σ, and the electron mobility μ in n‐type InSb samples with different degree of compensation have been investigated. Deep donor levels have been found at Ec −1.8 × 10−2eV and Ec −6.7 × 10−2eV. The parameters of these levels were estimated in terms of the many‐level model. The voltage‐current characteristics and the dependences RH = f(E) and RH σ = f(E) have been studied in detail at T=4.2 °K. It is shown that at E ≈︁ (2 to 8) × 10−2 V/cm a partial ionization of the shallow donor level Ec −6 × 10−4 eV can occur (tunnel transition), whereas at E > 10−1 V/cm an impact ionization of the donor level Ec − 1.8 × 10−2 eV is produced by electrons heated by the electric field.

Streptomycin-dependant mutants of Proteus mirabilis: their genetics, suppression and modification

1. Mutations toward streptomycin dependence occur in structural and functional heterogenous subunits of the streptomycin locus, which have been characterized as nonidentical alleles.str-d mutants of different, nonidentical alleles differ among themselves with respect to: a) the degree of depression of enzyme synthesis, the reaction toward sub-and superoptimal streptomycin concentrations, and their streptomycin and amino acid dependence, respectively, when grown in minimal medium; b) their allelespecific reaction with certain alleles of the genetic suppressorsu-str. They recombine among themselves with low frequency. 2. No specific subunits for thestr-d orstr-r mutation have been found within the complexstr-locus. It has been shown that a distinctstr-d allle may revert intraallelic to thestr-rd orstr-sd condition. In addition to intraallelic reversions, suppressor mutations cause streptomycin independence. 3. Thestr-d mutation is characterized by pleiotropic changes of the protein synthesis apparatus. It has been suggested that these changes affect the translation process and cause the dependence on streptomycin which acts as misreading inducing agent. 4. The function of streptomycin is allelespecific and represents a case of informational suppression. 5. The majority of streptomycin independent revertants is streptomycin-sensitive as the wild-type is. The major part of these revertants represent suppressor mutants genetically, and they are phenotypical either auxotrophic or noncompletely prototrophic. 6. Three groups of nonidentical suppressor alleles have been identified; between two of them (su-str A and-B) recombination occur with low frequency, while mutants of groupA orB recombine with mutants of groupsu-str C with higher frequency.A, B andC are thought to be components of one complex suppressor locussu-str. 7. The function of the suppressorsu-str is allelespecific, i.e. a certainsu-str allele either suppresses only distinctstr-d alleles or gives rise in combination with otherstr-d alleles to differences in the suppressor phenotype (auxotrophy or noncomplete prototrophy). 8. Auxotrophic and noncompletely prototrophic suppressor mutants differ in the effectivity of enzyme synthesis. These differences are interpreted as an expression of different degrees of compensation by the function of the suppressor of thestr-d allele induced misreading. Auxotrophy as well as noncomplete prototrophy are, therefore, integrated components or results of the allelespecific suppressor function. 9. The geneic suppressorsu-str has been characterized as an informational suppressor. 10. The suppressorsu-str interferes with the streptomycin indifference determining allelestr-r and with the allelesstr-s andstr-sd; in the first case the effect is a change in the degree of resistance, in the latter case, a depression of growth rate. 11. Suppressor mutants mutate with a generally high frequency to a limited streptomycin resistance. This change is caused by mutation of a modifier genemodstr. 12. Within the genome of suppressor mutants,modstr determines a limited streptomycin resistance; when being present in combination with the unsuppressedstr-d alleles,modstr acts in a similar way assu-str alleles; in the wild-type genomemodstr does not function as a resistance marker. These facts show that the function ofmodstr is genome specific. 13. It is supposed that the function ofmodstr like that ofsu-str is a result of a modification of certain ribosomal components.