Streptomycin-abhängige Mutanten vonProteus mirabilis: Ihre Genetik, Suppression und Modifikation

Summary

• 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-r ^d orstr-s ^d 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-s ^d; 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 genemod ^str.

• 12. Within the genome of suppressor mutants,mod ^str determines a limited streptomycin resistance; when being present in combination with the unsuppressedstr-d alleles,mod ^str acts in a similar way assu-str alleles; in the wild-type genomemod ^str does not function as a resistance marker. These facts show that the function ofmod ^str is genome specific.

• 13. It is supposed that the function ofmod ^str like that ofsu-str is a result of a modification of certain ribosomal components.