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Polar and antipolar mutants in the tryptophan operon of Salmonella typhimurium
Abstract:
Polar mutations in trpA, the first structural gene of the tryptophan operon of Salmonella typhimurium, have an uncoordinate effect on the expression of the distal genes, with trpB, the second gene, being more drastically affected than the last three. A number of these polar mutant strains grow very poorly on anthranilic acid-supplemented minimal medium. By selecting for more rapid growth in the presence of anthranilic acid, secondary mutant clones showing a correction of the polar effect were isolated. A few of these were analyzed and shown to contain deletions of various segments of the trpA gene. Ten randomly isolated deletion mutants missing various segments of the trp operon were analyzed for possible pleiotropic effects. Five of them showed a pleiotropic effect of some sort and five did not. Of those showing pleiotropic effects, one had lost the promotor-like elements necessary to initiate expression of the operon, three showed possible antipolar effects, and one showed both polar and antipolar effects simultaneously.
Insights
Secondary mutations corrected polar effects in Salmonella typhimurium
Area of Science:
- Microbiology
- Molecular Biology
- Bacterial Genetics
Background:
- Polar mutations in the trpA gene of Salmonella typhimurium disrupt tryptophan operon expression.
- These mutations disproportionately affect distal genes like trpB.
- Some mutant strains exhibit poor growth on specific media.
Purpose of the Study:
- Investigate the genetic basis of polar effects in the tryptophan operon.
- Identify mechanisms that correct polar mutations.
- Analyze pleiotropic effects of deletions within the tryptophan operon.
Main Methods:
- Isolation of secondary mutants with corrected polar effects through selection.
- Genetic analysis of deletion mutants within the trpA gene and wider trp operon.
- Assessment of pleiotropic effects in deletion mutants.
Main Results:
- Secondary mutations, including deletions in trpA, were found to correct polar effects.
- Analysis of ten deletion mutants revealed varied pleiotropic effects.
- Specific deletions resulted in loss of promoter elements, antipolar effects, or combined polar/antipolar effects.
Conclusions:
- Deletions within the trpA gene can restore coordinate expression of the tryptophan operon.
- The tryptophan operon exhibits complex regulatory interactions, with deletions causing diverse pleiotropic outcomes.
- Understanding these regulatory mechanisms is crucial for bacterial gene expression studies.