High School

A bacterial strain is able to grow on complete medium (all the amino acids) but not on minimal medium or on minimal medium supplemented with leucine. What could be the genotype(s) of this bacterial strain with respect to the arg and leu loci?

Answer :

Final answer:

The bacterial strain described is an auxotroph, meaning it cannot synthesize a certain necessary nutrient on its own, due to a mutation. It doesn't involve the leucine locus, but could possibly involve the arginine locus. Further testing with a range of amino acids in the minimal medium would be necessary.

Explanation:

The bacterium you're describing is known as an auxotroph: a mutant organism that requires a particular additional nutrient which the normal strain, called a prototroph, can synthesize on its own. In this case, the bacterium can grow in a complete medium (which contains all 20 amino acids), indicating that it may have a mutation in a gene involved in the synthesis of an essential compound.

As this bacterial strain is unable to grow on minimal medium or when supplemented with leucine alone, it suggests that the mutation does not involve leucine production. The gene responsible for the strain's auxotrophy could be linked to the arginine (arg) locus because only the addition of all the amino acids allows it to grow, but this is not guaranteed since we know it doesn't involve leucine production. Further testing on minimal medium supplemented with other amino acids would be needed to confirm this.

This scenario is similar to the Escherichia coli strains described above. E. coli Strain A can only grow in minimal medium when methionine and biotin are added, while Strain B requires threonine, leucine, and thiamine. This shows that each strain has a distinct auxotrophic mutant genotype that needs certain nutrients added to the medium for growth to occur. The mutants come from the original prototrophic strain, which can manufacture all necessary nutrients on its own.

Learn more about Auxotrophy here:

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The bacterial strain genotype could be leu- arg+. It cannot synthesize leucine but can synthesize arginine.

This can be inferred because the strain can grow on complete medium (which provides all the amino acids, including leucine) but not on minimal medium or minimal medium supplemented with leucine. Therefore, the genotype of the bacterial strain could be leu- (unable to synthesize leucine) and arg+ (able to synthesize arginine).