An E. coli strain requires exogenous tryptophan for growth at high temperatures. What type of mutation likely caused this?

The mutation that likely caused the E. coli strain to require exogenous tryptophan for growth at high temperatures is a missense mutation. Missense mutations result in a single amino acid change in a protein. Depending on where this change occurs within the protein sequence, it can significantly alter the protein's function.

In this context, the requirement for external tryptophan suggests that a protein involved in tryptophan synthesis, such as one encoding an enzyme in the biosynthetic pathway, has been affected. A missense mutation may have impaired the function of this enzyme, rendering it less effective or entirely non-functional, which in turn necessitates the addition of tryptophan from the environment for the organism's survival.

This type of mutation can be particularly relevant in adaptive strategies under stress conditions, such as high temperatures, where the fitness of a mutated strain may be challenged, leading to reliance on alternative nutrient sources. Hence, the missense mutation explains the phenotypic requirement observed in the E. coli strain.