You recently isolated two separate strains of yeast which have growth defects wh

Question

You recently isolated two separate strains of yeast which have growth defects when grown in uracil - lacking media. Normal (wildtype) yeast do not require uracil for growth. Mutant strain #1 requires uracil. Mutant strain #2 is able to grow in uracil lacking media at permissive temperatures (30 degree C) but is unable to grow at restrictive temperatures (35 degree C). All other processes in both mutant strains appear normal. Using Northern blot analysis you detect a 1.3 kDa mRNA species produced from gene X in both the wildtype and mutant strain #2 but a 1.7kDa species In mutant #1. You also have an antibody specific for Protein x. Western blot analysis detects a 60kDa species of Protein X in both the wild type and mutant #2 but a 15kDa species in mutant #2. Below Is the

Explanation / Answer

a. The mutant strain could have a mutation in splicing site, which results in the addition of intron sequences to the last exon of the gene. This might be the reason for larger mRNA compared to the wildtype mRNA.

However, the mRNA is large it would contain a mutation that produces a termination codon. The result is, truncated protein that is 15 kDa rather than 60 kDa.

b. The mutant strain could have a temperature sensitive mutant gene that takes on a normal functional phenotype when the temperature is permissive, but becomes nonfunctional when the temperature becomes restrictive.

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