Question

An E. coli strain contains a mutation in the malate dehydrogenase gene that results in a stop codon at amino acid position 14. From subcultures of this strain, a colony was obtained that was capable of making malate dehydrogenase. Your approach to examining this phenomenon should be to examine

A) the DNA sequence to see if a reversion has occurred.
B) the DNA sequence to see if a suppressor mutation has occurred.
C) other proteins to see if other stop codons are ignored, indicating the presence of a suppressor tRNA.
D) the DNA and plasmids for the incorporation of a second complete copy of the malate dehydrogenase gene.
E) All of these approaches need to be considered.

Answer 1

All proposed approaches, including examining the DNA for a reversion or suppressor mutation, checking for a suppressor tRNA or the incorporation of a functional gene copy, are valid and should be considered to understand the reemergence of malate dehydrogenase production in the mutant E. coli colony.

Step-by-step explanation:

If an E. coli strain contains a mutation in the malate dehydrogenase gene that results in a stop codon at amino acid position 14, but a colony is later found that is again capable of making malate dehydrogenase, several possibilities should be considered. One approach is to:

• Examine the DNA sequence to see if a reversion mutation has occurred, restoring the original gene function.
• Investigate if a suppressor mutation has occurred elsewhere in the genome that compensates for the original mutation.
• Look for the presence of a suppressor tRNA that reads the stop codon and allows for the translation to continue, potentially by checking if other proteins also ignore stop codons.
• Check for the incorporation of a second, functional copy of the malate dehydrogenase gene into the DNA or plasmids.

Therefore, all the approaches need to be considered to understand the exact mechanism that allowed the mutant strain to regain the ability to produce malate dehydrogenase.