A mutation in the elongation factor EF1-?, which is essential for protein synthesis, causes it to have a higher affinity for GDP than normal. This decreases the growth rate of the cells due to a decrease in protein synthesis. A secondary mutation in the cells increases the growth rate back toward normal. Which of the following is the secondary mutation?

A. Decreased affinity of elongation factor EF1-? for guanosine triphosphate
B. Decreased affinity of elongation factor EF1-? for guanosine triphosphate
C. Elimination of EF1-?
D. Elimination of EF1-?
E. Increased affinity of elongation factor EF1-? for the mutant EF1-?

The correct answer is E. Before binding mRNA, aminoacyl-tRNA first binds guanosine triphosphate (GTP) and elongation factor EF1-?. When tRNA binds to the A site of the ribosome, GTP is hydrolyzed to guanosine diphosphate (GDP). The GDP-EF1-? complex then binds elongation factor EF-?, allowing GDP to be released. Next, GTP binds the complex and EF-? disassociates, leaving GTP-EF1-? regenerated for another cycle. Therefore, increased affinity of EF1-? for the mutant EF1-? counteracts the increased affinity of EF1-? for GDP.

Answer A is not correct. Decreased affinity of elongation factor EF1-? for guanosine triphosphate would decrease protein synthesis even further.

Answer B is not correct. Decreased affinity of elongation factor EF1-? for guanosine triphosphate would decrease protein synthesis.

Answer C is not correct. Elimination of elongation factor EF1-? would completely halt protein synthesis.

Answer D is not correct. Elimination of elongation factor EF1-? would completely halt protein synthesis.

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