WORKED PROBLEMS

Problem

What would be the effect of a mutation that caused poly(A)-binding proteins to be nonfunctional?

Solution Strategy

What information is required in your answer to the problem?

A statement of the effect of a mutation that eliminated the function of the poly(A) binding protein.

What information is provided to solve the problem?

• A mutation occurs in the gene that encodes poly(A) binding protein.
• The mutation causes the poly(A) protein to be nonfunctional.

For help with this problem, review:

The Degradation of RNA in Section 17.4.

The Addition of the Poly(A) Tail in Section 14.2 (Chapter 14).

Solution Steps

Messenger RNA can be degraded from the 5′ end, the 3′ end, or through internal cleavage. Degradation from the 5′ end requires the removal of the 5′ cap and is usually preceded by the shortening of the poly(A) tail. Poly(A)-binding proteins bind to the poly(A) tail and prevent it from being shortened. Thus, the presence of these proteins on the poly(A) tail protects the 5′ cap, which prevents RNA degradation. If the gene for poly(A)-binding proteins were mutated in such a way that nonfunctional poly(A) proteins were produced, the proteins would not bind to the poly(A) tail. The tail would be shortened prematurely, the 5′ cap removed, and mRNA degraded more easily. The end result would be less mRNA and thus less protein synthesis.