Question

Which one of the following experimental results does not support the hypothesis that the ribosome is a ribozyme?

Group of answer choices

Peptidyl transferase activity is not inactivated by treating the ribosome with detergent.

There are no proteins observed within 18 angstroms of the peptidyl transferase active site in the crystal structure.

Peptidyl transferase activity can be inactivated by base changes in the rRNA.

Ribosomes from thermophilic organisms are more stable than from E. coli.

All of the answers support the hypothesis.

Answer 1

The most likelihood answer is

Ribosomes from thermophilic organisms are more stable than from E. coli.

Ribosomes from thermophilic organisms are more stable than from E. coli. There is evidence which suggest that Ribosomes of B. stearothermophilus are more heat stable than E. coli rRNA. It is predicted that this may be due to higher Tm (melting temperature) of B. stearothermophilus rRNA in comparison with E. coli rRNA. However, there is also possibility of the role of ribosome proteins as the stabilizing factors for ribosome against denaturation. The stabilizing factor may also be polyamines as evidences suggested. This creates the confusion on whether the peptidyl transferase is only played by robozyme or whether there is indirect stabilizing role of proteins or other agents ( such as polyamine). Please refer to FRIEDMAN2 et al, JOURNAL OF BACTERIOLOGY, May 1967, p. 1521-1526.

Why?

Reason

1. Peptidyl transferase activity is not inactivated by treating the ribosome with detergent. Proteins are denatured by detergent due to inactivation of charges amino acids by the hydrophilic end of the detergents.

2. There are no proteins observed within 18 angstroms of the peptidyl transferase active site in the crystal structure. This is remarkable evidence that peptidyl transferase activity is not catalyzed by the proteins. The absence of proteins in the vicinity of 18 angstroms suggests that they play no role in the enzymatic activity.

3. Peptidyl transferase activity can be inactivated by base changes in the rRNA. The mutation in the bases of rRNA may hinder the catalytic transfer and reaction specificity and thus may hinder its catalytic activity.