Chapter 8, Problem 57P

In certain bacterial species, pyrrolysine (Pyl), sometimes called amino acid 22, is incorporated into polypeptides through an unusual use of the genetic code: Pyl is specified by UAG triplets in the middle of the open reading frame of certain rare genes. These bacteria have a pyrrolysine tRNA synthetase that attaches Pyl to a tRNA with the anticodon 5′ CUA 3′ (see accompanying diagram). For Pyl to be incorporated into a protein, Pyl-charged tRNA^Pyl must arrive at the ribosome before translation is terminated.

 

a.	Explain two ways in which the mechanism for Pyl specification differs from that of selenocysteine (Sec) incorporation.
b.	How is the mechanism for Pyl specification similar to nonsense suppression? (See Fig. 8.34.)