Page 156 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

Problems 315

53. Yet another class of suppressor mutations not de- accompanying diagram). For Pyl to be incorporated
Pyl
scribed in the chapter are mutations in tRNA genes into a protein, Pyl-charged tRNA must arrive at the
that can suppress frameshift mutations. What would ribosome before translation is terminated.
have to be true about a tRNA that could suppress a
frameshift mutation involving the insertion of a single Pyl tRNA synthetase Pyl-charged tRNA Pyl
base pair? HC N H
—
H 2 C CH C NH (CH 2 ) 4 —C—NH 2
54. At least one nonsense suppressing tRNA is known HC N H — CH O = C —
—
that can suppress more than one type of nonsense H 2 C CH CH C O NH (CH 2 ) 4 — C—NH 2 O - – CH 3 O O
—
O =
C —
codon. CH 3 O–P O = 5'
–
O — Adenosine
a. What is the anticodon of such a suppressing tRNA? Pyrrolysine OH
(Pyl)
b. What stop codons would it suppress?
c. Could this tRNA possibly also function as a mis- 5' AUC
UAG
sense suppressor? mRNA
d. What are the amino acids most likely to be carried AUC
by this suppressing tRNA?
55. An investigator was interested in studying UAG non- a. Explain two ways in which the mechanism for Pyl
sense suppressor mutations in bacteria. In one species specification differs from that of selenocysteine
of bacteria, she was able to select two different mu- (Sec) incorporation.
tants of this type, one in a tRNA gene and the other b. How is the mechanism for Pyl specification similar
Tyr
in a tRNA gene, but in a second species, she was to nonsense suppression? (See Fig. 8.34.)
Gln
not able to obtain any such nonsense suppressor mu- 58. Canavanine is an amino acid similar to arginine (see
tations, even after very extensive effort. What could accompanying figure) that is normally synthesized by
explain the difference between the two species? some plants. Usually, in plants or animals that don’t
56. Brenner’s m mutant phages (m –m ) described in make canavanine, arginine aminoacyl-tRNA synthe-
6
1
Fig. 8.8 were suppressed when grown in suppressor tase cannot distinguish between canavanine and argi-
−
(su ) mutant bacteria; they produced full-length M nine, and tRNA Arg can be charged with canavanine.
proteins that functioned like wild-type M protein. Incorporation of canvanine in proteins in place of ar-
−
a. What gene do you think was mutant in the su ginine can cause misfolding and destroys protein
bacteria? structure and function.
−
−
b. When the m phages were propagated in the su a. Can you think of a reason why a plant might have
bacterial strain, not all of the proteins made by the evolved the ability to make canavanine?
mutant m alleles were identical to wild-type M b. How do you suppose plants that make canavanine
protein. How did some of them differ? escape its toxicity?
57. In certain bacterial species, pyrrolysine (Pyl), some- c. A particular vining legume called Dioclea mega-
times called amino acid 22, is incorporated into poly- carpa makes canavanine and yet still has a single
peptides through an unusual use of the genetic code: insect predator, a beetle, Caryedes brasiliensis.
Pyl is specified by UAG triplets in the middle of the The beetle lays its eggs on the ripe fruit of the
open reading frame of certain rare genes. These bac- vine, and after hatching, the beetle larvae live in
teria have a pyrrolysine tRNA synthetase that attaches the fruit until they mature into adults. How do you
Pyl to a tRNA with the anticodon 5′ CUA 3′ (see suppose that the beetle evades canavanine toxicity?

(middle photo): © Mark W. Skinner, hosted Canavanine
by the USDA-NRCS PLANTS Database; NH O
(right photo): Source: Sarah McCaffrey/ 2
Museum Victoria, http://www.padil.gov.au/ O
pests-and-diseases/pest/main/142145/ H N N OH
2
41386 CC-BY
NH 2
NH O

H N N OH
2
H NH 2 2.5 cm 0.5 mm
Arginine Dioclea megacarpa Caryedes brasiliensis