unc-54 encodes a muscle myosin in C. elegans (myosin is one of the major components of the thick filaments in muscle). Mutations in two different genes, sup-3 and sup-7, suppress the paralysis produced by mutations in unc-54. sup-3 mutations suppress the effects of a large number of unc-54 mutations as well as mutations in unc-1 5 and unc-22, two other genes that can be mutated to give muscle defects. sup-7 mutations suppress only a few of the unc-54 mutations but also suppression mutations in unc-15, dpy-18, and mec-8 (the dpy mutation makes the animals dumpy, and the mec mutation makes them touch insensitive). Both sup mutations suppress only when homozygous.
What type of suppressors are sup-3 and sup-7? Why?
Which is more likely to be a gene encoding a muscle protein and which is more likely to be a tRNA gene? Why?
A tRNA suppressor mutation in C. elegans suppresses mutations in a number of genes. For some mutations suppression is seen with only one copy of the suppressor, while for others suppression requires two copies of the suppressor. (Note: this i s an example of a mutation that, depending on the circumstances, can give rise to both dominant and recessive phenotypes.) Which of the two types of suppressible genes would you guess encodes structural proteins and which enzymatic proteins? Why?
A C. elegans strain carrying a suppressible dpy mutation (this makes the animals dumpy) and a tRNA suppressor mutation (two copies of the suppressor are needed to suppress the Dpy phenotype) is mutagenized to find F1 revertants (i.e. animals that are now Dpy). The majority of these are found to be in the suppressor gene.
Mutations in what other genes could have arisen?
How do you think the rate that the revertants were found compares to the rate that the original suppressor mutation was found?
Given the fact that there are multiple copies of each tRNA gene in C. elegans, what do you think the phenotype of a homozygote carrying the revertant mutation would be (remember the animals are still homozygous for the dpy mutation)?
Given your result in (c), do you think it would be possible to identify loss-of-function mutations for a tRNA gene from a wild-type stock?
You are asked to obtain and analyze mutants in C. elegans that are unable to move backward and you get the following.
|
Gene |
Total # Alleles |
# Recessive |
# Temperature Sensitive |
#Suppressed by sup-1 |
#Suppressed by sup-2 |
|
bac-1 |
20 |
20 |
1 |
2 |
20 |
|
bac-2 |
3 |
3 |
0 |
1 |
0 |
|
bac-3 |
15 |
0 |
0 |
0 |
0 |
|
bac-4 |
18 |
16 |
2 |
0 |
0 |
|
bac-5 |
16 |
16 |
16 |
1 |
0 |
How do you suspect the suppressor mutations, sup-1 and sup-2, act? Give reasons for your guesses and describe experiments you could do to test them.
For each gene predict, if possible, the phenotype produced from a complete loss-of-function allele. Give reasons to support your answer and indicate any experiments you would have to do to support your hypothesis. If you cannot predict the phenotype, give reasons why you cannot and say what experimental data you would need.
Compare and contrast the following.
An enhancer element and an enhancer mutation.
Physiological and informational suppression
Gene-specific and allele-specific suppression