Download Problem Set 4

Problem Set 4
1. Explain why I− mutations in the lac system are normally recessive to I+ mutations
and why I+ mutations are recessive to Is mutations.
2.
Note: the F' elements are shaded grey.
--------------------3.
I will solve A-D of this problem
4. The diagram below shows how much ("AP activity") different sugars induce
the S. meliloti agp operon, which is needed for utilization of agalactosides. The protein AgpT recognizes inducers and causes the agp genes
to be expressed. What can be said about how S. meliloti AgpT recognizes
molecules that act as inducers of the agp operon. What parts of the molecules
are important? Which can vary greatly? Can you draw a schematic of the
binding pocket of AgpT illustrating what is and what is not recognized?
5. IPTG can enter cells without the aid of membrane protein, while lactose
requires the aid of the membrane protein Lac permease. From the structures of
the two molecules, speculate on why this might be so.
6. In some cases gene systems are repressed when a particular compound is
present, and then become depressed when that compound is absent. This is the
opposite of the Lac system which is repressed when lactose absent, and
becomes depressed when lactose is around. Can you think of a way in which a
system could become derepressed when a compound was absent? For example, if
you found that cells made serine biosynthetic enzymes only when serine was
absent, and if you knew the controlling gene was a repressor, what kind of a
model would fit the data? What kind of mutants would support your model?
7. Fill in the following table by indicating whether b-gal and permease will
be made in each of the strains listed on the left. "P" stands for "promoter"
the region where RNAP binds to start transcription. As always, the "/"
indicates that the region following the slash is carried on something like a
F' plasmid.
8. Imagine that you are studying a newly discovered species of fungi
that forms fruiting bodies and spores. You have isolated a collection
of mutants that don’t sporulated when plated by themselves, but that
do sporulate when mixed with wild type. Having been and enthusiast of
the elegant genetics done in M. xanthus, your are ready to try putting
your mutants into complementation groups based on whether one mutant
forms spores when mixed with another mutant. The data in the table
below are the results of a mutant mixing experiment.
The “+” means spores formed and the “-“ means that spores were not
formed. How many complementation groups (genes or gene groups involved
in a process needed or sporulation) are there? Which mutants are in
each group?
9. Five new His- auxotrophs were isolated. Complementation analysis of each of
the mutants gave the following results (+ indicates growth on minimal medium
without histidine).
1
2
3
4
5
1
-
2
+
-
3
+
-
4
+
+
+
-
5
+
+
+
+
-
How many complementation groups are there?
How many genes do the five His- mutants affect?
10. Various nonsense mutants of phage T4 were tested for their ability to
grow on three different E. coli strains containing uncharacterized suppressor
mutations. The results are summarized in the table below. [+ = ability to
form plaques; 0 = no plaques are formed; supo = no nonsense suppressor]
Phage mutant
sup
o
amber-1
amber-2
amber-3
amber-4
amber-5
ochre-1
ochre-2
ochre-3
0
0
0
0
0
0
0
0
Host strain
supsupsup-63
61
62
+
0
+
+
+
+
+
0
0
0
0
0
+
+
+
0
+
0
0
+
0
0
0
0
Which of the host strains carry amber suppressors?
Which of the host strains carry ochre suppressors?
What anticodon would you expect for the suppressor tRNA of sup-62?
Give two independent explanations for the slight differences in the
suppression patterns of the sup-61 and sup-63 strains.
11. A variety of suppressor mutations have been isolated in chromosomal tRNA
genes in E. coli but tRNA suppressors which insert tryptophan are very rare.
Using a codon table, explain why this is true.