In a genetic screen using Neurospora crassa, you isolate six different His- muta
ID: 68393 • Letter: I
Question
In a genetic screen using Neurospora crassa, you isolate six different His- mutants (mutants unable to grow in the absence of a histidine supplement), all mutations target different genes (see Table 1 next page). Each mutant was tested on minimal media supplemented with a single precursor to histidine biosynthesis (indicated as a single letter). Results are presented. (+) indicates growth. (-) indicates a failure to grow.
TABLE 1
Precursors added to minimal medium
Strain
S
R
W
M
O
His*
none
Mutant 1
+
+
-
+
+
+
-
Mutant 2
+
-
-
-
-
+
-
Mutant 3
-
-
-
-
-
+
-
Mutant 4
+
-
-
+
-
+
-
Mutant 5
+
+
+
+
+
+
-
Mutant 6
+
+
-
+
-
+
-
WT
+
+
+
+
+
+
+
Mutant 4 and 6
*His=histidine
a) Draw a simple biochemical pathway that includes ordered substrates, arrows and genes (for simplicity, your gene name is the same as the mutant strain. For example, mutant 6 has a mutation in gene 6.
b) Which precursor should accumulate in mutant 6? __________
c) Which precursor should accumulate in mutant 4? __________
d) Which precursor should accumulate in the double mutant (4 and 6)? _____________.
e) Which gene is epistatic, 4 or 6? Explain how you know.
TABLE 1
Precursors added to minimal medium
Strain
S
R
W
M
O
His*
none
Mutant 1
+
+
-
+
+
+
-
Mutant 2
+
-
-
-
-
+
-
Mutant 3
-
-
-
-
-
+
-
Mutant 4
+
-
-
+
-
+
-
Mutant 5
+
+
+
+
+
+
-
Mutant 6
+
+
-
+
-
+
-
WT
+
+
+
+
+
+
+
Mutant 4 and 6
Explanation / Answer
TABLE 1
Precursors added to minimal medium
Strain
S
R
W
M
O
His*
none
Mutant 1
+
+
-
+
+
+
-
Mutant 2
+
-
-
-
-
+
-
Mutant 3
-
-
-
-
-
+
-
Mutant 4
+
-
-
+
-
+
-
Mutant 5
+
+
+
+
+
+
-
Mutant 6
+
+
-
+
-
+
-
WT
+
+
+
+
+
+
+
Mutant 4 and 6
*His=histidine
a) According to the data given above, we can come to the following conclusions:
Substrates: W-->O-->R-->M-->S-->Histidine
Genes: 5-->1-->6-->4-->2-->3
b) Precursor O will accumulate in mutant 6.
c) Precursor R will accumulate in mutant 4.
d) Precursor O will accumulate in double mutant 4 and 6.
e) Gene 6 is epistatic as the function of gene 4 depends on it.
TABLE 1
Precursors added to minimal medium
Strain
S
R
W
M
O
His*
none
Mutant 1
+
+
-
+
+
+
-
Mutant 2
+
-
-
-
-
+
-
Mutant 3
-
-
-
-
-
+
-
Mutant 4
+
-
-
+
-
+
-
Mutant 5
+
+
+
+
+
+
-
Mutant 6
+
+
-
+
-
+
-
WT
+
+
+
+
+
+
+
Mutant 4 and 6
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